Physiology of Liver, Biliary Tract, Exocrine Pancreas Flashcards
What are the mixing contractions of the small intestine called?
segmentation contractions. These are slow rhythmic waves. The duodenum and jejunum are the fastest at about 10/min.
How long does it take to move chyme from the pylorus to the ileocecal valve?
3-5 hours at about 1cm/min
What controls peristalsis?
- NERVOUS signals initiated by chyme entry in the duodenum (stretches duodenal wall).
- HORMONAL control: gastrin, CCK, insulin, motilin, and serotonin increase motility. Secretin and glucagon inhibit motility.
What is peristaltic rush?
intense irritation of intestinal mucosa causing powerful and rapid peristalsis initiated by a combination of the autonomic nervous system, brain stem, and intrinsic enhancement of the myenteric plexus.
Can individual fibers of the muscularis mucosa extend into intestinal villi and cause them to contract intermittently?
YES. Initiated by local nervous reflexes in submucosal nerve plexus.
What is the function of the ileocecal valve?
- prevents backflow of fecal contents from colon into small intestine via ileocecal sphincter (normally mildly constricted).
- it is forcibly closed wen excess pressure builds up in the cecum.
What is the gastroileal reflex?
after a meal, the presence of food in the stomach intensifies peristalsis in the ileum, emptying ileal contents into the cecum to make room for more food.
What controls the degree of ileocecal sphincter contraction?
reflexes form the cecum and intensity of peristalsis in the terminal ileum.
What happens when the cecum is distended?
contraction of the ileocecal sphincter is intensified, while ileal peristalsis is inhibited, delaying emptying into the cecum.
*irritants in the cecum also delay emptying
What mediates the reflexes from the cecum to the ileocecal sphincter and ileum?
the myenteric plexus in the gut wall and extrinsic autonomic nerves (via prevertebral sympathetic ganglia)
What are the 2 major tissue types of the pancreas?
- acini (exocrine)= secrete digestive juices into the duodenum (enzymes and sodium bicarb flow through pancreatic duct that joins the hepatic duct and empties into the duodenum via the papilla of Vater, surrounded by the sphincter of Oddi).
- islets of Langerhans (endocrine)= secrete insulin and glucagon.
What causes pancreatic secretions in the duodenum?
chyme
What are the most important pancreatic digestive enzymes?
trypsin, chymotrypsin, and carboxypolypeptidase
*they are synthesized as inactive zymogen forms remember (trypsinogen, chymotrypsinogen, procarboxypolypeptidase), and activated by enzymes upon secretion into the duodenum.
What prevents premature trypsinogen activation in the pancreas?
trypsin inhibitor
What pancreatic enzymes are secreted for protein digestion?
- trypsin and chymotrypsin= split proteins into peptides.
- carboxypolypeptidase= splits peptides into individual amino acids.
What pancreatic enzymes are secreted for carbohydrate digestion?
pancreatic amylase= hydrolyzes starch, glycogen, and other carbs, forming di- and trisaccharides.
What pancreatic enzymes are secreted for fat digestion?
- pancreatic lipase= hydrolyzes fat into fatty acids and monoglycerides.
- cholesterol esterase= hydrolyzes cholesterol esters.
- phospholipase= splits fatty acids from phospholipids.
What enzyme acitivates trypsinogen to trypsin?
enterokinase (enteropeptidase)
What does trypsin activate?
chymotrypsinogen to chymotrypsin and procarboxypolypeptidase to carboxypolypeptidase
What is the mechanism by which the pancreas secretes Na+ bicarb into the pancreatic ductules and ducts in order to create osmotic pressure gradient for water to enter?
CO2 diffuses into the cell from the blood, where it combines with water via carbonic anhydrase to form carbonic acid. Carbonic acid will dissociate into H+ and HCO3- for the bicarb to be actively transported to the lumen of the duct, while H+ is exchanged for Na+ out the basolateral membrane via secondary active transport. The Na+ will then diffuse down its gradient into the lumen of the pancreatic duct to combine with HCO3-.
What are the 3 phases of pancreatic secretion (similar to that of gastric secretion)?
- CEPHALIC phase= Ach release by vagal nerve endings in pancreas.
- GASTRIC phase= nervous stimulation of enzyme secretion continues.
- INTESTINAL phase= after chyme leaves stomach and enters small intestine (secretion is copious) mainly in response to SECRETIN.
What is secretin?
hormone secreted from S cells in mucosa of duodenum and jejunum into the blood that stimulates copious secretion of sodium-bicarb ions from pancreas to neutralize acidic stomach chyme.
At what pH do pancreatic enzymes function?
7-8
What is cholecystokinin (CCK)?
hormone secreted by I cells of duodenum and jejunum into the blood that induces secretion of more pancreatic digestive enzymes (by acinar cells remember) due to presence of partially digested protein and long chain fatty acids in chyme.
