Digestion

The digestive tract is a muscular tube, approximately 18-27 feet in length, that extends from the mouth to the anus. It is composed of seven compartments and four accessory organs. The compartments house the food as it passes through the digestive tract while the accessory organs secrete enzymes or produce molecules that aid in digestion. The accessory organs are connected to the main digestive tract by a series of ducts. The compartments occur in the following order: mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus. The accessory organs consist of the paired salivary glands, the pancreas, the liver, and the biliary system.

The gastrointestinal tract (the gut) is composed of three microscopic layers. Each layer is important for either maintaining peristalsis--the squeezing motion of the intestine--or the digestive functions of the gut. The innermost layer is the mucosa. The mucosa consists of specialized cells known as epithelial cells. These cells can be arranged in a single layer as seen in the esophagus, or in multiple layers as seen in the stomach and intestine. The epithelium serves to reduce friction and provide a protective barrier from the concentrated enzymes that are released into the inside of the intestine known as the lumen. It is the first layer that nutrients must pass through to reach the blood stream. Without this barrier, other layers of the gut would be autodigested by enzymes and toxic substances would have free passage into the blood stream.

The mucosal layer also consists of a thin layer of muscle tissue. This is referred to as the muscularis mucosa. The main function of this portion of the mucosa is to aid in propelling nutrients in a uniform direction from the lumen to the submucosa. Finally, there is some connective tissue in the mucosa that serves to keep all the structures together and in somewhat fixed positions.

The next layer encountered--moving from inside the lumen to the outside of the gut--is the submucosa. This is not considered a separate layer from the mucosa, but it does have some distinct properties. This layer is made up of connective tissue that contains blood vessels, nerves and lymphatics, vessels that carry lymph material. Absorption into the blood stream takes place in this layer. Once the nutrients have successfully passed through the mucosal layer, they will come in contact with the blood vessels here. Through passive and active diffusion, the nutrients will be absorbed into the blood stream and carried through the circulatory system to the rest of the body. The nerves in this layer help coordinate peristalsis and absorption. Meanwhile, the lymphatic vessels carry immune system cells that help fight infection. When bacteria enters the gut and gets past the mucosal barrier into the submucosa, the cells in the lymphatic tissue will identify the foreign substance and attack and destroy the bacteria, thereby preventing the spread of infection.

The second true layer encountered is the muscular layer of the gut (muscularis externa). This is composed of two discrete layers: an inner layer of muscle that runs in an up and down fashion (longitudinal muscle); and an outer layer of muscle that runs in a circular fashion (circular muscle). The main function of these two layers is to provide peristalsis to the gut. The peristaltic movement is like that of an inchworm, creating a slow wave of contractions that starts in the esophagus and continues through the rectum. The two muscular layers work in a complimentary fashion to squeeze the food and products of digestion through the digestive tract. The slow waves of peristalsis are known as the migrating motor complex (MMC). These waves are constant, continue between meals, and are only interrupted by mass movements. The mass movements propel the food rapidly through the gut when challenged with food and products of digestion.

The third and final layer of the gut is the serosa. This layer is mostly composed of connective tissue and gives strength to the long digestive tract. It helps suspend the gut in the thoracic (chest) and abdominal cavities by attaching itself to surrounding structures. Although not rigidly fixed, the organs and compartments of the gastrointestinal tract will remain in constant relationships to one another and surrounding organs thanks to their serosal attachments.