Digestive System

Vagus nerve and motility
One function of the vagus nerve is to stimulate digestion by GI secretion and peristalsis throughout the digestive tract. The vagal nerve fiber myenteric plexus influences motility, and is strategically located between the circular and longitudinal muscle layers. The Meissners' plexus influence secretion and lies near the glandular regions.

Digestion and absorption

Micelle scheme by SuperManu

Carbohydrates are broken down to monosaccharides in the mouth, and are absorbed in the small intestine. intestine and pancreas. Intestinal and pancreatic glands secrete enzymes that also hydrolyze starches. (Different intestinal and pancreatic enzymes digest carbohydrate, protein and fat).

Protein digestion begins in the stomach and ends in the small intestine, where proteins are broken down to amino acids and small peptides.

Chylomicron illustration by OpenStax College

Fat is emulsified, or broken into small droplets, by the stomach. Triglycerides, cholesterol esters and phospholipids are broken down in the intestine, and then combine with bile salts to form micelles. Micelles carry the digested lipid products to be absorbed into the intestinal epithelial cell, where the triglycerides, cholesterol esters and phospholipids are reformed. These molecules are combined with apoproteins to form chylomicrons, which exit the cell into the lymphatics.

Chylomicrons travel to the liver, where by further processing, they become lipoprotein vehicles VLDL, LDL, IDL or HDL. Triglycerides are most concentrated in VLDL, which is transported to fat, skeletal and heart muscle cells. Cholesterol is found in all lipoprotein vehicles, but is most concentrated in LDL and HDL.

Table of digestive chemicals

Organ	Cell type	Produces	Function
Mouth	Salivary gland	Salivary amylase	Partially breaks down starch.
Stomach	Chief cell (Stimulated by gastrin, pepsin and HCl).	Pepsinogen	In the presence of acid, changes to pepsin and break down proteins to peptides.
	Parietal cell (Stimulated by vagus, gastrin, stomach stretch and presence of peptides. Inhibited by acidity, secretin, GIP, somatostatin and cholecystokinin).	HCl	Creates an acidic environment for pepsin, and has a small effect on starch hydrolysis.
		Intrinsic factor	Binds to vitamin B12 to aid its absorption in the small intestine.
	Gastrin cell (Stimulated by vagus, duodenum stretch and presence of peptides. Inhibited by acidity, secretin and GIP).	Gastrin	Stimulates parietal cell to secrete HCl, and increases intestinal motility.
Duodenum / Small intestine	Epithelial cells of Crypts of Lieberkuhn (Stimulated by vagus and direct contact with food. Secretin, GIP and cholecystokinin secretion are stimulated by fat and acid).	Maltase, Lactase, Sucrase	Splits maltose to glucose, lactose to glucose and galactose, and sucrose to glucose and fructose, respectively.
		Peptidase	Breaks down peptides to amino acids.
		Intestinal lipase, Intestinal enterase	Breaks down fats to glycerol and fatty acids, and cholesterol esters to free cholesterol and fatty acids, respectively.
		Secretin, Gastric inhibitory peptide (GIP), Cholecystokinin	Decrease gastrin secretion and motility.
		Enterokinase	A catalyst for the activation of pancreas peptides: trypsinogen, chymotrypsinogen, and procarboxypolypeptidase.
Pancreas (exocrine)	Exocrine cell (Stimulated by vagus, cholecystokinin and secretin).	Trypsinogen, Chymotrypsinogen, Procarboxypolypeptidase	Break down proteins to peptides and amino acids.
		Pancreatic lipase, Cholesterol esterase, Phospholipase	Breaks down triglycerides to fatty acids and monoglycerides, cholesterol esters to free cholesterol and fatty acids, and phospholipids to lysophospholids and fatty acids, respectively.
Pancreas (endocrine)	Beta islet cell	Insulin	Increases intestinal motility.
	Alpha islet cell	Glucagon	Decreases intestinal motility.
	Delta islet cell	Somatostatin	Decreases digestion and absorption, and insulin and glucagon secretion.
Liver	Liver cell	Bile salt	Emulsifies fat to smaller particles that can be digested by pancreatic enzymes.