Select the letter of the organ with the following function: Muscular propulsion of materials into the esophagus.

A. Motility is under the control of the sympathetic nervous system:Sympathetic innervation primarily inhibits gastrointestinal motility and induces sphincter contraction. Parasympathetic fibers from the vagus nerve and sacral plexus are the primary stimulators of motor activity. The sympathetic system acts as a functional brake.

B. Motility is movement of the GI tract:This term encompasses the various mechanical actions of the smooth muscle layers, including peristalsis and segmentation. These movements are essential for mixing and propelling luminal contents. It is a fundamental physiological process.

C. Peristalsis pushes food particles through the GI tract:Peristalsis involves a wave of proximal contraction followed by distal relaxation of the circular and longitudinal muscles. This coordinated sequence ensures the unidirectional, anterograde movement of the bolus. It is the primary propulsive motor pattern.

D. Digestion is the process of breaking food into nutrients:Digestion includes both mechanical reduction of food size and chemical hydrolysis of macromolecules into monomers. These processes occur along the entire length of the alimentary canal. It prepares molecules for epithelial absorption.

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.