Structure and Function of the Digestive System


The stomach stores food and mixes it with acidic secretions to produce chyme.

The stomach stores food after it passes through the esophagus. Once in the stomach, the food mass, or bolus, is liquified as it mixes with secretions of the stomach, and proteins begin to be chemically digested. No absorption of nutrients occurs in the stomach, but water, alcohol, and some drugs are absorbed. The stomach has a curved shape. The upper concave curvature is called the lesser curvature and the bottom convex, the greater curvature. The main parts of the stomach are the cardia, fundus, body, and pylorus. Food leaves the esophagus and enters the stomach at the cardia region. The fundus is the dome-shaped region above the gastroesophageal sphincter. The body of the stomach lies between the fundus and the pylorus, which connects the stomach to the small intestine.The lumen of the stomach contains rugae, a series of ridges produced by longitudinal folding of the stomach wall. These folds allow the stomach to expand if large quantities of food are consumed.

The fundus and body regions of the stomach are dotted with gastric pits that open into gastric glands. These glands contain different types of secretory cells, including mucous neck cells, chief cells, parietal cells, and enteroendocrine cells. A mucous neck cell is a cell in the neck, or upper part of the gastric pits, that secretes mucus. Lower regions of the gastric pits contain chief cells and parietal cells. A chief cell secretes pepsinogen, the precursor for pepsin, an enzyme that breaks down proteins. A parietal cell secretes hydrochloric acid (HCl), which is responsible for the stomach's high acidity and activates pepsinogen into pepsin. These cells also secrete intrinsic factor, which is a glycoprotein necessary for the absorption of vitamin B12. An enteroendocrine cell secretes hormones such as gastrin, which stimulates parietal cells to secrete HCl.

The conditions of the stomach are highly acidic with pH levels ranging between one and three. This low pH is thought to be a protective measure against potentially dangerous pathogens ingested with food. The lining of the stomach protects itself to prevent tissue damage from this corrosive pH level. The mucus that lines the stomach is alkaline to neutralize the chyme (liquified food) immediately adjacent to the walls. The cells in the mucosal layer are joined by tight junctions to prevent fluid from leaking out of the stomach. If any damage does occur to the cells of the stomach walls, they are quickly replaced by stem cells.

Strong peristaltic contractions occur in the stomach as the food becomes liquified into chyme. Three layers of smooth muscles in the stomach wall contract and relax in waves to mix the food with stomach secretions. When this mechanical digestion is complete, peristaltic waves begin to slowly move the mixture of stomach secretions and partially broken down food out of the stomach. As waves of stomach muscle contractions move toward the small intestine, the pyloric sphincter opens allowing small volumes of chyme to pass through the sphincter.
The stomach contains four main regions-the cardia, which connects to the esophagus, the fundus at the most superior region, the body, and the pylorus, which attaches to the small intestine. The mucosa layer of the stomach contains gastric pits, invaginations that lead to gastric glands. Gastric glands contain different cell types to produce the gastric fluids in the stomach.