N3 Anatomy and Physiology AY 2023-2024

How does the food change in consistency and form?

• Food changes its form as it passes along the pathways of the digestive system. First, it is formed into a bolus, a ball-like mixture of saliva and food mass, when it reaches the buccal phase and then forced into the pharynx by the tongue. The bolus is transported involuntarily by peristalsis during the pharyngeal-esophageal phase. Second, the stomach mixes the food with an enzyme-containing gastric juice, forming the chyme, a liquid type that resembles heavy cream. It passes through the small intestine undergoing water absorption. Third, the semisolid chyme becomes the feces in the large intestine. The presence of feces when it reaches the rectum will cause a defecation reflex. 

How could the body absorb the nutrients from the foods we eat? 

• Although the stomach only serves as a temporary “storage tank” for food, it is also the site for food breakdown. The hydrochloric acid makes the content of the stomach very acidic; as a result of increased acidity, there is an activation of pepsinogen to pepsin for protein digestion and provision of a hostile environment for microorganisms. Consequently, the small intestine accounts for the most digestion and absorption processes. The intestinal enzymes from the brush border (microvilli-covered surface of enterocytes) function to break double sugars into simple sugars and complete some protein digestion. Moreover, the intestinal enzymes and pancreatic enzymes help to complete the digestion of all food. The hormones secretin and CCK target the liver and gallbladder to release bile, which is a fat emulsifier needed for fat absorption and fat-soluble vitamin absorption. Then, water is absorbed along the length of the small intestine through osmosis. Lastly, the large intestine absorbs nutrients left behind by the intestine. No digestive enzymes are produced. Water, vitamins, ions, and remaining water are absorbed and the remaining materials are eliminated via feces.