The enzyme ___ found in saliva begins the chemical digestion of carbohydrates.

A. Ferment:Fermentation of undigested carbohydrates primarily occurs in the large intestine via anaerobic bacteria. The stomach is an acidic environment unsuitable for significant bacterial fermentation. Its primary role is mechanical and initial chemical breakdown.

B. Secrete:Secretion of gastric juice is a function of the mucosal glandular epithelium, not the muscularis externa. While muscle contraction helps mix secretions, the muscles themselves do not produce acid or enzymes. Secretion is an epithelial process.

C. Churn:The stomach possesses an inner oblique muscle layer in addition to circular and longitudinal layers. This allows for complex, multi-directional contractions that physically break down food into chyme. This vigorous mechanical mixing is unique to gastric physiology.

D. Absorb:The stomach has a limited capacity for absorption, primarily taking in lipid-soluble substances like alcohol and aspirin. Absorption is mainly a function of the villi in the small intestine. The muscular layers do not facilitate the absorptive process.

A. Because mucus is rapidly lost from the surface:While mucus is constantly secreted, its loss is not the primary reason for cellular turnover. The mucus layer is a protective barrier for the cells beneath it. Cell replacement is a response to physical damage.

B. To keep gastric juice production constant:Gastric juice is produced by the deep glandular cells in the gastric pits, not the surface epithelium. The surface cells primarily provide a protective lining. Their turnover does not directly regulate the volume of acid.

C. To increase the absorption of nutrients:The stomach is not a primary site for nutrient absorption. Frequent epithelial turnover is a defensive strategy rather than an absorptive one. The villi of the small intestine handle the majority of nutrient uptake.

D. Because the cells are often digested by stomach acid:The harsh luminal environment, characterized by a pH of 1 to 2 and proteolytic pepsin, inevitably damages surface cells. Rapid mitosis allows the stomach to maintain its integrity. The entire lining is replaced every 3 to 6 days.