Essentials of Anatomy and Physiology

types of cells; their collective secretions are called gas-
tric juice. Mucous cellssecrete mucus, which coats
the stomach lining and helps prevent erosion by the
gastric juice. Chief cellssecrete pepsinogen, an inac-
tive form of the enzyme pepsin. Parietal cellspro-
duce hydrochloric acid (HCl); these cells have
enzymes called proton pumps, which secrete H+ions
into the stomach cavity. The H+ions unite with Cl–
ions that have diffused from the parietal cells to form
HCl in the lumen of the stomach. HCl converts
pepsinogen to pepsin, which then begins the digestion

of proteins to polypeptides, and also gives gastric juice

its pH of 1 to 2. This very acidic pH is necessary for

pepsin to function and also kills most microorganisms

that enter the stomach. The parietal cells also secrete

intrinsic factor, which is necessary for the absorption

of vitamin B 12. Enteroendocrine cells called G cells

secrete the hormone gastrin.

Gastric juice is secreted in small amounts at the

sight or smell of food. This is a parasympathetic

response that ensures that some gastric juice will be

present in the stomach when food arrives. The pres-

The Digestive System 377

Esophagus Fundus of stomach

Longitudinal muscle

layer

Circular muscle layer

Oblique muscle layer

Body

Greater curvature

Rugae

Pylorus

Duodenum

Pyloric sphincter

Lesser curvature

Cardiac orifice

Mucous cell

Parietal cell

Chief cell

G cell

B

A

Figure 16–5. (A) The stomach in anterior view. The stomach wall has been sectioned to

show the muscle layers and the rugae of the mucosa. (B) Gastric pits (glands) showing the

types of cells present. See text for functions.

QUESTION:What is the function of the pyloric sphincter?