The stomach is a J‐shaped, baglike organ that expands to store food (Figure 1). Typical of that of the entire digestive tract, the wall of the stomach contains four layers. However, the inner layer, the mucosa, is modified for the specialized functions of the stomach. In particular, the innermost layer of the mucosa (facing the lumen) contains a layer of simple columnar epithelium consisting of goblet cells. Gastric pits on the surface penetrate deep into the layer, forming ducts whose walls are lined with various gastric glands. A summary of the glands in the mucosa follows:
- Mucous surface cells are the goblet cells that make up the surface layer of the simple columnar epithelium. These cells secrete mucus, which protects the mucosa from the action of acid and digestive enzymes.
- Parietal (oxyntic) cells are scattered along the neck and lower walls of the ducts. They secrete hydrochloric acid (HC) and intrinsic factor. Intrinsic factor is necessary for the absorption of vitamin B 12 in the small intestine.
- Chief (zymogenic) cells also line the lower walls of the ducts. They secrete pepsinogen, the inactive form of pepsin. Pepsin is a protease, an enzyme that breaks down proteins.
- Enteroendocrine cells secrete various hormones that diffuse into nearby blood vessels. One important hormone, gastrin, stimulates other glands in the stomach to increase their output.
Figure 1. The parts of the digestive system.
The first three glands listed in the preceding list are exocrine glands, whose secretions, collectively called gastric juice, enter the stomach and mix with food. The last gland is an endocrine gland, whose hormone secretions enter the blood supply.
The stomach serves a variety of functions:
- Storage. Because of its accordionlike folds (called rugae), the wall of the stomach can expand to store two to four liters of material. Temporary storage is important because you eat considerably faster than you can digest food and absorb its nutrients.
- Mixing. The stomach mixes the food with water and gastric juice to produce a creamy medium called chyme.
- Physical breakdown. Three layers of smooth muscles (rather than the usual two) in the muscularis externa churn the contents of the stomach, physically breaking food down into smaller particles. In addition, HCl denatures (or unfolds) proteins and loosens the cementing substances between cells (of the food). The HCl also kills most bacteria that may accompany the food.
- Chemical breakdown. Proteins are chemically broken down by the enzyme pepsin. Chief cells, as well as other stomach cells, are protected from self‐digestion because chief cells produce and secrete an inactive form of pepsin, pepsinogen. Pepsinogen is converted to pepsin by the HCl produced by the parietal cells. Only after pepsinogen is secreted into the stomach cavity can protein digestion begin. Once protein digestion begins, the stomach is protected by the layer of mucus secreted by the mucous cells.
- Controlled release. Movement of chyme into the small intestine is regulated by a sphincter at the end of the stomach, the pyloric sphincter.