NCERT Solutions for Class 11 Biology Gastrointestinal Hormones notes
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Gastrointestinal Hormones (G I Hormones)
The food that we ingest is broken down into simple and soluble forms by our digestive system/ alimentary canal/ gastrointestinal tract. This simple and soluble food material is absorbed into the bloodstream. Through the blood, the food material is taken up by the various cells of our body and gives energy. Gastrointestinal System can be understood as a system of cells, tissues, organs, and biochemical molecules that help in the digestion and absorption of food into the body. GI hormones are chemical messengers that are implicated in many aspects of physiological functions of the gastrointestinal tract, including the regulation of secretion, absorption and digestion, and gut motility. GI hormones are a large family of peptides and are secreted by endocrine cells that are widely distributed throughout the GI mucosa and pancreas. Gastrin, secretin, and cholecystokinin (CCK) were the first discovered gut hormones, and as of today, there are more than 50 gut hormone genes and a multitude of bioactive peptides, hence the gut is considered as the largest endocrine organ of the body.
The Gastrointestinal Hormones (GI or gut hormones) constitute a group of hormones secreted by enteroendocrine cells (cells that secrete their chemicals through ducts) in the stomach, pancreas, and small intestine that control various functions of the digestive organs.
- The digestive hormones should not be confused with digestive enzymes. The hormones coordinate the digestive process by acting as a chemical messenger, whereas, enzymes are a part of chemical digestion by acting as a catalyst to break down food within the gut.
GI hormones may act on an entire organ, part of the organ or specific tissues and cells in order to help following activities
a. Secrete enzymes for digestion – chemical digestion.
b. Stimulate muscle contraction/relaxation – open/close sphincters and move food through the gut.
c. Increase blood flow to the gut – absorption of nutrients.
d. Regulate the flow of water and electrolytes – between the gut and bloodstream.
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Ten major known GI Hormones are enlisted here; however, new hormones are also being discovered.
- Cholecystokinin Pancreozymin
- Pancreatic Polypeptide.
- Vasoactive Intestinal Peptide
Cholecystokinin: Cholecystokinin (CCK), formerly called pancreozymins, a digestive hormone released with secretin when food from the stomach reaches the first part of the small intestine (duodenum). It is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein by stimulating the gallbladder to contract and release stored bile into the intestine. Cholecystokinin (CCK or CCK-PZ; from Greek chole, “bile”; cysto, “sac”; kinin, “move”; hence, move the bile-sac (gallbladder). Released by the duodenum in response to fats and acid, cholecystokinin causes the gallbladder to squeeze bile into the duodenum and stimulates the production of pancreatic enzymes, which pass into the duodenum through the pancreatic duct.
Duocrinin: It is secreted by the duodenal epithelium and stimulates the Brunner’s glands to release mucus and enzymes into the intestinal juice. Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in that portion of the duodenum which is above the hepatopancreatic sphincter (a.k.a. sphincter of Oddi). The main function of these glands is to produce a mucus-rich alkaline secretion (containing bicarbonate) in order to protect the duodenum from the acidic content of chyme (which is introduced into the duodenum from the stomach). Hence it provides an alkaline condition for the intestinal enzymes to be active, thus enabling absorption to take place. The secretions of Brunner’s glands also lubricate the intestinal walls. They also secrete urogastrone, which inhibits parietal and chief cells of the stomach from secreting acid and their digestive enzymes. This is another form of protection for the duodenum. They are the distinguishing feature of the duodenum and are named for the Swiss physician who first described them, Johann Conrad Brunner.
Enterocrinin: It is secreted by the epithelium of the entire small intestine. It stimulates the Crypts of Lieberkuhn to release enzymes into the intestinal juice.In histology, an intestinal gland (also crypt of Lieberkühn and intestinal crypt) is a gland found in the intestinal epithelium lining of the small intestine and large intestine (colon).
Enterogastrone: (= Gastric Inhibitory Peptide— GIP). It is secreted by the duodenal epithelium. It inhibits gastric secretion and motility. It slows gastric contraction; hence it is also called a gastric inhibitory peptide.
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Gastrin: Gastrin is a peptide hormone that stimulates the secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and pancreas. When food enters the stomach, G cells trigger the release of gastrin in the blood. As blood levels of gastrin rise, the stomach releases acid (gastric acid, HCl) that helps break down and digest food. The acid also kills the harmful bacteria that enter the gut through food. Gastrin also increases the motility of the stomach, thereby helping to churn food and eventually to empty the stomach; to a lesser degree, gastrin also increases the motility of the upper small intestine and the gallbladder.
Pancreatic Polypeptide: It is secreted by the pancreatic polypeptide cells (also ailed PP cells or F-cells) of islets of Langerhans(pancreatic cells). It inhibits the release of pancreatic juice from the pancreas.PP is rapidly released after a meal but remains elevated for 4-6 hours in humans with the Vagus nerve being the major stimulator. PP has effects on GI motility, metabolism and food intake.
Secretin: It was the first hormone to be discovered by scientists. It is a polypeptide of 27 amino acids. It is secreted by cells in the duodenum when they are exposed to the acidic contents of the emptying stomach. It stimulates the exocrine portion of the pancreas to secrete bicarbonate into the pancreatic fluid (thus neutralizing the acidity of the intestinal contents). It releases bi-carbonates in the pancreatic juice. It increases secretion of bile. It decreases gastric secretion and motility.
Somatostatin: Somatostatin secreted by the Delta cells of islets of Langerhans of the pancreas. It inhibits the secretion of glucagon by alpha cells of the pancreas and insulin by beta cells of the pancreas. Finally, in the gastrointestinal tract, the hormone reduces gastric secretion and the emission of gastrointestinal hormones, such as secretin and gastrin.
Vasoactive Intestinal Peptide (VIP): VIP is produced in many tissues of vertebrates including the gut, pancreas, and suprachiasmatic nuclei of the hypothalamus in the brain. With respect to the digestive system, VIP seems to induce smooth muscle relaxation (lower esophageal sphincter, stomach, gallbladder), stimulate secretion of water into pancreatic juice and bile, and cause inhibition of gastric acid secretion and absorption from the intestinal lumen. Its role in the intestine is to greatly stimulate secretion of water and electrolytes, as well as relaxation of the enteric smooth muscle, dilating peripheral blood vessels, stimulating pancreatic bicarbonate secretion, and inhibiting gastrin-stimulated gastric acid secretion. These effects work together to increase motility. It also has the function of stimulating pepsinogen secretion by chief cells.It also has the function of stimulating pepsinogen secretion by chief cells.
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Villikini: It is secreted by the epithelium of the entire small intestine. It accelerates the movement of villi. In addition to the above hormones, some other important hormones are- Motilin, Glucose-dependent insulin-tropic polypeptide GIP, Ghrelin, Gut hormones.
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