- Understand the physiologic role of gastric acid secretion, as well as that of other gastric secretory products
- Identify the regions of the stomach and cell types from which the various gastric secretions originate
- Understand how gastric secretion is initiated in response to anticipation of a meal, and how secretion is amplified once the meal has been ingested
- Define cephalic, gastric, and intestinal phases of the secretory response
- Describe how secretion is terminated once the meal has left the stomach
- Define the cellular basis for acid secretion and the morphologic changes that take place in parietal cells to achieve this
- Identify clinical correlates of abnormal acid secretion
The stomach is a muscular reservoir into which the meal enters after being swallowed. While limited digestion may begin in the oral cavity as a result of enzymes contained in saliva, the gastric juices represent the first significant source of digestive capacity. However, the digestive functions of the stomach are not necessary for assimilation of a mixed meal, and indeed, surgical removal of the majority of the stomach because of disease, or in an effort to treat morbid obesity, is not incompatible with adequate nutrition. However, some degree of gastric secretory function is critically required for the absorption of an essential vitamin, B12
, and gastric acid may also be important in the absorption of dietary nonheme iron.
Gastric secretions also serve to sterilize the meal. Other than those receiving irradiated meals for medical reasons, humans may ingest significant quantities of microbes with their meals. This is particularly the case in developing countries, where sanitation is inadequate and many do not have access to clean food or water. However, assuming the microbial load ingested is not overwhelming, the secretions of the stomach can kill many of the microorganisms contained therein, and thus maintain the relative sterility of the small intestine.
Gastric Secretory Products
The functions outlined in the previous section are subserved by a number of products secreted by the stomach (Table 3–1). The most characteristic secretory product of the stomach is hydrochloric acid, which is not secreted in such large quantities anywhere else in the body. The acidity of the gastric secretions begins the digestive process via simple hydrolysis. This acid also contributes to the ability of the gastric juices to sterilize the meal.
Table 3–1. Important Gastric Secretory Products ||Download (.pdf)
Table 3–1. Important Gastric Secretory Products
|Hydrochloric acid||Parietal cell||Hydrolysis; sterilization of meal|
|Intrinsic factor||Parietal cell||Vitamin B12 absorption|
|Pepsinogen||Chief cell||Protein digestion|
|Mucus, bicarbonate||Surface mucous cells||Gastroprotection|
|Trefoil factors||Surface mucous cells||Gastroprotection|
|Histamine||ECL cells||Regulation of gastric secretion|
|Gastrin||G cells||Regulation of gastric secretion|
|Gastrin-releasing peptide||Nerves||Regulation of gastric secretion|
|Acetylcholine (ACh)||Nerves||Regulation of gastric secretion|