Digestion Overview

salivary α-amylase, mucins - parotid, submandibular, sublingual glandslingual lipase - lingual serous

bicarbonate ion, mucus - mucosal cellsHCl, intrinsic factor – parietal cellspepsinogen / pepsin – chief cellsgastrin – G-cells

trypsinogen, chymotrypsinogen, carboxypeptidase, collagenase, pancreatic α-amylase, ribonuclease, deoxyribonuclease, phospholipase, pancreatic lipase, colipase…

amino peptidase, dipeptidase, monoglyceride lipase, maltase, sucrase, lactase…

bile acids:- glycholic acid- glycochenodeoxycholic acid- taurocholic acid- taurochenodeoxycholic acid

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.

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.

Release of digestive enzymes are controlled by a variety of hormones & neurotransmitters…

Villi

Duodenum(small intestine)

Where most of the action really happens

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 …

Absorption from lumen

To venous circulation

Brush Border

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 . . . . . . . . . .