GI Physiology

·         Motilin is released during the interdigestive period and is believed to be involved in the initiation of the migrating motor complex.

·         Enteropeptidase contains 41% polysaccharide. It is this high level of polysaccharide that protects enteropeptidase from digestion.

·         Gastric compliance is a property of the proximal stomach.

Peristalsis, trituration (grinding), and retropulsion (mixing) are terms referring to the contractile activity and functions of the distal stomach.

Segmentation is the primary contractile pattern of the small intestine during the digestive period.

·         Inflammation or removal of the upper small intestine leads to a decrease in pancreatic and hepatobiliary function.

·         Gas within the colon is derived primarily from fermentation of undigested material by intestinal bacteria to produce CO2, H2, and methane.

The digestive tract normally contains about 150 to 200 mL of gas, most of which is in the colon (100-150 mL).

Most of the gas in the stomach is derived from air swallowed during eating or in periods of anxiety.

 Gas is produced in the small intestine by interaction of gastric acid and bicarbonate in the intestinal and pancreatic secretions but does not accumulate because it is either reabsorbed or quickly passed into the colon.

·         Absorption of bicarbonate from the small intestine occurs primarily in the jejunum and is the result of bicarbonate-stimulated Na/H exchange.

·         Facilitated diffusion is the major transport route for fructose.

·         Absorption of vitamin D increases linearly as the intraluminal concentration increases, suggesting absorption by a nonsaturable passive-diffusion mechanism.

·         Orad stomach accommodation depends exclusively upon an intact vago-vagal reflex. Secondary esophageal peristalsis, intestinal segmentation, and migrating motor complexes are unaffected by vagotomy, whereas caudad stomach peristalsis is decreased but not abolished by vagotomy.

·         The migrating motor complex is the “intestinal housekeeper”, which cleanses the gastrointestinal tract.

·         In the ascending colon, peristalsis is followed by antiperistalsis, which allow time for absorption of water and electrolytes.

·         Approximately half of the total lymph produced in the body is liver lymph, although the liver is only 1.5 kg of the total body weight. Chylomicrons filled with lipids reach the blood via the liver lymph and liver.

·         In humans, gastric acid secretion by the parietal cell occurs in response to excitatory neural (acetylcholine), hormonal (gastrin), and paracrine (histamine) stimuli. Inhibitory feedback regulation of acid output also involves neural (enterogastric reflex), hormonal (enterogastrone), and paracrine (somatostatin) influences.

·         Important inhibitory neurotransmitters in the gastrointestinal tract include vasoactive intestinal peptide and nitric oxide. Relaxation of gastrointestinal smooth muscle occurs following activation of nonadrenergic, noncholinergic (NANC) enteric nerve fibers. Acetylcholine, substance P, and dopamine are excitatory neurotransmitters. Somatostatin is a paracrine secretory product with multiple effects on gastrointestinal function.

·         cholera toxin specifically inhibits neutral NaCl absorption

·         Both the absorption of Na+ and secretion of K+ from the colon are affected by changes in circulating levels of aldosterone. The major route of absorption of sodium in the colon is electrogenic transport. Because of the “tight” nature of the tight junctions that connect cells in the colon, a relatively large potential difference exists between the mucosal (negative) and serosal (positive) surfaces of the absorptive cells. This electrical difference favors the net secretion of K+ into the lumen. Secretion of HCO3− occurs in exchange for absorption of Cl−. No counterbalancing cation exchange pumps are present in the colon.

·         Intestinal crypt cells are characterized by the transport of chloride and water into the intestinal lumen. Secretion of chloride across the apical cell membrane is believed to involve the cystic fibrosis transmembrane receptor (CFTR) chloride channel. Crypt cells are the proliferative cells of the intestinal mucosa. As crypt cells migrate up the villus axis, they undergo significant morphological and biochemical differentiation. They change from short, cuboidal cells with minimal microvilli and few apical membrane transporters or brush border hydrolases into mature villus tip cells prepared for the role of nutrient, water, and electrolyte absorption.

·         Absorption of vitamin D increases linearly as the intraluminal concentration increases, suggesting absorption by a nonsaturable passive-diffusion mechanism.

·         The acidic environment of the gastric lumen favors the binding of cobalamin to R protein–type binding proteins that originate from salivary and gastric secretions. Pancreatic proteases in the small intestine degrade the R proteins, and the rise in pH favors rapid and complete transfer of the vitamin to intrinsic factor.

·         Absorption of bicarbonate from the small intestine occurs primarily in the jejunum and is the result of bicarbonate-stimulated Na/H exchange.

·         The transport protein responsible for the sodium-dependent glucose transport in the small intestine is termed the SGLT1. The absorption of glucose occurs through the coordinated action of transport proteins located in the brush border and basolateral membranes of the enterocyte. Glucose uptake into the enterocyte occurs primarily via the sodium-dependent SGLT1 secondary active transport mechanism. Exit from the enterocyte occurs by facilitated diffusion and is mediated by the membrane transporter, Glut-2. Glut-5 is the membrane transporter located on the apical portion of the enterocyte responsible for the facilitated entry of fructose into the cell.

·         The colon is the major site for the generation and absorption of short-chain fatty acids. They are products of bacterial metabolism of undigested complex carbohydrates derived from fruits and vegetables. In addition to exhibiting trophic effects on the colonic mucosa, they are believed to promote sodium absorption from the colon. The mechanism of action remains controversial.