Gastrointestinal Hormones and Functions

Gastrointestinal Physiology

• The gastrointestinal (GI) tract comprises both extrinsic and intrinsic nervous systems.
• The GI tract has four official hormones: gastrin, cholecystokinin (CCK), secretin, and glucose-dependent insulinotropic peptide (GIP).
• Gastrin increases H+ secretion by gastric parietal cells.
• Stimuli for gastrin secretion include small peptides and amino acids in the stomach lumen, distention of the stomach, and vagal stimulation (mediated by gastrin-releasing peptide (GRP)).
• CCK stimulates gallbladder contraction and relaxation of the sphincter of Oddi, pancreatic enzyme secretion, and inhibits gastric emptying.
• CCK is released from I cells of the duodenal and jejunal mucosa in response to fatty acids and monoglycerides.
• Secretin is released by S cells of the duodenum in response to H+ and fatty acids in the duodenum lumen. Increased pancreatic bicarbonate (HCO3−) secretion and inhibited gastric acid secretion are secretin's effects.
• GIP stimulates insulin release in the presence of oral glucose and inhibits gastric acid secretion.
• GIP is released by the duodenum and jejunum in response to fat, protein, carbohydrates, and glucose.
• The small intestine mixes nutrients with digestive enzymes, exposes nutrients to the absorptive mucosa, and propels unabsorbed material to the large intestine.
• Slow waves set the basic electrical rhythm (frequency of 12 waves/minute) in the small intestine.
• Action potentials on top of slow waves lead to contractions.
• Peristalsis is a highly coordinated movement that moves chyme through the small intestine toward the large intestine, ideally after digestion and absorption.
• Contraction behind the bolus and simultaneous relaxation in front of the bolus cause chyme propulsion.
• Segmentation contractions mix intestinal contents by contracting a section of the small intestine in both the orad and caudad directions.
• The contents then move back into the segment.
• The gastroileal reflex is mediated by extrinsic ANS and possibly by gastrin, increasing ileum peristalsis and ileocecal sphincter relaxation.

Gastrointestinal Motility

• The contractile tissue of the GI tract is primarily smooth muscle except for the pharynx, upper one-third of the esophagus, and the external anal sphincter, which are striated muscles.
• Depolarization of circular muscle in the GI tract causes contraction and a decrease in diameter.
• Depolarization of longitudinal muscle causes contraction in the longitudinal direction and a decrease in length.
• Chewing lubricates food with saliva, decreasing food particle size to facilitate swallowing and begin digestion.
• The swallowing reflex is coordinated in the medulla and involves actions in the pharynx, glottis, and larynx; and in the upper esophageal sphincter.
• Esophageal motility propels swallowed food into the stomach. Sphincters at both ends of the esophagus prevent air and stomach acid entry.
• Gastroesophageal reflux (heartburn) may occur due to a decreased tone of the lower esophageal sphincter.
• Achalasia may occur if the lower esophageal sphincter does not relax during swallowing.
• The orad region of the stomach relaxes to accommodate the ingested meal.
• CCK participates in "receptive relaxation" by increasing the distensibility of the orad region of the stomach.
• The caudad region of the stomach contracts to propel food into the duodenum.
• Gastric emptying occurs fastest with isotonic stomach contents; hypertonic or hypotonic contents cause slower gastric emptying.
• Fats and H+ in the duodenum inhibit gastric emptying.
• The process of defecation is initiated by rectal filling, causing relaxation of the internal anal sphincter, with subsequent relaxation of the external anal sphincter allowing defecation to occur.
• The gastrocolic reflex increases colon motility and frequency of mass movements when the stomach is stretched by food.

Gastrointestinal Secretion

• Saliva functions in initial starch digestion by amylase and initial triglyceride digestion by lingual lipase.
• Saliva is high in bicarbonate and potassium, and is hypotonic.
• Saliva is formed by three major glands: the parotid, submandibular, and sublingual glands.
• Saliva production is controlled by parasympathetic and sympathetic nervous systems and increased by both.
• Parasympathetic activity is more important.
• Gastric secretion is mediated by parietal cells (HCI and intrinsic factor), chief cells (pepsinogen), and G cells (gastrin).
• Gastric H+ secretion occurs due to the conversion of CO2 and H2O to H+ and HCO3− by carbonic anhydrase in parietal cells.
• Pancreatic secretions include enzymes such as pancreatic lipase, amylase, proteases, and other components.
• Bile contains bile salts, phospholipids, cholesterol, and pigments (especially bilirubin).
• Bile salts are amphipathic molecules that emulsify lipids, helping in lipid digestion and absorption.
• The pancreatic lipase, cholesterol ester hydrolase, and phospholipase A2 hydrolyze lipids into fatty acids, monoglycerides, cholesterol and lysolecithin.

Digestion and Absorption

• Carbohydrates, proteins, and fats are digested and absorbed in the small intestine. The small intestine has an increased surface area for digestion and absorption.
• Carbohydrates must be broken down to monosaccharides (glucose, galactose, and fructose).
• Amylases hydrolyze bonds in starches, yielding maltose, maltotriose, and α-limit dextrins.
• Maltase, dextrinase, and sucrase hydrolyze oligosaccharides to glucose.
• Lactase, trehalase, and sucrase degrade other disaccharides to various monosaccharides for absorption.
• Proteins are converted into amino acids, dipeptides, and tripeptides, and are absorbed.
• Pepsinogen is converted into pepsin by gastric acid. Pepsin hydrolyzes proteins into smaller peptides.
• Pancreatic enzymes like trypsin, chymotrypsin, elastase, and carboxypeptidase break down peptides into free amino acids.
• Free amino acids, dipeptides, and tripeptides are absorbed by Na+-dependent cotransport and facilitated diffusion.
• Fats are emulsified by bile salts in the small intestine, breaking them down into smaller components.
• Pancreatic lipases hydrolyze lipids to fatty acids, monoglycerides, cholesterol, and lysolecithin.
• The small intestine absorbs glycerol, products of digested lipids.
• Micelles carry fats across the cell membrane and the rest are repackaged into chylomicrons which are carried into the lympathic system.
• Electrolytes are absorbed via both cellular and paracellular routes in the intestinal cells.
• Water absorption is dependent on solute absorption.
• The GI tract also secretes electrolytes and H2O from blood to the lumen via the crypt.
• Cl- is the primary ion secreted into the intestinal lumen.
• Fat-soluble vitamins are incorporated into micelles and absorbed along with other lipids.
• Water-soluble vitamins are absorbed by sodium-dependent cotransport mechanisms.
• Vitamin B12 requires intrinsic factor for absorption in the ileum.
• Calcium absorption relies on 1,25-dihydroxycholecalciferol which is produced in the kidneys.
• Vitamin D deficiency or renal failure may cause vitamin deficiency.
• Iron is absorbed as heme iron or free iron.
• Free iron binds to transferrin which transports iron to storage sites.

Liver Physiology

• The liver performs gluconeogenesis, stores glucose as glycogen, and releases glucose into the circulation.
• The liver synthesizes nonessential amino acids and plasma proteins.
• The liver participates in fatty acid oxidation, synthesizes lipoproteins, cholesterol, and phospholipids, and performs detoxification.
• Potentially toxic substances enter the liver via the portal circulation and are modified in a "first pass" metabolism; often catalyzed by cytochrome P-450 enzymes which followed by phase II reactions that conjugate the substances.

This quiz focuses on the crucial hormones and mechanisms that control secretion and blood flow in the gastrointestinal tract. Test your knowledge on hormones like gastrin, secretin, and cholecystokinin, as well as their roles and regulatory functions within the digestive system.