GIT physiology Flashcards
Gastric Secretion
What factors regulate gastric secretion?
Can divide this answer into either neural and hormonal or do it via stages i.e.
cephalic/gastric/intestinal. We have done it in order of what happens as you eat food
because its easier to remember.
Cephalic phase - mediated via the vagus nerve. The sight, smell or taste of food excites
the vagus nuclei, which excites parasympathetic neurons in the stomach that release
ACh onto parietal cells to stimulate secretion. Vagal efferents also stimulate gastrin
release from G cells.
Gastric Phase - food in the stomach triggers local receptors which activate post
ganglionic neurons. These neurons stimulate parietal cells to produce acid.
Intestinal phase - fats, carbohydrates and acid in the duodenum inhibit gastric acid
secretion and pepsin secretion as well as motility by neural and hormonal mechanisms
What does the parietal cell do?
● Parietal cell is stimulated by histamine (from ECL cells) and gastrin (from G cells)
as well as vagal efferents
● H+ ions are pumped out by H/K/ATPase
● Cl- ions follow to combine with H+ in the lumen
Digestion of carbohydrates
Describe the enzymes required for the digestion of carbohydrates and their
location
● Mouth - salivary amylase
● Duodenum - Pancreatic amylase
● Brush border of the small intestine - oligosaccharidases i.e. lactase, sucrase,
maltase, isomaltase
● The final oligosaccharides are metabolised to one of the final hexoses (i.e.
glucose, fructose) in the small intestine
Please describe how carbohydrates are absorbed from the GI tract
Two phases - first into the mucosal cell from the lumen, then into the ECF and blood
from the cell.
● Glucose and galactose undergo ‘secondary active transport’ with sodium via
SGLT1 and SGLT2 co transporters. A low concentration of sodium inhibits
transport.
● Glucose and Galactose can also go via ‘facilitated diffusion’ into the cells via
GLUT-2
● Fructose can go via ‘facilitated diffusion into the cell via GLUT-5 and then
GLUT-2 to blood
Digestion and absorption of lipids
Describe the enzymes required for the digestion of lipids and their location
● Lingual lipase - active in the stomach on triglycerides
● Pancreatic lipase - requires colipase for maximal activity, acts on triglycerides
● Pancreatic bile-salt activated lipase - triglycerides, cholesterol esters, some
vitamins and phospholipids
● Cholesteryl ester hydrolase
What other process is involved in the digestion of lipids?
● Emulsification
● Micelle formation - these are formed from bile salts, lecithin and monoglycerides
surrounding fatty acids and cholesterol
● Transport of lipids through the unstirred layer to the brush border of mucosal cells
Please describe how lipids are absorbed through the GI tract
● Two phases
● First phase - lipids go into the intestinal mucosal cell and second phase into the
interstitial fluid and thus into the capillaries and portal blood as free fatty acids
(FFAs) or into lymphatics as chylomicrons
● They move into the enterocytes via passive diffusion and carriers
● They move out of enterocytes into the interstitial fluids depending on the size
○ <12 carbons goes directly into portal blood
○ >12 carbons are re-esterified to triglycerides or cholesterol esters &
packaged in chylomicrons
Digestion of Proteins
Describe the enzymes required for digestion of protein in the gastrointestinal tract
and their location
● In the stomach - pepsinogens are activated by gastric acid to produce pepsins
and these cleave the bond between amino acids to yield polypeptides
● Small intestine - proteins are digested by powerful proteolytic enzymes from the
pancreas and intestinal mucosa. These include endopeptidases (trypsin,
chymotrypsin & elastase - which recognize specific amino acids in the middle of
the peptide) and exopeptidases which yield amino acids (by recognizing and
acting on one or two terminal amino acids)
● Brush border - amino, carboxy, endo and dipeptidases cleave peptides into
amino acids
How are proteins absorbed from the GI tract?
● Two phases
○ 1 Into the mucosal cell
○ 2 Into interstitial fluid and then into capillaries and portal blood
● There are seven transport systems for moving amino acids into enterocytes: five
require sodium co-transport and there are two sodium independent transporters
● Absorption of protein is rapid in the duodenum and jejunum and then slow in the
ileum
How does protein absorption and digestion differ in infa nts and young children
compared to adults?
Infants absorb more undigested protein, which results in passive immunity via absorption
of antibodies. However, this also results in more food allergies in this age group.
Glucose
What factors influence glucose homeostasis?
● Glucose absorption from the intestine
● Uptake from the periphery - muscle, fat, brain, RBCs, liver
● Reabsorption in the kidney
● Gluconeogenesis in the liver - as determined by the actions of insulin and
glucagon
What happens to glucose homeostasis in the absence of insulin?
● Hyperglycaemia due to a number of factors
○ decreased peripheral uptake of glucose into muscle and fat
○ reduced glucose uptake by the liver
○ increased glucose output by the liver and lack of glycogen synthesis
● The intake of glucose by the brain, GIT , kidney and RBCs remains unchanged
By what mechanism does glucose cause the release of insulin?
● Taken up by specific GLUT 2 transporter in beta cells of the pancreas
● Glucose is converted to pyruvate, then metabolised to glutamate via the citric
acid cycle which primes insulin granules for release
● Production of ATP triggers (via K efflux) a Ca influx which causes the granules to
be released
Iron
How is iron absorbed from the GIT?
● In the stomach, gastric acid causes a reduction of Fe3+ (ferric form) to Fe2+
(ferrous form) and formation of soluble complexes
● The duodenum is the major site of absorption
● Fe3+ is converted to Fe2+ by ferric reductase
● Fe2+ is transported into enterocytes via the apical membrane iron transported
(DMT1)
● Dietary haeme is transported into the enterocyte by heme transporter
● Heme oxidase releases Fe2+ from heme
● Some intracellular Fe2+ is converted to Fe3+ and bound to ferritin
● The remainder binds to basolateral Fe2+ transporter ferroportin and transported
to interstitial fluid aided by hephaestin (Hp)
● Then it is converted to the ferric form (Fe3+) and bound to transferrin
What factors reduce iron absorption from the GIT?
● Dietary factors - phytic acids in cereals, oxalates and phosphates bind to Fe to
produce insoluble compounds
● Surgical factors - partial gastrectomy (via reduction of gastric acid), duodenal
surgery or illness e.g. ulcers (via reduced site of absorption)
● Physiological - high iron stores, recent high iron diet, degree of erythropoiesis
● Drugs - antacids
