L30 - Digestion 3 Flashcards
What is the net result of the gastric phase?
digestion of proteins by pepsin, formation of chyme by action of pepsin, acid & peristaltic contractions and controlled entry of chyme to small intestine
When does the intestinal phase begin?
begins once chyme enters small intestine
most digestion and absorption occurs here
Intestinal phase triggers reflexes that…
feed back - regulate delivery of chyme from stomach
feed forward - promote digestion, motility and utilisation of nutrients
Chemical digestion of which nutrients will have begun by the time chyme reaches the small intestine?
starch/carbohydrates, fats and proteins
How is acid neutralised in the small intestine?
secretin is released which is a hormone that mediates the release of bicarbonate from the pancreas
Which hormone is released upon entry of fats and proteins into the small intestine?
CCK which mediates the release of pancreatic enzymes that break down fats and proteins
also triggers contraction of the gallbladder which releases bile salts
Which compounds are released upon entry of carbohydrates into the small intestine?
GIP and GLP-1 which both trigger insulin signalling
Which hormone inhibits gastric motility and acid secretion?
CCK which is released when fats and proteins move into the small intestine
What is the role of intestinal motility?
mix chyme with enzymes for digestion, expose digested nutrients to mucosa for absorption, slowly propel food forward (stimulated by PNS and inhibited by SNS)
What are characteristics of the microbiome?
includes bacteria and fungi, 10 times more cells than host, 100 times more genes than human genome, dynamic ecosystem which reflects diet, sometimes symbiotic, often parasitic and detrimental
What does bile contain?
bile salts and drugs cleared from the body
What is the hepatic portal system?
everything absorbed from the GIT is delivered directly to the liver e.g. glucose and toxins
recirculates bile salts (twice for typical meal)
What is the role of the pancreas?
has endocrine and exocrine functions
endocrine: insulin in fed state and glucagon in fasted state
exocrine: pancreatic enzymes and alkaline solution
What is the role of the exocrine function in the pancreas?
proteolytic enzymes (inactive enzymes) digest protein pancreatic amylase (active) converts polysaccharides into disaccharides pancreatic lipase (active) digests fat
How are pancreatic enzymes activated?
inactive enzymes are released into the small intestine e.g. trypsin -> enteropeptidase in the wall of the SI converts trypsinogen into trypsin which activates other proenzymes
Where does does digestion occur in the GI tract?
some amylase in saliva: carbohydrate
some protein digestion in stomach
but most digestion occurs in small intestine
Into which system are fats absorbed?
the lymphatic system
What is the role of amylase?
breaks polysaccharides down into disaccharides maltose, sucrose and lactose
Which enzymes break down disaccharides?
disaccharidases which are found on the intestinal brush-border
include maltase, sucrase and lactase
What is carbohydrate absorption in the small intestine driven by?
Na+-K+ ATPase on the basolateral membrane and transporters on the apical membrane
e.g. glucose enters the cell with Na+ on the SGLT symporter and exits on GLUT 2
What is protein absorption in the small intestine driven by?
proteins are broken down into peptides -> di- and tripeptides cotransport with H+, amino acids cotransport with Na+ and small peptides are carried intact across the cell by transcytosis
A decrease in luminal [Na+] will decrease intestinal reabsorption of which compounds?
glucose, galactose and amino acids
Which fats are digested by the body?
triglycerides, cholesterol, phospholipids and fat soluble vitamins
What is the role of bile salts and colipase in fat digestion?
since fats are not water soluble bile salts emulsify fat and colipase facilitates lipase access
What are chylomicrons?
combination of triglycerides, cholesterol and protein which are absorbed into the lymphatic system
Of the 8-10 L of fluid entering the GI tract daily…
only 100-200 mL are excreted in the stool
How is fluid balance in the digestive system maintained?
water absorbed by osmosis: gradient created by absorption of ions and nutrients into interstitial fluid
What is osmotic diarrhoea?
unabsorbed osmotically active solutes hold water in the lumen e.g. lactose, sorbitol
What is secretory diarrhoea?
bacterial toxins enhance colonic secretion of Cl-
intestinal infection e.g. cholera toxin, E. coli
