Human Digestive System: Stomach and Intestine Functions

The Small Intestine: Structure and Function

Structure

It is divided into three parts: the Duodenum, Jejunum, and Ileum. The small intestine represents the largest portion of the digestive tube. Its length varies with the type of feed: it is longer in herbivores and shorter in carnivores.

Function

Its primary function is to perform most of the chemical digestion and absorption of nutrients. For this, it requires that the secretions discharged by the pancreas, liver, and intestinal glands are mixed with chyme through sawing motions and peristaltic waves. After this mixture, chyme becomes chyle, consisting of H₂O, mineral salts, monosaccharides, and amino acids, which are absorbed by the intestinal mucosa to be incorporated into the circulatory system.

Accessory Glands and Intestinal Secretions

Intestinal Glands

These glands secrete intestinal juice and are of two types: Brunner's Glands and the Crypts of Lieberkühn.

Liver

It is a gland that produces and secretes bile, which is stored in the gallbladder. Its function is to contribute to emulsifying lipids, aiding fat digestion by dispersing them into smaller droplets.

Pancreas

It is a mixed gland that secretes hormones like insulin and glucagon, as well as pancreatic juice. The most important enzymes in pancreatic juice are:

• Amylase: Hydrolyzes starch
• Lipases: Transform fats into fatty acids and glycerol
• Trypsin

The Stomach: Structure and Function

Structure

The stomach is a dilated portion of the digestive tract located between the esophagus and the intestine. In primitive fish, it may be less developed. In birds, it undergoes a major transformation, dividing into a muscular part specialized in food trituration called the gizzard, and a glandular stomach.

Function

It stores food, regulates its passage to the bowel, and performs a crucial part of digestion.

Chyme and Gastric Secretions

Chyme

When ingested food reaches the stomach, it is beaten and mixed with gastric juices, thereby forming a semi-liquid slurry.

Gastric Secretions

They are secreted by glands lining the walls of the stomach. They consist of:

• Water
• Mucin: Provides a protective function
• Pepsinogen: A precursor of pepsin, which degrades proteins into peptides
• Renin: Acts on milk proteins
• Hydrochloric Acid: Maintains the acidic pH

Regulation of Gastric Secretions

This process has both nervous and hormonal components. When food is ingested, the brain sends a message to gastric cells to produce the secretion of gastric juice. Additionally, when food reaches the stomach, it secretes gastrin, which travels through the blood to reach the cells of the stomach, further stimulating secretion.