What Is It?<\/h1>\r\n<\/div>\r\nFigure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.\r\n\r\nOrgans of the Lower Gastrointestinal Tract<\/h1>\r\n<\/div>\r\nMost of the bacteria that normally live in the\u00a0[pb_glossary id=\"4575\"]lower gastrointestinal (GI) tract[\/pb_glossary]<\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary]. The latter is arguably the most important organ of the digestive system. It is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all absorption of nutrients take place.\r\n\r\nSmall Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4559\"]small intestine[\/pb_glossary]<\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary], as shown in Figure 15.5.2 and described below.\r\n\r\n[caption id=\"attachment_4631\" align=\"aligncenter\" width=\"600\"]![\"15.5.2\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0817_SmallIntestine_Anatomy-2.png\")
Figure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em>[\/caption]\r\n\r\nThe [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] lining the small intestine is highly enfolded and covered with the finger-like projections called [pb_glossary id=\"4543\"]villi[\/pb_glossary]. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous [pb_glossary id=\"4593\"]surface area[\/pb_glossary] for the absorption of nutrients. Inside each of the villi is a network of tiny [pb_glossary id=\"5835\"]blood[\/pb_glossary] and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.\r\n\r\n[caption id=\"attachment_4632\" align=\"aligncenter\" width=\"640\"]![\"15.5.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Human_jejunum_microvilli_1_-_TEM-2.jpg\")
Figure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em>[\/caption]\r\nDuodenum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4576\"]duodenum[\/pb_glossary]<\/strong> is the first part of the small intestine, directly connected to the [pb_glossary id=\"4558\"]stomach[\/pb_glossary]. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary], and it is where most of the chemical digestion in the entire GI tract takes place.\r\n\r\n[caption id=\"attachment_4635\" align=\"alignleft\" width=\"326\"]![\"15.5.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-2422_Accessory_Organs-2.jpg\")
Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em>[\/caption]\r\n\r\nThe duodenum receives partially digested, semi-liquid [pb_glossary id=\"4582\"]chyme[\/pb_glossary] from the stomach. It receives digestive enzymes and alkaline bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] through the [pb_glossary id=\"5461\"]pancreatic duct[\/pb_glossary], and it receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] through the [pb_glossary id=\"5497\"]common bile duct[\/pb_glossary] (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete [pb_glossary id=\"4571\"]mucous[\/pb_glossary] and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.\r\nDigestion of Carbohydrates in the Duodenum<\/h3>\r\nComplex [pb_glossary id=\"5655\"]carbohydrates[\/pb_glossary] (such as [pb_glossary id=\"5459\"]starches<\/span>[\/pb_glossary]<\/span>) are broken down by the digestive enzyme [pb_glossary id=\"4578\"]amylase[\/pb_glossary] from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple [pb_glossary id=\"316\"]sugars[\/pb_glossary]). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span>\r\nDigestion of Proteins in the Duodenum<\/h3>\r\nIn the duodenum, the pancreatic enzymes [pb_glossary id=\"4597\"]trypsin[\/pb_glossary] and [pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary] cleave [pb_glossary id=\"5813\"]proteins<\/span>[\/pb_glossary]<\/span>\u00a0into peptides. Then, these smaller molecules are broken down into [pb_glossary id=\"5707\"]amino acid<\/span>s<\/span>[\/pb_glossary]<\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span>\r\nDigestion of Lipids in the Duodenum<\/h3>\r\nPancreatic [pb_glossary id=\"4602\"]lipase[\/pb_glossary] breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of [pb_glossary id=\"4599\"]bile[\/pb_glossary] secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.\r\nJejunum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4605\"]jejunum[\/pb_glossary]<\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] (water and fatty acids), [pb_glossary id=\"5703\"]facilitated diffusion[\/pb_glossary] (the simple sugar fructose), or [pb_glossary id=\"5689\"]active transport[\/pb_glossary] (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the [pb_glossary id=\"2702\"]blood[\/pb_glossary], except for fatty acids and fat-soluble vitamins, which are absorbed into the [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Although most nutrients are absorbed in the jejunum, there are a few exceptions:\r\n\r\n \t- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\nParts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"]![\"15.5.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1024px-Blausen_0603_LargeIntestine_Anatomy-2.png\")
Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"]![\"15.5.6](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/512px-Ds00070_an01934_im00887_divert_s_gif.webp-1-e1592580013661-2.png\")
Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n\r\n\r\n15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\nTED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":" <\/p>\n![\"15.5.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/1024px-Bacteroides_biacutis_01-2.jpg\")
Figure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n\nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\nFigure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\nThe mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\nFigure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\nDuodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\nFigure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\nThe duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\nDigestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\nDigestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\nDigestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\nJejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n\n- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\nParts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\nFigure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\nFigure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\nFunctions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
<\/p>\n
\n\n\n15.5 Summary<\/span><\/h1>\n<\/header>\n\n\n- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.5 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Organs of the Lower Gastrointestinal Tract<\/h1>\r\n<\/div>\r\nMost of the bacteria that normally live in the\u00a0[pb_glossary id=\"4575\"]lower gastrointestinal (GI) tract[\/pb_glossary]<\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary]. The latter is arguably the most important organ of the digestive system. It is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all absorption of nutrients take place.\r\n\r\nSmall Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4559\"]small intestine[\/pb_glossary]<\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary], as shown in Figure 15.5.2 and described below.\r\n\r\n[caption id=\"attachment_4631\" align=\"aligncenter\" width=\"600\"] Figure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em>[\/caption]\r\n\r\nThe [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] lining the small intestine is highly enfolded and covered with the finger-like projections called [pb_glossary id=\"4543\"]villi[\/pb_glossary]. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous [pb_glossary id=\"4593\"]surface area[\/pb_glossary] for the absorption of nutrients. Inside each of the villi is a network of tiny [pb_glossary id=\"5835\"]blood[\/pb_glossary] and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.\r\n\r\n[caption id=\"attachment_4632\" align=\"aligncenter\" width=\"640\"] Figure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em>[\/caption]\r\nDuodenum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4576\"]duodenum[\/pb_glossary]<\/strong> is the first part of the small intestine, directly connected to the [pb_glossary id=\"4558\"]stomach[\/pb_glossary]. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary], and it is where most of the chemical digestion in the entire GI tract takes place.\r\n\r\n[caption id=\"attachment_4635\" align=\"alignleft\" width=\"326\"] Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em>[\/caption]\r\n\r\nThe duodenum receives partially digested, semi-liquid [pb_glossary id=\"4582\"]chyme[\/pb_glossary] from the stomach. It receives digestive enzymes and alkaline bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] through the [pb_glossary id=\"5461\"]pancreatic duct[\/pb_glossary], and it receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] through the [pb_glossary id=\"5497\"]common bile duct[\/pb_glossary] (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete [pb_glossary id=\"4571\"]mucous[\/pb_glossary] and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.\r\nDigestion of Carbohydrates in the Duodenum<\/h3>\r\nComplex [pb_glossary id=\"5655\"]carbohydrates[\/pb_glossary] (such as [pb_glossary id=\"5459\"]starches<\/span>[\/pb_glossary]<\/span>) are broken down by the digestive enzyme [pb_glossary id=\"4578\"]amylase[\/pb_glossary] from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple [pb_glossary id=\"316\"]sugars[\/pb_glossary]). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span>\r\nDigestion of Proteins in the Duodenum<\/h3>\r\nIn the duodenum, the pancreatic enzymes [pb_glossary id=\"4597\"]trypsin[\/pb_glossary] and [pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary] cleave [pb_glossary id=\"5813\"]proteins<\/span>[\/pb_glossary]<\/span>\u00a0into peptides. Then, these smaller molecules are broken down into [pb_glossary id=\"5707\"]amino acid<\/span>s<\/span>[\/pb_glossary]<\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span>\r\nDigestion of Lipids in the Duodenum<\/h3>\r\nPancreatic [pb_glossary id=\"4602\"]lipase[\/pb_glossary] breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of [pb_glossary id=\"4599\"]bile[\/pb_glossary] secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.\r\nJejunum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4605\"]jejunum[\/pb_glossary]<\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] (water and fatty acids), [pb_glossary id=\"5703\"]facilitated diffusion[\/pb_glossary] (the simple sugar fructose), or [pb_glossary id=\"5689\"]active transport[\/pb_glossary] (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the [pb_glossary id=\"2702\"]blood[\/pb_glossary], except for fatty acids and fat-soluble vitamins, which are absorbed into the [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Although most nutrients are absorbed in the jejunum, there are a few exceptions:\r\n\r\n \t- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\nParts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"] Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"] Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n\r\n\r\n15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\nTED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":" <\/p>\nFigure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n\nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\nFigure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\nThe mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\nFigure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\nDuodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\nFigure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\nThe duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\nDigestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\nDigestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\nDigestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\nJejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n\n- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\nParts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\nFigure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\nFigure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\nFunctions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
<\/p>\n
\n\n\n15.5 Summary<\/span><\/h1>\n<\/header>\n\n\n- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.5 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Small Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4559\"]small intestine[\/pb_glossary]<\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary], as shown in Figure 15.5.2 and described below.\r\n\r\n[caption id=\"attachment_4631\" align=\"aligncenter\" width=\"600\"] Figure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em>[\/caption]\r\n\r\nThe [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] lining the small intestine is highly enfolded and covered with the finger-like projections called [pb_glossary id=\"4543\"]villi[\/pb_glossary]. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous [pb_glossary id=\"4593\"]surface area[\/pb_glossary] for the absorption of nutrients. Inside each of the villi is a network of tiny [pb_glossary id=\"5835\"]blood[\/pb_glossary] and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.\r\n\r\n[caption id=\"attachment_4632\" align=\"aligncenter\" width=\"640\"] Figure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em>[\/caption]\r\nDuodenum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4576\"]duodenum[\/pb_glossary]<\/strong> is the first part of the small intestine, directly connected to the [pb_glossary id=\"4558\"]stomach[\/pb_glossary]. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary], and it is where most of the chemical digestion in the entire GI tract takes place.\r\n\r\n[caption id=\"attachment_4635\" align=\"alignleft\" width=\"326\"] Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em>[\/caption]\r\n\r\nThe duodenum receives partially digested, semi-liquid [pb_glossary id=\"4582\"]chyme[\/pb_glossary] from the stomach. It receives digestive enzymes and alkaline bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] through the [pb_glossary id=\"5461\"]pancreatic duct[\/pb_glossary], and it receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] through the [pb_glossary id=\"5497\"]common bile duct[\/pb_glossary] (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete [pb_glossary id=\"4571\"]mucous[\/pb_glossary] and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.\r\nDigestion of Carbohydrates in the Duodenum<\/h3>\r\nComplex [pb_glossary id=\"5655\"]carbohydrates[\/pb_glossary] (such as [pb_glossary id=\"5459\"]starches<\/span>[\/pb_glossary]<\/span>) are broken down by the digestive enzyme [pb_glossary id=\"4578\"]amylase[\/pb_glossary] from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple [pb_glossary id=\"316\"]sugars[\/pb_glossary]). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span>\r\nDigestion of Proteins in the Duodenum<\/h3>\r\nIn the duodenum, the pancreatic enzymes [pb_glossary id=\"4597\"]trypsin[\/pb_glossary] and [pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary] cleave [pb_glossary id=\"5813\"]proteins<\/span>[\/pb_glossary]<\/span>\u00a0into peptides. Then, these smaller molecules are broken down into [pb_glossary id=\"5707\"]amino acid<\/span>s<\/span>[\/pb_glossary]<\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span>\r\nDigestion of Lipids in the Duodenum<\/h3>\r\nPancreatic [pb_glossary id=\"4602\"]lipase[\/pb_glossary] breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of [pb_glossary id=\"4599\"]bile[\/pb_glossary] secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.\r\nJejunum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4605\"]jejunum[\/pb_glossary]<\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] (water and fatty acids), [pb_glossary id=\"5703\"]facilitated diffusion[\/pb_glossary] (the simple sugar fructose), or [pb_glossary id=\"5689\"]active transport[\/pb_glossary] (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the [pb_glossary id=\"2702\"]blood[\/pb_glossary], except for fatty acids and fat-soluble vitamins, which are absorbed into the [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Although most nutrients are absorbed in the jejunum, there are a few exceptions:\r\n\r\n \t- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\nParts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"] Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"] Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n\r\n\r\n15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\nTED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":" <\/p>\nFigure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n\nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\nFigure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\nThe mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\nFigure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\nDuodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\nFigure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\nThe duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\nDigestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\nDigestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\nDigestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\nJejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n\n- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\nParts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\nFigure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\nFigure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\nFunctions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
<\/p>\n
\n\n\n15.5 Summary<\/span><\/h1>\n<\/header>\n\n\n- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.5 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em>[\/caption]\r\n\r\nThe duodenum receives partially digested, semi-liquid [pb_glossary id=\"4582\"]chyme[\/pb_glossary] from the stomach. It receives digestive enzymes and alkaline bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] through the [pb_glossary id=\"5461\"]pancreatic duct[\/pb_glossary], and it receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] through the [pb_glossary id=\"5497\"]common bile duct[\/pb_glossary] (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete [pb_glossary id=\"4571\"]mucous[\/pb_glossary] and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.\r\nDigestion of Carbohydrates in the Duodenum<\/h3>\r\nComplex [pb_glossary id=\"5655\"]carbohydrates[\/pb_glossary] (such as [pb_glossary id=\"5459\"]starches<\/span>[\/pb_glossary]<\/span>) are broken down by the digestive enzyme [pb_glossary id=\"4578\"]amylase[\/pb_glossary] from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple [pb_glossary id=\"316\"]sugars[\/pb_glossary]). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span>\r\nDigestion of Proteins in the Duodenum<\/h3>\r\nIn the duodenum, the pancreatic enzymes [pb_glossary id=\"4597\"]trypsin[\/pb_glossary] and [pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary] cleave [pb_glossary id=\"5813\"]proteins<\/span>[\/pb_glossary]<\/span>\u00a0into peptides. Then, these smaller molecules are broken down into [pb_glossary id=\"5707\"]amino acid<\/span>s<\/span>[\/pb_glossary]<\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span>\r\nDigestion of Lipids in the Duodenum<\/h3>\r\nPancreatic [pb_glossary id=\"4602\"]lipase[\/pb_glossary] breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of [pb_glossary id=\"4599\"]bile[\/pb_glossary] secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.\r\nJejunum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4605\"]jejunum[\/pb_glossary]<\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] (water and fatty acids), [pb_glossary id=\"5703\"]facilitated diffusion[\/pb_glossary] (the simple sugar fructose), or [pb_glossary id=\"5689\"]active transport[\/pb_glossary] (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the [pb_glossary id=\"2702\"]blood[\/pb_glossary], except for fatty acids and fat-soluble vitamins, which are absorbed into the [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Although most nutrients are absorbed in the jejunum, there are a few exceptions:\r\n\r\n \t- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\nParts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"] Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"] Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n\r\n\r\n15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\nTED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":" <\/p>\nFigure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n\nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\nFigure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\nThe mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\nFigure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\nDuodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\nFigure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\nThe duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\nDigestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\nDigestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\nDigestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\nJejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n\n- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\nParts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\nFigure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\nFigure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\nFunctions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
<\/p>\n
\n\n\n15.5 Summary<\/span><\/h1>\n<\/header>\n\n\n- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.5 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Digestion of Lipids in the Duodenum<\/h3>\r\nPancreatic [pb_glossary id=\"4602\"]lipase[\/pb_glossary] breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of [pb_glossary id=\"4599\"]bile[\/pb_glossary] secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.\r\nJejunum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4605\"]jejunum[\/pb_glossary]<\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple [pb_glossary id=\"1655\"]diffusion[\/pb_glossary] (water and fatty acids), [pb_glossary id=\"5703\"]facilitated diffusion[\/pb_glossary] (the simple sugar fructose), or [pb_glossary id=\"5689\"]active transport[\/pb_glossary] (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the [pb_glossary id=\"2702\"]blood[\/pb_glossary], except for fatty acids and fat-soluble vitamins, which are absorbed into the [pb_glossary id=\"4552\"]lymph[\/pb_glossary]. Although most nutrients are absorbed in the jejunum, there are a few exceptions:\r\n\r\n \t- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\nParts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"] Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"] Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n\r\n\r\n15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\nBlausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\nTED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":" <\/p>\nFigure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n\nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\nFigure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\nThe mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\nFigure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\nDuodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\nFigure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\nThe duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\nDigestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\nDigestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\nDigestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\nJejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n\n- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\nParts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\nFigure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\nFigure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\nFunctions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
<\/p>\n
\n\n\n15.5 Summary<\/span><\/h1>\n<\/header>\n\n\n- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.5 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
- \r\n \t
- Iron is absorbed in the duodenum.<\/li>\r\n \t
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\r\n \t
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\r\n<\/ul>\r\n
Ileum<\/h2>\r\nThe\u00a0[pb_glossary id=\"4606\"]ileum[\/pb_glossary]<\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.\r\n
\r\nLarge Intestine<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4560\"]large intestine[\/pb_glossary]\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid [pb_glossary id=\"4636\"]feces[\/pb_glossary]. Feces is stored in the large intestine until it leaves the body during [pb_glossary id=\"4638\"]defecation[\/pb_glossary].\r\n
Parts of the Large Intestine<\/h2>\r\nLike the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the [pb_glossary id=\"4639\"]colon[\/pb_glossary].<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the [pb_glossary id=\"4640\"]cecum[\/pb_glossary]. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0[pb_glossary id=\"4641\"]rectum[\/pb_glossary]<\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0[pb_glossary id=\"4556\"]anus[\/pb_glossary]<\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.\r\n\r\n[caption id=\"attachment_4642\" align=\"aligncenter\" width=\"600\"]
Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em>[\/caption]\r\n\r\nThe diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the [pb_glossary id=\"4643\"]appendix[\/pb_glossary].<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.\r\n\r\n[caption id=\"attachment_4645\" align=\"aligncenter\" width=\"339\"] Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em>[\/caption]\r\nFunctions of the Large Intestine<\/h2>\r\nThe removal of water from [pb_glossary id=\"4582\"]chyme[\/pb_glossary] to form [pb_glossary id=\"4636\"]feces[\/pb_glossary] starts in the ascending [pb_glossary id=\"4639\"]colon[\/pb_glossary] and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.\r\n\r\nThe large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.\r\n
\r\nFeature: My Human Body<\/h1>\r\n<\/div>\r\nSerotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the \"happy chemical,\" it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this \"happy chemical\" function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin's main target is brain function, you may be surprised to learn that 90% of the human body's supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.\r\n\r\nThe enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the \"second brain\".\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!\r\n\r\nIn a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.\r\n\r\n \r\n
\r\n\r\n 15.5 Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- The [pb_glossary id=\"4575\"]lower GI tract[\/pb_glossary] includes the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] and [pb_glossary id=\"4560\"]large intestine[\/pb_glossary]. The small intestine is where most [pb_glossary id=\"4551\"]chemical digestion[\/pb_glossary] and virtually all [pb_glossary id=\"4553\"]absorption[\/pb_glossary] of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\r\n \t
- The small intestine consists of three parts: the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary], [pb_glossary id=\"4605\"]jejunum[\/pb_glossary], and [pb_glossary id=\"4606\"]ileum[\/pb_glossary]. All three parts of the small intestine are lined with [pb_glossary id=\"4561\"]mucosa[\/pb_glossary] that is highly enfolded and covered with [pb_glossary id=\"4543\"]villi[\/pb_glossary] and [pb_glossary id=\"4583\"]microvilli[\/pb_glossary], giving the small intestine a huge surface area for digestion and absorption.<\/li>\r\n \t
- The duodenum secretes digestive enzymes, and also receives [pb_glossary id=\"4599\"]bile[\/pb_glossary] from the [pb_glossary id=\"2989\"]liver[\/pb_glossary] via the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and digestive enzymes and bicarbonate from the [pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These digestive substances neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\r\n \t
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\r\n \t
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\r\n \t
- The large intestine consists of the [pb_glossary id=\"4639\"]colon[\/pb_glossary] (which, in turn, includes the [pb_glossary id=\"4640\"]cecum[\/pb_glossary], ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the [pb_glossary id=\"4643\"]appendix[\/pb_glossary] is attached to the cecum of the colon.<\/li>\r\n \t
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid [pb_glossary id=\"4636\"]feces[\/pb_glossary] from the body through the anus. The large intestine is also the site where trillions of [pb_glossary id=\"2588\"]bacteria[\/pb_glossary] help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 15.5 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- [h5p id=\"623\"]<\/li>\r\n \t
- How is the mucosa of the small intestine specialized for digestion and absorption?<\/li>\r\n \t
- What digestive substances are secreted into the duodenum? What compounds in food do they help digest?<\/li>\r\n \t
- What is the main function of the jejunum?<\/li>\r\n \t
- What roles does the\u00a0ileum play?<\/li>\r\n \t
- How do beneficial bacteria in the large intestine help the human organism?<\/li>\r\n \t
- When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer<\/li>\r\n \t
- What causes intestinal gas, or flatulence?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 15.5 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/GTvnjaUU6Xk\r\nWhy do we pass gas? - Purna Kashyap, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/0IVO50DuMCs\r\n
What causes constipation? - Heba Shaheed, TED-Ed, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 15.5.1<\/strong>\r\n\r\n1024px-Bacteroides_biacutis_01<\/a> by CDC\/Dr. V.R. Dowell, Jr. Public Health Image Library (PHIL) ID#3087)<\/a> on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.2<\/strong>\r\n\r\nBlausen_0817_SmallIntestine_Anatomy<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.3<\/strong>\r\n\r\nHuman_jejunum_microvilli_1_-_TEM<\/a> by Louisa Howard, Katherine Connollly \/ E. M. Facility, Dartmouth<\/a>, on Wikimedia Commons is in the public domain<\/a> (https:\/\/en.wikipedia.org\/wiki\/public_domain).<\/i>\r\n\r\nFigure 15.5.4<\/strong>\r\n\r\n512px-2422_Accessory_Organs<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.5<\/strong>\r\n\r\n1024px-Blausen_0603_LargeIntestine_Anatomy<\/a> by\u00a0BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 15.5.6<\/strong>\r\n\r\n512px-Ds00070_an01934_im00887_divert_s_gif.webp (1)<\/a> by Lfreeman04<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n
References<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span><\/span>23.24<\/span><\/span>\u00a0<\/span>Accessory organs [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 23.6). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder<\/span><\/span><\/p>\r\n
Blausen.com Staff. (2014). Medical gallery of Blausen Medical 2014. WikiJournal of Medicine 1<\/em> (2). DOI:10.15347\/wjm\/2014.010. ISSN 2002-4436. \/<\/p>\r\n
TED-Ed. (2018, May 7). What causes constipation? - Heba Shaheed. YouTube. https:\/\/www.youtube.com\/watch?v=0IVO50DuMCs&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, September 8). Why do we pass gas? - Purna Kashyap. YouTube. https:\/\/www.youtube.com\/watch?v=GTvnjaUU6Xk&feature=youtu.be<\/p>\r\n
Yano, J. M., Yu, K., Donaldson, G. P., Shastri, G. G., Ann, P., Ma, L., Nagler, C. R., Ismagilov, R. F., Mazmanian, S. K., & Hsiao, E. Y. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell, 161<\/em>(2), 264\u2013276. https:\/\/doi.org\/10.1016\/j.cell.2015.02.047<\/p>\r\n ","rendered":"
<\/p>\n
Figure 15.5.1 Are you a human with bacteria, or bacteria wearing a human?<\/em><\/figcaption><\/figure>\n \nWhat Is It?<\/h1>\n<\/div>\n
Figure 15.5.1 shows some of the cells of what has been called \u201cthe last human organ to be discovered.\u201d This \u201corgan\u201d weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It\u2019s the mass of bacteria that live in our lower gastrointestinal tract.<\/p>\n
\nOrgans of the Lower Gastrointestinal Tract<\/h1>\n<\/div>\n
Most of the bacteria that normally live in the\u00a0lower gastrointestinal (GI) tract<\/a><\/strong>\u00a0live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine<\/a>. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion<\/a> and virtually all absorption of nutrients take place.<\/p>\n
\nSmall Intestine<\/h1>\n<\/div>\n
The\u00a0small intestine<\/a><\/strong> (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called \u201csmall\u201d because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m2<\/sup>. Structurally and functionally, the small intestine can be divided into three parts, called the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>, as shown in Figure 15.5.2 and described below.<\/p>\n
Figure 15.5.2 The three parts of the small intestine are colour coded in this diagram, with yellow for the duodenum, blue for the jejunum, and pink for the ileum.<\/em><\/figcaption><\/figure>\n The mucosa<\/a> lining the small intestine is highly enfolded and covered with the finger-like projections called villi<\/a>. In fact, each square inch (about 6.5 cm2<\/sup>) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections \u2014 the microvilli<\/a>, shown in Figure 15.5.3. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme<\/a> to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area<\/a> for the absorption of nutrients. Inside each of the villi is a network of tiny blood<\/a> and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.<\/p>\n
Figure 15.5.3 The fringe-like structures in this photomicrograph are microvilli lining the inside of the small intestine.<\/em><\/figcaption><\/figure>\n Duodenum<\/h2>\n
The\u00a0duodenum<\/a><\/strong> is the first part of the small intestine, directly connected to the stomach<\/a>. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion<\/a>, and it is where most of the chemical digestion in the entire GI tract takes place.<\/p>\n
Figure 15.5.4 This figure shows the liver, gallbladder and pancreas, along with the ducts that carry their secretions to the duodenum (labeled small intestine).<\/em><\/figcaption><\/figure>\n The duodenum receives partially digested, semi-liquid chyme<\/a> from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas<\/a> through the pancreatic duct<\/a>, and it receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> through the common bile duct<\/a> (see Figure 15.5.4). In addition, the lining of the duodenum secretes digestive enzymes and contains glands \u2014 called Brunner\u2019s glands \u2014 that secrete mucous<\/a> and bicarbonate. The bicarbonate from the pancreas and Brunner\u2019s glands \u2014 along with bile from the liver \u2014 neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.<\/p>\n
Digestion of Carbohydrates in the Duodenum<\/h3>\n
Complex carbohydrates<\/a> (such as starches<\/span><\/a><\/span>) are broken down by the digestive enzyme amylase<\/a> from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars<\/a>). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.<\/span><\/p>\n
Digestion of Proteins in the Duodenum<\/h3>\n
In the duodenum, the pancreatic enzymes trypsin<\/a> and chymotrypsin<\/a> cleave proteins<\/span><\/a><\/span>\u00a0into peptides. Then, these smaller molecules are broken down into amino acid<\/span>s<\/span><\/a><\/span>. Their digestion is catalyzed by pancreatic enzymes called peptidases.<\/span><\/p>\n
Digestion of Lipids in the Duodenum<\/h3>\n
Pancreatic lipase<\/a> breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile<\/a> secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.<\/p>\n
Jejunum<\/h2>\n
The\u00a0jejunum<\/a><\/strong> is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption<\/a> of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion<\/a> (water and fatty acids), facilitated diffusion<\/a> (the simple sugar fructose), or active transport<\/a> (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood<\/a>, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph<\/a>. Although most nutrients are absorbed in the jejunum, there are a few exceptions:<\/p>\n
- \n
- Iron is absorbed in the duodenum.<\/li>\n
- Vitamin B12 and bile salts are absorbed in the ileum.<\/li>\n
- Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.<\/li>\n<\/ul>\n
Ileum<\/h2>\n
The\u00a0ileum<\/a><\/strong> is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme<\/a> that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.<\/p>\n
\nLarge Intestine<\/h1>\n<\/div>\n
The\u00a0large intestine<\/a>\u00a0<\/strong>\u2014 also called the large bowel \u2014 is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces<\/a>. Feces is stored in the large intestine until it leaves the body during defecation<\/a>.<\/p>\n
Parts of the Large Intestine<\/h2>\n
Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 15.5.5. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon<\/a>.<\/strong>\u00a0The first part of the colon, where chyme enters from the small intestine, is called the cecum<\/a>. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The\u00a0rectum<\/a><\/strong> stores feces until elimination occurs. It\u00a0transitions to the final part of the large intestine, called the\u00a0anus<\/a><\/strong>,\u00a0which has an opening to the outside of the body for feces to pass through.<\/p>\n
Figure 15.5.5 The parts of the large intestine include the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.<\/em><\/figcaption><\/figure>\n The diagram below (Figure 15.5.6) shows a projection from the cecum of the colon that is known as the appendix<\/a>.<\/strong> The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon\u2019s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.<\/p>\n
Figure 15.5.6 The appendix is a projection of the cecum of the large intestine.<\/em><\/figcaption><\/figure>\n Functions of the Large Intestine<\/h2>\n
The removal of water from chyme<\/a> to form feces<\/a> starts in the ascending colon<\/a> and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts \u2014 as well as water \u2014 to be recycled in the body.<\/p>\n
The large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria<\/a> are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.<\/p>\n
\nFeature: My Human Body<\/h1>\n<\/div>\n
Serotonin is a neurotransmitter with a wide variety of functions in the body.\u00a0 Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.\u00a0 While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.\u00a0 Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.<\/p>\n
The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.\u00a0 Interestingly, the ENS can act independently from both the sympathetic\/parasympathetic nervous systems and the brain\/spinal cord, which is why some scientists refer to the ENS as the “second brain”.\u00a0 The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function \u2014 meaning that your gut may actually be partly dictating your mood!\u00a0 Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibres are dedicated to sending information too<\/em> the brain, and only 10% of the fibres receiving information from<\/em> the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!<\/p>\n
In a 2015 study by researchers at Caltech,\u00a0 it was shown that gut bacteria promote serotonin production by enterochromaffin cells.\u00a0 The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.\u00a0 The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.\u00a0 Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.\u00a0 This lead researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.<\/p>\n
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\n\n\n 15.5 Summary<\/span><\/h1>\n<\/header>\n
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- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The duodenum secretes digestive enzymes, and also receives bile<\/a> from the liver<\/a> via the gallbladder<\/a> and digestive enzymes and bicarbonate from the pancreas<\/a>. These digestive substances neutralize acidic chyme<\/a> and allow for the chemical digestion of carbohydrates, proteins, lipids, and nucleic acids in the duodenum.<\/li>\n
- The jejunum carries out most of the absorption of nutrients in the small intestine, including the absorption of simple sugars, amino acids, fatty acids, and many vitamins.<\/li>\n
- The ileum carries out any remaining digestion and absorption of nutrients, but its main function is to absorb vitamin B12 and bile salts.<\/li>\n
- The large intestine consists of the colon<\/a> (which, in turn, includes the cecum<\/a>, ascending colon, transverse colon, descending colon, and sigmoid colon), rectum, and anus. The vestigial organ called the appendix<\/a> is attached to the cecum of the colon.<\/li>\n
- The main function of the large intestine is to remove water and salts from chyme for recycling within the body and eliminating the remaining solid feces<\/a> from the body through the anus. The large intestine is also the site where trillions of bacteria<\/a> help digest certain compounds, produce vitamins, stimulate the immune system, and break down toxins, among other important functions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
\n\n 15.5 Review Questions<\/span><\/h1>\n<\/header>\n
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- The small intestine consists of three parts: the duodenum<\/a>, jejunum<\/a>, and ileum<\/a>. All three parts of the small intestine are lined with mucosa<\/a> that is highly enfolded and covered with villi<\/a> and microvilli<\/a>, giving the small intestine a huge surface area for digestion and absorption.<\/li>\n
- The lower GI tract<\/a> includes the small intestine<\/a> and large intestine<\/a>. The small intestine is where most chemical digestion<\/a> and virtually all absorption<\/a> of nutrients occur. The large intestine contains huge numbers of beneficial bacteria and removes water and salts from food waste before it is eliminated.<\/li>\n
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