Physiology of the small intestine Flashcards
What does the small intestine secrete?
Na+, Cl- and HCO3- (water follows by osmosis)
Mucus (alkaline)
Hormones
What secretes mucus into the small intestine?
Brunner’s glands in the duodenal mucosa
What hormones does the small intestine secrete?
Secretin
Cholecystokinin
Motilin
Vasoactive intestinal peptide (VIP)
Gastric inhibitory peptide (GIP)
What does chyme trigger the release of in the small intestine?
Hormones
Alkali (to correct acidity)
Water (helps correct acidity)
Mucous (alkali)
Digestive enzymes
What is the hormone motilin responsible for?
Stimulates migrating motor complexes via the enteric NS and ANS for rapid peristaltic movement.
Acts between periods of digestion to clear undigestible products from the GI tract
What is the hormone vasoactive intestinal peptide (VIP) responsible for?
Increasing blood flow to GI tract
What is gastric inhibitory peptide (GIP) responsible for?
Inhibits gastric secretion, stimulates insulin secretion
What is cholecystokinin responsible for?
Stimulation of gall bladder contraction
Stimulation of pancreatic enzyme secretion from exocrine pancreas.
What is secretin responsible for?
Stimulation of alkali release from exocrine pancreas
Stimulation of bile production in the liver
What is alkali?
Isotonic solution rich in HCO3-. Neutralises duodenal contents.
What digestive enzymes are secreted from the exocrine pancreas?
- Proteolytic enzymes (peptidases):
- Trypsin
- Chymotrypsin
- Carboxypeptidase
(proteolytic enzymes- cleave proteins and peptides) -Secreted in inactive forms
- Pancreatic amylase (breakdown of starch)
- Lipases (breakdown of fats)
What neutralises chyme in the small intestine?
Alkali from exocrine pancreas
Water secreted from intestinal mucosa
Alkaline bile
Mucus from duodenal mucosa
How are the inactive pancreatic enzyme precursors activated?
The pancreas releases inactive enzymes including trypsinogen.
Trypsinogen is converted into trypsin by membrane-bound enterokinase enzyme.
Trypsin activates inactive enzymes which are released into the intestinal lumen
Describe the mechanism of pancreatic HCO3- secretion
CO2 and H2O are converted by carbonic anhydrase to HCO3 and H+.
HCO3- is released into the duct lumen
H+ is pumped out of the duct cell into the extracellular fluid.
(ions moved across opposite membranes than in gastric parietal cells)
How are fats (triglycerides) digested in the duodenum and proximal jejunum?
Fats (triglycerides) emulsified by bile salts and phospholipids into emulsion droplets.
Digested by pancreatic lipase to monoglycerides and fatty acids.
Held in micelles, combined with bile salts and phospholipids.
Micelles diffuse into an ‘unstirred layer’ next to the surface of epithelial cells.
Monoglycerides and fatty acids diffuse into cell membrane.
Inside the cell monoglycerides and fatty acids are reassembled into triglycerides and packaged into chylomicrons
Exported across the basolateral membrane and leave the intestinal villus via the lacteals of its lymph system.