It is secreted by the salivary glands and starts working on starch immediately, even while still in the buccal cavity. The two types of muscles that propel food down the esophagus are skeletal in the upper third of the esophagus and smooth in the lower two-thirds. The rhythmic contractions of these muscles that propel food down the esophagus are termed peristalsis. The parietal cells secrete hydrochloric acid. Pepsinogen is secreted by the chief cells.
Hydrochloric acid activates pepsinogen and turns it into the active protein-digesting enzyme pepsin.
Villi are projections of the mucosal wall into the lumen of the digestive tract that increase its overall surface area. Microvilli are smaller fingerlike projections that occur across the surface of the epithelial cells of the mucosa, forming a brush border. This greatly extends the surface area of the intestine to facilitate absorption. The liver produces bile salts, cholesterol, and lecithin, all of which solubilize fats. The gallbladder stores bile.