57 Digestive Homeostasis
With the exception of oxygen from the respiratory system and sunlight (for vitamin D formation) from the integumentary system, we obtain all of the materials we need for growth and energy from the digestive system. It is through the digestive system that we obtain water, nutrients and other essential building blocks for metabolism. However, as you know from day to day eating habits, the input into the digestive system can change greatly in amount, frequency and content. So it is up to the digestive system to have complex processes in place that help maintain homeostasis at various levels in the body.
Hormonal and Paracrine Control of Digestion
Hormonal control (for long distance regulation) and paracrine control (for short distance, or local, regulation) help maintain processes throughout the digestive system that regulate secretion of chemicals, mechanical changes and absorption of nutrients.
| Hormone/Paracrine | Production Site | Production Stimulus | Target Organ | Action |
|---|---|---|---|---|
| Cholecystokinin (CCK) | Duodenal mucosa | Fatty chyme, partially digested proteins | Liver, pancreas, gall bladder, Hepato-pancreatic sphincter | Potentiates activity of secretin; increases production of enzyme-rich pancreatic juice; contracts gall bladder and releases bile; relaxes sphincter, permitting bile and pancreatic juice to enter the duodenum |
| Gastrin | Stomach mucosa | Presence of food in stomach; release of acetylcholine by axons | Stomach, small intestine, ileocecal valve, large intestine | Increased secretion by gastric glands; provokes gastric emptying; provokes intestinal muscle contraction; relaxes valve; provokes mass movements |
| Histamine | Stomach mucosa | Presence of food in stomach | Stomach | Stimulates parietal cells to release HCl |
| Intestinal gastrin | Duodenal mucosa | Presence of acidic and partially digested foods in duodenum | Stomach | Activates gastric glands; stimulates motility |
| Motilin | Duodenal mucosa | Fasting; neural stimuli activate release every 1.5 to 2 hours | Proximal duodenum | Activates migrating motility complex |
| Secretin | Duodenal mucosa | Primarily acidic chyme | Stomach, pancreas, liver | Restricts gastric gland secretion and gastric motility in gastric phase of secretion; increases production of pancreatic juice rich in bicarbonate ions; enhances activity of CCKEnhances bile production |
| Serotonin | Stomach mucosa | Presence of food in stomach | Stomach | Contracts stomach muscle |
| Somatostatin | Mucosa of stomach and duodenum, pancreas | Presence of food in stomach; sympathetic axon stimulation | Stomach, pancreas, small intestine | Restricts all gastric secretions; restricts gastric motility and emptying; restricts secretion; inhibits intestinal absorption by restricting GI blood flow |
| Vasoactive intestinal peptide | Duodenal mucosa | Chyme with partially digested foods | Duodenum, stomach, small intestine | Activates buffer secretion; dilates intestinal capillaries; restricts HCl production; relaxes intestinal smooth muscle |
Nerve Control of Digestion
Neural innervation of the GI tract is provided intrinsically by the enteric nervous system and extrinsically by the autonomic nervous system. The enteric nervous system contains around 100 million motor neurons, sensory neurons, and interneurons that run from the esophagus to the anus. These neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis externa layer of the intestinal wall while the submucosal plexus (plexus of Meissner) lies in the submucosal layer. The myenteric plexus is responsible for GI tract motility (spontaneous movement), especially the rhythm and force of contractions of the muscularis. The submucosal plexus regulates digestive secretions and reacts to the presence of food.
The autonomic nervous system provides parasympathetic and sympathetic innervation to the GI tract. In general, parasympathetic nerves increase GI secretion and motility by stimulating neurons of the enteric nervous system, and sympathetic nerves decrease GI secretion and motility by restricting the activity of enteric nervous system neurons. When we are angry, frightened, or anxious, sympathetic innervation of the GI tract is stimulated, slowing digestive activity. Many people feel ‘butterflys’ in their stomach (or regurgitate) when they are nervous.
Many enteric nervous system neurons are part of the GI reflex pathways responsible for controlling secretion and motility in response to GI tract stimuli. In these pathways, enteric nervous system sensory neuronal axons synapse with neurons of the enteric, central, or autonomic nervous system, either activating or inhibiting the activities of GI glands and smooth muscle.
Dysfunctions of Digestive Homeostasis
Homeostasis of Water
As an example of homeostasis maintained in the digestive system, we will consider the homeostasis of water. In the introductory units, we covered all of the important functions that water plays in our bodies. We get water from drinking liquids and from most of the foods that we eat. Water is necessary for normal functioning of most of our tissue systems including cardiovascular fluids (blood). We lose water by exhalation (respiratory system), urination (urinary system) and through feces in the digestive system. A fraction of water is retained in the digestive system to maintain movement, and water is both absorbed and secreted in the large intestine link to section above on water absorption.
Infections of the gastrointestinal tract leading to diarrhea: Diarrhea is a gastrointestinal disorder characterized by frequent watery stools. Diarrhea can occur secondary to a number of problems, including bacterial or viral infection of the gastrointestinal tract. The most common causes of these infectious diarrheas are infections with the Escherichia coli or Vibrio cholera bacteria or from Rotavirus.. In the large intestine, 98% of digestive system water is reabsorbed along with electrolytes and nutrients. Bacterial or viral infection interferes with the processes that control fluid reabsorption, leading to frequent, watery stools.
Infectious diarrheal disease usually occurs in older infants and children, particularly in situations where safe sources of drinking water are not available. Most children can recover from these gastrointestinal infections; however, diarrhea can lead to loss of substantial body fluids, sodium, chloride and other electrolytes. The loss of just 10% of body fluids can prevent maintenance of blood pressure. Diarrhea leads to 2.2 million deaths per year throughout the world, most in children in developing countries.
Cholera is a severe diarrheal disease caused by the Vibrio cholerae bacterium. This bacterium produces a toxin that binds tightly to cells of the large intestine and prevents water absorption. Simply providing water to cholera victims does not prevent dehydration, but adds to the volume of diarrhea. Proper treatment solutions contain water, salts and glucose since all of these are responsible for absorption of fluids.
