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.