Jaundiced Eyes<\/h1>\r\n<\/div>\r\nDid you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.\r\n\r\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_4657\" align=\"alignright\" width=\"438\"]![\"15.6.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0428_Gallbladder-Liver-Pancreas_Location-1-2.png\")
Figure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em>[\/caption]\r\n\r\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the [pb_glossary id=\"2989\"]liver[\/pb_glossary], the major accessory organs of digestion are the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary] \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.\r\n\r\nLiver<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"2989\"]liver[\/pb_glossary]<\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the [pb_glossary id=\"4292\"]diaphragm[\/pb_glossary], to the right of the [pb_glossary id=\"4558\"]stomach[\/pb_glossary]. The liver plays an important role in digestion by secreting [pb_glossary id=\"4599\"]bile[\/pb_glossary], but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.\r\nStructure of the Liver<\/h2>\r\nThe liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.\r\n\r\nAs shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.\r\n\r\n[caption id=\"attachment_4658\" align=\"aligncenter\" width=\"495\"]![\"15.6.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Diagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg_-2.png\")
Figure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em>[\/caption]\r\nFunctions of the Liver<\/h2>\r\nThe main digestive function of the liver is the production of bile.\u00a0[pb_glossary id=\"4599\"]Bile[\/pb_glossary]<\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by [pb_glossary id=\"4659\"]hepatocytes<\/span>[\/pb_glossary]<\/span>. The rest are extracted from the\u00a0blood.<\/span>\r\n\r\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.\r\n\r\n[caption id=\"attachment_4660\" align=\"aligncenter\" width=\"789\"]![\"15.6.4\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gallbladder-by-NIH-Image-Gallery-on-Flickr-CC-BY-NC-24312875104_e6ab50150b_h-2.jpg\")
Figure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em>[\/caption]\r\n\r\nBesides its roles in digestion, the liver has many other vital functions:\r\n\r\n \t- The liver synthesizes [pb_glossary id=\"327\"]glycogen[\/pb_glossary] from [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\r\n \t
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\r\n \t
- The liver synthesizes numerous [pb_glossary id=\"5813\"]proteins[\/pb_glossary]\u00a0and many of the [pb_glossary id=\"5707\"]amino acids[\/pb_glossary]\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\r\n \t
- The liver synthesizes many important\u00a0lipids, including [pb_glossary id=\"5531\"]cholesterol[\/pb_glossary],\u00a0triglycerides, and lipoproteins.<\/li>\r\n \t
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\r\n<\/ul>\r\nThe liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.\r\n\r\n
Gallbladder<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4586\"]gallbladder[\/pb_glossary]<\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.\r\n\r\n[caption id=\"attachment_4662\" align=\"aligncenter\" width=\"563\"]![\"15.6.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Gallbladder_organ-1-2.png\")
Figure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em>[\/caption]\r\n\r\nPancreas<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]<\/strong> is a glandular organ that is part of both the [pb_glossary id=\"5969\"]digestive system[\/pb_glossary] and the [pb_glossary id=\"5985\"]endocrine system[\/pb_glossary]. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.\r\n\r\n[caption id=\"attachment_4665\" align=\"aligncenter\" width=\"519\"]![\"15.6.6\"](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Blausen_0698_PancreasAnatomy-2.png\")
Figure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em>[\/caption]\r\n\r\nAs an endocrine gland, the pancreas secretes several [pb_glossary id=\"5661\"]hormones[\/pb_glossary], including [pb_glossary id=\"2590\"]insulin[\/pb_glossary] and [pb_glossary id=\"6043\"]glucagon[\/pb_glossary], which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] after it enters the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary]. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:\r\n\r\n \t- [pb_glossary id=\"4578\"]Amylase[\/pb_glossary]<\/strong>, which helps digest starch and other carbohydrates.<\/li>\r\n \t
- [pb_glossary id=\"4597\"]Trypsin[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary]<\/strong>, which help digest proteins.<\/li>\r\n \t
- [pb_glossary id=\"4602\"]Lipase[\/pb_glossary]<\/strong>, which helps digest lipids.<\/li>\r\n \t
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\r\n<\/ul>\r\n
\r\n15.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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\nReferences<\/h2>\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\nnature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\n![\"15.6.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/Scleral_Icterus-2.jpg\")
Figure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n\nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\nFigure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\nLiver<\/span><\/p>\nThe\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\nStructure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\nFigure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\nFunctions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\nFigure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\nBesides its roles in digestion, the liver has many other vital functions:<\/p>\n
\n- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\nFigure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\nPancreas<\/span><\/p>\nThe\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\nFigure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\nAs an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n\n- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n\n
\n15.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
What Are\u00a0Accessory Organs of Digestion?<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_4657\" align=\"alignright\" width=\"438\"] Figure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em>[\/caption]\r\n\r\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the [pb_glossary id=\"2989\"]liver[\/pb_glossary], the major accessory organs of digestion are the [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] and\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary] \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.\r\n\r\nLiver<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"2989\"]liver[\/pb_glossary]<\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the [pb_glossary id=\"4292\"]diaphragm[\/pb_glossary], to the right of the [pb_glossary id=\"4558\"]stomach[\/pb_glossary]. The liver plays an important role in digestion by secreting [pb_glossary id=\"4599\"]bile[\/pb_glossary], but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.\r\nStructure of the Liver<\/h2>\r\nThe liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.\r\n\r\nAs shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.\r\n\r\n[caption id=\"attachment_4658\" align=\"aligncenter\" width=\"495\"] Figure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em>[\/caption]\r\nFunctions of the Liver<\/h2>\r\nThe main digestive function of the liver is the production of bile.\u00a0[pb_glossary id=\"4599\"]Bile[\/pb_glossary]<\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by [pb_glossary id=\"4659\"]hepatocytes<\/span>[\/pb_glossary]<\/span>. The rest are extracted from the\u00a0blood.<\/span>\r\n\r\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.\r\n\r\n[caption id=\"attachment_4660\" align=\"aligncenter\" width=\"789\"] Figure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em>[\/caption]\r\n\r\nBesides its roles in digestion, the liver has many other vital functions:\r\n\r\n \t- The liver synthesizes [pb_glossary id=\"327\"]glycogen[\/pb_glossary] from [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\r\n \t
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\r\n \t
- The liver synthesizes numerous [pb_glossary id=\"5813\"]proteins[\/pb_glossary]\u00a0and many of the [pb_glossary id=\"5707\"]amino acids[\/pb_glossary]\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\r\n \t
- The liver synthesizes many important\u00a0lipids, including [pb_glossary id=\"5531\"]cholesterol[\/pb_glossary],\u00a0triglycerides, and lipoproteins.<\/li>\r\n \t
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\r\n<\/ul>\r\nThe liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.\r\n\r\n
Gallbladder<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4586\"]gallbladder[\/pb_glossary]<\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.\r\n\r\n[caption id=\"attachment_4662\" align=\"aligncenter\" width=\"563\"] Figure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em>[\/caption]\r\n\r\nPancreas<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]<\/strong> is a glandular organ that is part of both the [pb_glossary id=\"5969\"]digestive system[\/pb_glossary] and the [pb_glossary id=\"5985\"]endocrine system[\/pb_glossary]. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.\r\n\r\n[caption id=\"attachment_4665\" align=\"aligncenter\" width=\"519\"] Figure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em>[\/caption]\r\n\r\nAs an endocrine gland, the pancreas secretes several [pb_glossary id=\"5661\"]hormones[\/pb_glossary], including [pb_glossary id=\"2590\"]insulin[\/pb_glossary] and [pb_glossary id=\"6043\"]glucagon[\/pb_glossary], which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] after it enters the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary]. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:\r\n\r\n \t- [pb_glossary id=\"4578\"]Amylase[\/pb_glossary]<\/strong>, which helps digest starch and other carbohydrates.<\/li>\r\n \t
- [pb_glossary id=\"4597\"]Trypsin[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary]<\/strong>, which help digest proteins.<\/li>\r\n \t
- [pb_glossary id=\"4602\"]Lipase[\/pb_glossary]<\/strong>, which helps digest lipids.<\/li>\r\n \t
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\r\n<\/ul>\r\n
\r\n15.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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\nReferences<\/h2>\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\nnature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n\nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\nFigure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\nLiver<\/span><\/p>\nThe\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\nStructure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\nFigure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\nFunctions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\nFigure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\nBesides its roles in digestion, the liver has many other vital functions:<\/p>\n
\n- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\nFigure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\nPancreas<\/span><\/p>\nThe\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\nFigure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\nAs an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n\n- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n\n
\n15.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
Figure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em>[\/caption]\r\nFunctions of the Liver<\/h2>\r\nThe main digestive function of the liver is the production of bile.\u00a0[pb_glossary id=\"4599\"]Bile[\/pb_glossary]<\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by [pb_glossary id=\"4659\"]hepatocytes<\/span>[\/pb_glossary]<\/span>. The rest are extracted from the\u00a0blood.<\/span>\r\n\r\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.\r\n\r\n[caption id=\"attachment_4660\" align=\"aligncenter\" width=\"789\"] Figure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em>[\/caption]\r\n\r\nBesides its roles in digestion, the liver has many other vital functions:\r\n\r\n \t- The liver synthesizes [pb_glossary id=\"327\"]glycogen[\/pb_glossary] from [pb_glossary id=\"5451\"]glucose[\/pb_glossary] and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\r\n \t
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\r\n \t
- The liver synthesizes numerous [pb_glossary id=\"5813\"]proteins[\/pb_glossary]\u00a0and many of the [pb_glossary id=\"5707\"]amino acids[\/pb_glossary]\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\r\n \t
- The liver synthesizes many important\u00a0lipids, including [pb_glossary id=\"5531\"]cholesterol[\/pb_glossary],\u00a0triglycerides, and lipoproteins.<\/li>\r\n \t
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\r\n<\/ul>\r\nThe liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.\r\n\r\n
Gallbladder<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4586\"]gallbladder[\/pb_glossary]<\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.\r\n\r\n[caption id=\"attachment_4662\" align=\"aligncenter\" width=\"563\"] Figure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em>[\/caption]\r\n\r\nPancreas<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]<\/strong> is a glandular organ that is part of both the [pb_glossary id=\"5969\"]digestive system[\/pb_glossary] and the [pb_glossary id=\"5985\"]endocrine system[\/pb_glossary]. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.\r\n\r\n[caption id=\"attachment_4665\" align=\"aligncenter\" width=\"519\"] Figure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em>[\/caption]\r\n\r\nAs an endocrine gland, the pancreas secretes several [pb_glossary id=\"5661\"]hormones[\/pb_glossary], including [pb_glossary id=\"2590\"]insulin[\/pb_glossary] and [pb_glossary id=\"6043\"]glucagon[\/pb_glossary], which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] after it enters the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary]. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:\r\n\r\n \t- [pb_glossary id=\"4578\"]Amylase[\/pb_glossary]<\/strong>, which helps digest starch and other carbohydrates.<\/li>\r\n \t
- [pb_glossary id=\"4597\"]Trypsin[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary]<\/strong>, which help digest proteins.<\/li>\r\n \t
- [pb_glossary id=\"4602\"]Lipase[\/pb_glossary]<\/strong>, which helps digest lipids.<\/li>\r\n \t
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\r\n<\/ul>\r\n
\r\n15.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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\nReferences<\/h2>\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\nnature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n\nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\nFigure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\nLiver<\/span><\/p>\nThe\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\nStructure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\nFigure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\nFunctions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\nFigure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\nBesides its roles in digestion, the liver has many other vital functions:<\/p>\n
\n- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\nFigure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\nPancreas<\/span><\/p>\nThe\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\nFigure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\nAs an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n\n- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n\n
\n15.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
Gallbladder<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"4586\"]gallbladder[\/pb_glossary]<\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.\r\n\r\n[caption id=\"attachment_4662\" align=\"aligncenter\" width=\"563\"] Figure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em>[\/caption]\r\n\r\nPancreas<\/span>\r\n\r\nThe\u00a0[pb_glossary id=\"3197\"]pancreas[\/pb_glossary]<\/strong> is a glandular organ that is part of both the [pb_glossary id=\"5969\"]digestive system[\/pb_glossary] and the [pb_glossary id=\"5985\"]endocrine system[\/pb_glossary]. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.\r\n\r\n[caption id=\"attachment_4665\" align=\"aligncenter\" width=\"519\"] Figure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em>[\/caption]\r\n\r\nAs an endocrine gland, the pancreas secretes several [pb_glossary id=\"5661\"]hormones[\/pb_glossary], including [pb_glossary id=\"2590\"]insulin[\/pb_glossary] and [pb_glossary id=\"6043\"]glucagon[\/pb_glossary], which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] after it enters the [pb_glossary id=\"4576\"]duodenum[\/pb_glossary]. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:\r\n\r\n \t- [pb_glossary id=\"4578\"]Amylase[\/pb_glossary]<\/strong>, which helps digest starch and other carbohydrates.<\/li>\r\n \t
- [pb_glossary id=\"4597\"]Trypsin[\/pb_glossary]<\/strong>\u00a0and\u00a0[pb_glossary id=\"4598\"]chymotrypsin[\/pb_glossary]<\/strong>, which help digest proteins.<\/li>\r\n \t
- [pb_glossary id=\"4602\"]Lipase[\/pb_glossary]<\/strong>, which helps digest lipids.<\/li>\r\n \t
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\r\n<\/ul>\r\n
\r\n15.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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\nReferences<\/h2>\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\nnature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n\nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\nFigure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\nLiver<\/span><\/p>\nThe\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\nStructure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\nFigure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\nFunctions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\nFigure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\nBesides its roles in digestion, the liver has many other vital functions:<\/p>\n
\n- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\nFigure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\nPancreas<\/span><\/p>\nThe\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\nFigure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\nAs an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n\n- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n\n
\n15.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
15.6 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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\nReferences<\/h2>\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\nnature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n\nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\nFigure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\nAccessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\nLiver<\/span><\/p>\nThe\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\nStructure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\nFigure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\nFunctions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\nAs shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\nFigure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\nBesides its roles in digestion, the liver has many other vital functions:<\/p>\n
\n- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\nFigure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\nPancreas<\/span><\/p>\nThe\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\nFigure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\nAs an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n\n- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n\n
\n15.6 Summary<\/span><\/h1>\n<\/header>\n\n\n- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n15.6 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
- \r\n \t
- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\r\n \t
- The [pb_glossary id=\"2989\"]liver[\/pb_glossary] is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or [pb_glossary id=\"4659\"]hepatocytes[\/pb_glossary]. The liver receives oxygen in blood from the [pb_glossary id=\"4413\"]aorta[\/pb_glossary] through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the [pb_glossary id=\"4497\"]spleen[\/pb_glossary] through the portal vein.<\/li>\r\n \t
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. [pb_glossary id=\"4599\"]Bile[\/pb_glossary] is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic [pb_glossary id=\"4582\"]chyme[\/pb_glossary] that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\r\n \t
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\r\n \t
- The [pb_glossary id=\"4586\"]gallbladder[\/pb_glossary] is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\r\n \t
- The [pb_glossary id=\"3197\"]pancreas[\/pb_glossary] is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 15.6 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\r\n \t
- [h5p id=\"624\"]<\/li>\r\n \t
- Describe the liver and its blood supply.<\/li>\r\n \t
- Explain the main digestive function of the liver and describe the components of bile and it's importance in the digestive process.<\/li>\r\n \t
- What type of secretions does the pancreas release as part of each body system?<\/li>\r\n \t
- List pancreatic enzymes that work in the duodenum, along with the substances they help digest.<\/li>\r\n \t
- What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?<\/li>\r\n \t
- People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?<\/li>\r\n \t
- Which accessory organ of digestion synthesizes cholesterol?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 15.6 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/8dgoeYPoE-0\r\nWhat does the pancreas do? - Emma Bryce, TED-Ed. 2015.<\/p>\r\nhttps:\/\/youtu.be\/wbh3SjzydnQ\r\n
What does the liver do? - Emma Bryce, TED-Ed, 2014.<\/p>\r\nhttps:\/\/youtu.be\/a0d1yvGcfzQ\r\n
Scar wars: Repairing the liver, nature video, 2018.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n
Attributions<\/h2>\r\nFigure 15.6.1<\/strong>\r\n\r\nScleral_Icterus<\/a> by Sheila J. Toro<\/a> on Wikimedia Commons is used under a \u00a0CC BY 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\n\r\nFigure 15.6.2<\/strong>\r\n\r\nBlausen_0428_Gallbladder-Liver-Pancreas_Location<\/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\n\r\nFigure 15.6.3<\/strong>\r\n\r\nDiagram_showing_the_two_lobes_of_the_liver_and_its_blood_supply_CRUK_376.svg<\/a> by Cancer Research UK<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> \u00a0(https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license.\r\n\r\nFigure 15.6.4<\/strong>\r\n\r\nGallbladder<\/a> by NIH Image Gallery<\/a> on Flickr<\/a> is used CC BY-NC 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-nc\/2.0\/) license.\r\n\r\nFigure 15.6.5<\/strong>\r\n\r\nGallbladder_(organ) (1)<\/a> by BruceBlaus<\/a> on Wikimedia Commons is used under a CC BY-SA 4.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0) license. (See a\u00a0full animation<\/a> of this medical topic at blausen.com.)\r\n\r\nFigure 15.6.6<\/strong>\r\n\r\nBlausen_0698_PancreasAnatomy<\/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
References<\/h2>\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
nature video. (2018, December 19). Scar wars: Repairing the liver. YouTube. https:\/\/www.youtube.com\/watch?v=a0d1yvGcfzQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2014, November 25). What does the liver do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=wbh3SjzydnQ&feature=youtu.be<\/p>\r\n
TED-Ed. (2015, February 19). What does the pancreas do? - Emma Bryce. YouTube. https:\/\/www.youtube.com\/watch?v=8dgoeYPoE-0&feature=youtu.be<\/p>","rendered":"
<\/p>\n
Figure 15.6.1 “Look deep into my eyes.”<\/em><\/figcaption><\/figure>\n \nJaundiced Eyes<\/h1>\n<\/div>\n
Did you ever hear of a person looking at something or someone with a \u201cjaundiced eye\u201d? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.<\/p>\n
\nWhat Are\u00a0Accessory Organs of Digestion?<\/h1>\n<\/div>\n
Figure 15.6.2 The liver, gallbladder, and pancreas are the major accessory organs of digestion. In this figure, the pink tubular structure that starts at the lower stomach and wraps around the pancreas is the duodenum of the small intestine. This is where the accessory organs secrete their digestive substances.<\/em><\/figcaption><\/figure>\n Accessory organs of digestion\u00a0are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver<\/a>, the major accessory organs of digestion are the gallbladder<\/a> and\u00a0pancreas<\/a>. These organs secrete or store substances that are needed for digestion in the first part of the\u00a0small intestine\u00a0\u2014 the duodenum<\/a> \u2014 where most chemical digestion takes place. You can see the three organs and their locations in Figure 15.6.2.<\/p>\n
Liver<\/span><\/p>\n
The\u00a0liver<\/a><\/strong>\u00a0is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm<\/a>, to the right of the stomach<\/a>. The liver plays an important role in digestion by secreting bile<\/a>, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.<\/p>\n
Structure of the Liver<\/h2>\n
The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.<\/p>\n
As shown in Figure 15.6.3, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.<\/p>\n
Figure 15.6.3 The portal vein supplies the liver with wastes filtered out of the blood in the spleen, as well as nutrients from the gastrointestinal tract. Oxygen-rich blood enters the liver via the hepatic artery.<\/em><\/figcaption><\/figure>\n Functions of the Liver<\/h2>\n
The main digestive function of the liver is the production of bile.\u00a0Bile<\/a><\/strong>\u00a0is a yellowish alkaline\u00a0liquid\u00a0that consists of\u00a0water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes<\/span><\/a><\/span>. The rest are extracted from the\u00a0blood.<\/span><\/p>\n
As shown in Figure 15.6.4, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.<\/p>\n
Figure 15.6.4 The common bile duct carries bile from the liver and gallbladder to the duodenum.<\/em><\/figcaption><\/figure>\n Besides its roles in digestion, the liver has many other vital functions:<\/p>\n
- \n
- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
- The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the\u00a0minerals\u00a0iron and copper.<\/li>\n
- The liver synthesizes numerous proteins<\/a>\u00a0and many of the amino acids<\/a>\u00a0needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.<\/li>\n
- The liver synthesizes many important\u00a0lipids, including cholesterol<\/a>,\u00a0triglycerides, and lipoproteins.<\/li>\n
- The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the\u00a0kidneys, which excrete them in urine.<\/li>\n<\/ul>\n
The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic\u00a0location\u00a0and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.<\/p>\n
\nGallbladder<\/h1>\n<\/div>\n
The\u00a0gallbladder<\/a><\/strong> is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 15.6.5). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.<\/p>\n
Figure 15.6.5 The gallbladder is connected to the common duct by the cystic duct. It stores bile secreted by the liver.<\/em><\/figcaption><\/figure>\n Pancreas<\/span><\/p>\n
The\u00a0pancreas<\/a><\/strong> is a glandular organ that is part of both the digestive system<\/a> and the endocrine system<\/a>. As shown in Figure 15.6.6, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.<\/p>\n
Figure 15.6.6 Pancreatic digestive enzymes and bicarbonate travel to the duodenum through the pancreatic ducts. The main pancreatic duct joins with the common bile duct before the latter enters the duodenum.<\/em><\/figcaption><\/figure>\n As an endocrine gland, the pancreas secretes several hormones<\/a>, including insulin<\/a> and glucagon<\/a>, which circulate in the blood. The\u00a0endocrine hormones\u00a0are secreted by clusters of\u00a0cells\u00a0called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive\u00a0enzymes\u00a0and also bicarbonate, which helps neutralize acidic chyme<\/a> after it enters the duodenum<\/a>. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down\u00a0carbohydrates,\u00a0proteins,\u00a0lipids, and\u00a0nucleic acids\u00a0in chyme. The pancreatic digestive enzymes include:<\/p>\n
- \n
- Amylase<\/a><\/strong>, which helps digest starch and other carbohydrates.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- Lipase<\/a><\/strong>, which helps digest lipids.<\/li>\n
- Deoxyribonucleases<\/strong>\u00a0and\u00a0ribonucleases<\/strong>, which help digest nucleic acids.<\/li>\n<\/ul>\n
\n\n 15.6 Summary<\/span><\/h1>\n<\/header>\n
\n- \n
- Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. The accessory organs include the liver, gallbladder, and pancreas. These organs secrete or store substances that are carried to the duodenum of the\u00a0small intestine\u00a0as needed for digestion.<\/li>\n
- The liver<\/a> is a large organ in the abdomen that is divided into lobes and smaller lobules, which consist of metabolic\u00a0cells\u00a0called hepatic cells, or hepatocytes<\/a>. The liver receives oxygen in blood from the aorta<\/a> through the hepatic artery. It receives\u00a0nutrients\u00a0in blood from the GI tract and wastes in blood from the spleen<\/a> through the portal vein.<\/li>\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Other vital functions of the liver include regulating blood sugar levels by storing excess sugar as glycogen, storing many\u00a0vitamins and minerals, synthesizing numerous proteins and lipids, and breaking down waste products and toxic substances.<\/li>\n
- The gallbladder<\/a> is a small pouch-like organ near the liver. It stores and concentrates bile from the liver until it is needed in the duodenum to neutralize chyme and help digest lipids.<\/li>\n
- The pancreas<\/a> is a glandular organ that secretes both\u00a0endocrine hormones\u00a0and digestive\u00a0enzymes. As an endocrine gland, the pancreas secretes insulin and glucagon to regulate blood sugar. As a digestive organ, the pancreas secretes digestive enzymes into the duodenum through ducts. Pancreatic digestive enzymes include amylase (starches) trypsin and chymotrypsin (proteins), lipase (lipids), and ribonucleases and deoxyribonucleases (RNA\u00a0and DNA).<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
\n\n 15.6 Review Questions<\/span><\/h1>\n<\/header>\n
\n- \n
- Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?<\/li>\n
- \n\n
- The main digestive function of the liver is the production of the alkaline\u00a0liquid\u00a0called bile. Bile<\/a> is carried directly to the duodenum by the common bile duct or to the gallbladder first for storage. Bile neutralizes acidic chyme<\/a> that enters the duodenum from the stomach, and also emulsifies fat globules into smaller particles (micelles) that are easier to digest chemically.<\/li>\n
- Trypsin<\/a><\/strong>\u00a0and\u00a0chymotrypsin<\/a><\/strong>, which help digest proteins.<\/li>\n
- The liver synthesizes glycogen from glucose<\/a> and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.<\/li>\n
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