Human Digestive System

The human digestive system is a complex process that consists of breaking down large organic masses into smaller particles that the body can use as fuel. The breakdown of the nutrients requires the coordination of several enzymes secreted from specialized cells within the mouth, stomach, intestines, and liver. The major organs or structures that coordinate digestion within the human body include the mouth, esophagus, stomach, small and large intestine, and liver.



In the mouth, food is moistened by saliva, a sticky fluid that binds food particles together into a soft mass. Three pairs of salivary glands—the parotid glands, the submaxillary glands, and the sublingual glands—secrete saliva into the mouth. The saliva contains an enzyme called amylase, which digests starch molecules into smaller molecules of the disaccharide maltose.

During chewing, the tongue moves food about and manipulates it into a mass called abolus. The bolus is pushed back into the pharynx (throat) and is forced through the opening to the esophagus.







Small intestine

The soupy mixture called chyme spurts from the stomach through a sphincter into the small intestine. An adult's small intestine is about 23 feet long and is divided into three sections: the first 10 to 12 inches form the duodenum; the next 10 feet form thejejunum; and the final 12 feet form the ileum. The inner surface of the small intestine contains numerous fingerlike projections called villi. Each villus has projections of cells called microvilli to increase the surface area.

Most chemical digestion takes place in the duodenum. In this region, enzymes digest nutrients into simpler forms that can be absorbed. Intestinal enzymes are supplemented by enzymes from the pancreas, a large, glandular organ lying near the stomach. In addition, bile enters the small intestine from the gall bladder to assist in fat digestion.

The enzymes functioning in carbohydrate digestion include amylase (for starch), maltase (for maltose), sucrase (for sucrose) and lactase (for lactose). For fats, the principal enzyme is lipase. Before this enzyme can act, the large globules of fat must be broken into smaller droplets by bile. Bile is a mixture of salts, pigments, and cholesterol that is produced by the liver and stored in the gall bladder, a saclike structure underneath the liver.


Most absorption in the small intestine occurs in the jejunum. The products of digestion enter cells of the villi, move across the cells, and enter blood vessels called capillaries. Diffusion accounts for the movement of many nutrients, but active transport is responsible for the movement of glucose and amino acids. The products of fat digestion pass as small droplets of fat into lacteals, which are branches of the lymphatic system.


Large intestine

The small intestine joins the large intestine in the lower right abdomen of the body. The two organs meet at a blind sac called the cecum and a small fingerlike process called the appendix. Evolutionary biologists believe the cecum and appendix are vestiges of larger organs that may have been functional in human ancestors.

The large intestine is also known as the colon. It is divided into ascending, transverse, and descending portions, each about one foot in length. The colon's chief functions are to absorb water and to store, process, and eliminate the residue following digestion and absorption. The intestinal matter remaining after water has been reclaimed is known asfeces. Feces consist of nondigested food (such as cellulose), billions of mostly harmless bacteria, bile pigments, and other materials. The feces are stored in the rectum and passed out through the anus to complete the digestion process.


