Digestion in the Small Intestine

Digestion in the Stomach

Motor Functions of the Stomach

The motor functions of the stomach

Is represented by three types:

 (1)storage of large quantities of food until the food can be processed in the stomach, duodenum, and lower intestinal tract;

(2) mixing of this food with gastric secretions until it forms a semifluid mixture called chyme;

 (3) slow emptying (peristaltic) of the chyme from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.

Stomach Emptying

Each strong peristaltic wave forces up to several milliliters of chyme into the duodenum. Thus, the peristaltic waves, in addition to causing mixing in the stomach, also provide a pumping action called the “pyloric pump.”

Hunger contractions often occur when the stomach is empty for several hours or more. This rhythmic peristaltic contractions in the body of the stomach. They sometimes become very strong and last 2 to 3 minutes.

Gastric Secretion
In the stomach there are glands of the bottom of the stomach(also called gastric glands), pyloric and mucous, which cover its entire surface.

The gastric glands secrete hydrochloric acid, pepsinogen, internal factor and mucus.

The pyloric glands secrete mainly mucus and also hormone gastrin.

Pepsin functions as an active proteolytic enzyme in a highly acidic medium (optimal pH from 1.8 to 3.5), it breaks down proteins to oligopeptides, (Pepsin А -hydrolyzes egg albumin, muscle protein, Pepsin С - gastricin clives connective tissue protein, Pepsin B

An internal factor is necessary for the absorption of vitamin B12 in the ileum

In close connection with the release of hydrochloric acid, histamine release occurs.

Proteins of food have a special stimulating effect on gastrin cells in pyloric glands, causing the release of gastrin into the digestive juices of the stomach.

Gastrin, getting into the gastric glands, causes the secretion of histamine, and it stimulates the secretion of hydrochloric acid.

Regulation of Pepsinogen Secretion
Regulation of pepsinogen secretion occurs in response to two types of signals:

(1) stimulation of peptide cells

acetylcholine released from vagus nerves or from the gastric intestinal plexus, (2) stimulation of the secretion of peptide cells by acid.

Phases of gastric secretion

Gastric secretion is said to occur in three “phases”: a cephalic phase, a gastric phase, and an intestinal phase.

Cephalic phase.

It  arises on the appearance, smell, thoughts of food. Neurogenic signals appear in the cerebral cortex and in the appetizing centers of the hypothalamus. They are transmitted along the vagus to the stomach. The volume of juice is 20% of the total.

Gastric phase.

 As soon as the food enters the stomach, it stimulates (1) the vagovagal reflexes (2) the local intestinal reflexes and (3) the gastrin mechanism.

All this causes the secretion of the juice for several hours while the food is in the stomach. The volume of juice is 70%.

Intestinal Phase.

The intake of chyme in the duodenum will continue to cause the secretion of a small amount of gastric juice, probably because of the small amount of gastrin secreted by the mucosa of the duodenum.

Inhibition of gastric secretion

The entry of chyme into duodenum causes the release of intestinal hormones: secretin, an inhibiting peptide of the stomach, a vasoactive intestinal polypeptide, and somatostatin. They inhibit the secretion of the stomach.

Digestion in the Small Intestine

Movements of the Small Intestine

Mixing Contractions (Segmentations Contractions). They divide the intestine into separate segments, which look like a chain of sausages. Then other transverse muscles contract, and the previous ones relax.

Propulsive Movements - Peristalsis in the Small Intestine

Chyme is propelled through the small intestine by peristaltic waves. These can occur in any part of the small intestine, and they move toward the anus at a velocity of 0.5 to 2.0 cm/sec, faster in the proximal intestine and slower in the terminal intestine.

 

Movements Caused by the Muscles Mucosae and Muscle Fibers of the Villi.

Muscles of the mucous membrane can cause the appearance of short folds in the intestinal mucosa. They increase the surface area for contact with the chyme. In addition, the reduction of the villi, allows the lymph to freely flow through the capillaries of the villi into the lymphatic system.

 

The excretory ducts of the pancreas and the hepatic duct flow into the duodenum (the beginning of the small intestine), these juices take part in digestion along with the thin juice. intestines

Pancreatic Secretion

Pancreatic juice contains enzymes for digesting proteins, carbohydrates and fats. It also contains a large amount of bicarbonate ions, which play an important role in neutralizing the acidity of chyme from the stomach.

The most important of the enzymes for digesting proteins are trypsin, chymotrypsin and carboxypolypeptidase. They are excreted in an inactive state, activated in the duodenum.

The enzyme for digesting carbohydrates is amylase, which hydrolyzes starches, glycogen and most other carbohydrates, forming mainly disaccharides

The main enzymes for fat digestion are (1) pancreatic lipase, which is capable of hydrolyzing neutral fat into fatty acids and monoglycerides; (2) cholesterol esterase, which causes hydrolysis of cholesterol esters; and (3) phospholipase, which splits fatty acids from phospholipids. The pH of the juice is 8.