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Digestion Overview
Digestion involves the coordinated breakdown of the huge macromolecules in the food that we eat
into their molecular components by a large array of digestive enzymes and “facilitative” compounds.
salivary α-amylase, mucins - parotid, submandibular,
sublingual glands
lingual lipase - lingual serous
bile acids:
- glycholic acid
- glycochenodeoxycholic acid
- taurocholic acid
- taurochenodeoxycholic acid
trypsinogen, chymotrypsinogen, carboxypeptidase,
collagenase, pancreatic α-amylase, ribonuclease,
deoxyribonuclease, phospholipase, pancreatic
lipase, colipase…
bicarbonate ion, mucus - mucosal cells
HCl, intrinsic factor – parietal cells
pepsinogen / pepsin – chief cells
gastrin – G-cells
amino peptidase, dipeptidase,
monoglyceride lipase, maltase,
sucrase, lactase…
Release of digestive enzymes are controlled by a variety
of hormones & neurotransmitters…
Entry of the bolus into the stomach
(stretching) stimulates the release of gastrin
from G-cells, a hormone that stimulates chief
cells and parietal cells in the stomach to
release pepsinogen and hydrochloric acid,
respectively. Gastrin also stimulates mast
cells to release histamine (other effects of
gastrin are being ignored).
Acetylcholine released from the vagus nerve
and histamine released from mast cells also
stimulate release of HCl from parietal cells.
The resulting chyme enters into the duodenum
and stimulates release of CCK by the I-cells.
CCK stimulates release of pancreatic enzymes,
HCO 3, and bile into duodenum via bile /
pancreatic duct and additional HCO-3 from
biliary and duodenal secretion.
Secretin is released from S-cells in the first few
centimeters of the duodenum in response to
acid in the chyme.
Secretin stimulates the pancreas to release
bicarbonate and water to further increase pH of
the duodenum and to inhibit HCl secretion in
the stomach.
Where most of the action really happens
Villi
Duodenum
(small intestine)
Absorption from lumen
Absorption of nutrients occurs across the brush border
of the epithelial cells. Many nutrients are transported
across the cell membrane by various versions of sodium
co-transporters and then “released” to be taken up into
the venous circulation …
Brush Border
To venous circulation
Nutrients Tend To Be Absorbed at “Specific Sites” Throughout The Entire GI Tract
Nutrients are “transported to
the liver” through the portal
vein and are picked up by the
liver cells (and the rest of the
body’s cells for those that the
liver doesn’t get) for
processing . . .
Liver releases nutrients to the
venous circulation and they
are transported to the rest of
the body through the arterial
circulation . . . . . . . . . .