The digestive tract includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The pancreas, liver, and gallbladder are accessory organs of the digestive tract that have many functions, one of which is to produce digestive juices and enzymes that aid in digestion. The digestive tract alters food into readily absorbed nutrients and eliminates waste products. Digestion begins in the mouth. Saliva moistens the food and amylase, an enzyme in saliva, begins the process of breaking it down. Food moves through the esophagus and enters the stomach, where digestive enzymes and stomach acid continue breaking it down. The resulting breakdown product, called chyme, contains carbohydrates, small proteins, minerals, fats, vitamins, and water. Chyme exits the stomach and enters the small intestine. The small intestine digests and absorbs each component of chyme. Pancreatic enzymes eventually break down carbohydrates into several simple sugars called monosaccharides. Sodium-glucose transporters are responsible for transporting monosaccharides across the intestinal cell membrane into the cell using active transport. After transport into the cell, glucose transporters move the monosaccharides out of the cell and eventually into the bloodstream for use by the body. Enzymes called proteases break down proteins into amino acids. Like carbohydrates, amino acids such as glycine are co-transported with sodium ions via the sodium-glucose transporter. Various amino acid transporters then transfer amino acids into the interstitial fluid where they are available to build proteins needed by the body. Minerals such as sodium are co-transported with carbohydrates and amino acids. Liver bile salts emulsify fats. Then pancreatic and intestinal enzymes digest them into fatty acids and diglyceride. Bile acid droplets called micelles absorb the fatty acids and diglyceride as well as any fat-soluble vitamins in the chyme and deliver them to the intestinal cell wall for absorption. Within the cell, the lipids and fat-soluble vitamins are packaged into chylomicrons, which are delivered to the lacteals or lymphatic capillaries in the intestinal villi for transport to the lymphatic system and eventual return to the blood.
