What is the Stomach?

The stomach is a muscular, hollow organ located in the upper left part of the abdomen, just beneath the diaphragm. It is a key part of the digestive system and functions mainly as a site for the temporary storage and partial digestion of food. After chewing and swallowing, food passes down the esophagus and enters the stomach.

Structure of the Stomach

The stomach has five main regions:

1. Cardia – This is the entry point where food from the esophagus enters the stomach. The lower esophageal sphincter at the cardia prevents the contents of the stomach from flowing back up into the esophagus.

2. Fundus – This is the dome-shaped upper portion that stores undigested food and gas released during digestion.

3. Body (Corpus) – This is the central and largest part of the stomach, where most of the digestion and mixing takes place.

4. Antrum – The lower portion of the stomach, where food is ground into a thick liquid.

5. Pylorus – This is the final section that connects the stomach to the small intestine. The pyloric sphincter controls the passage of the digested food (called chyme) into the small intestine.

Layers of the Stomach Wall

The wall of the stomach is made up of four main layers:

• The mucosa is the innermost layer. It contains glands that secrete mucus, digestive enzymes, and hydrochloric acid. The mucus protects the lining of the stomach from the acid. The enzymes and acid help break down food, especially proteins.

• The submucosa lies just beneath the mucosa and contains blood vessels, lymph vessels, and nerves.

• The muscularis externa is made of three layers of muscle: longitudinal, circular, and oblique. These muscles contract in different directions to churn and mix the stomach contents.

• The serosa is the outermost layer, a thin membrane that protects the stomach and reduces friction between the stomach and surrounding organs.

Functions of the Stomach

The stomach performs both mechanical and chemical digestion. Mechanically, it churns the food to break it into smaller particles. Chemically, it uses gastric juices that include hydrochloric acid and digestive enzymes like pepsin. Hydrochloric acid kills many harmful microorganisms and provides the acidic environment needed for enzymes to work effectively. Pepsin begins the digestion of proteins by breaking them into smaller peptides.

The stomach also regulates the rate at which food is passed into the small intestine. This is important because it ensures that the small intestine has enough time to properly digest and absorb nutrients.

Specialized Cells in the Stomach

Several types of specialized cells in the stomach wall perform different functions:

• Parietal cells produce hydrochloric acid and intrinsic factor. Intrinsic factor is important for the absorption of vitamin B12 in the small intestine.

• Chief cells produce pepsinogen, an inactive enzyme that becomes active pepsin in the presence of stomach acid.

• Mucous cells produce mucus to protect the stomach lining.

• G cells, located in the antrum, secrete gastrin, a hormone that stimulates acid production.

What Happens to Food in the Stomach

After swallowing, food enters the stomach and is mixed with gastric juices. The strong muscular contractions of the stomach wall help break the food down into smaller particles and mix it with the enzymes and acids. This process creates a thick liquid called chyme. Once the chyme is well mixed, the pyloric sphincter opens slightly to allow small amounts of it to pass into the small intestine for further digestion and absorption.

Common Stomach Disorders

Several issues can affect the stomach. These include:

• Gastritis, which is inflammation of the stomach lining, often caused by infection, alcohol, or certain medications.

• Peptic ulcers, which are sores in the stomach lining caused by too much acid or infection with the bacteria Helicobacter pylori.

• Acid reflux, where stomach acid flows back into the esophagus, causing heartburn.

• Stomach cancer, though less common, can also develop, especially in cases of chronic inflammation or infection.