Stomach

The stomach is a curved J-shaped muscular organ, positioned in the left-hand side of the abdominal cavity below the diaphragm.

Food enters the stomach via the cardiac sphincter which is a strong circular muscle at the junction of the stomach and the oesophagus. Its function is to control the entry of food into the stomach. The layers of the stomach are as follows:

Stomach layer

Description

Peritoneum

Serous membrane that lines the abdominal cavity, supporting the alimentary canal and secretes a serous fluid which prevents friction

Muscular coat

Consists of longitudinal, circular and oblique fibres which assist the mechanical breakdown of food

Sub-mucous coat

Made up of areolar tissue containing blood vessels and lymphatics

Mucous coat

Secretes mucous to protect the stomach lining from the damaging effects of the acidic gastric juice

Surface epithelium

Infolded into numerous tubular gastric glands which secrete gastric juice

The main constituents of gastric juice, produced and secreted by cells in the stomach wall are as follows:

Pepsin

An enzyme which starts the breakdown of proteins

Hydrochloric acid

Provides the acidic conditions needed for pepsin to become active, kill germs present in food and prepares it for intestinal digestion

Mucus

Secreted by the neck cells in the stomach wall. It protects the stomach lining from the damaging effects of the acidic gastric juice

Rennin

An enzyme found in the gastric juices of infants that curdles milk protein.

Gastrin

A hormone released by endocrine cells in the stomach wall and is stimulated by the presence of food This hormone circulates in the blood stream, stimulating the further release of gastric juice

The functions of the stomach are to:

• churn and break up large particles of food mechanically

• mix food with gastric juice to begin the chemical breakdown of food

• commence the digestion of protein

Food stays in the stomach for approximately five hours until it has been churned down to a liquid state called chyme. Chyme is then released at intervals into the first part of the small intestine. The exit from the stomach is controlled by the pyloric sphincter which sits at the junction of the stomach and the duodenum. Its function is to relax and release chyme at intervals into the small intestine.

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