Physiology Of The Digestive System

The unique physiological processes that take place in the digestive system are digestion, absorption, secretion, motility and excretion.

Digestion is the process whereby large food molecules are broken down to smaller ones. Food is ingested as large pieces of matter containing substances such as protein and starch which are unable to cross the cell membranes of the gut epithelium. Before these complex molecules can be utilised they are degraded to smaller molecules, such as glucose and amino acids, which can be absorbed from the gastrointestinal system into the bloodstream.

The mixture of ingested material and secretions in the gastrointestinal tract contains water, minerals and vitamins as well as fats, carbohydrates and proteins. The products of digestion, other small dissolved molecules, ions and water are transported across the epithelial cell membranes, mainly in the small intestine. This is the process of absorption. The transported molecules enter the blood or lymph for circulation to the tissues. This process is central to the digestive system, and the other physiological processes of the gastrointestinal tract, such as elimination, subserve it.

Food which is ingested travels along the gastrointestinal tract to the appropriate sites for mixing, digestion and absorption to occur. Most of the gastrointestinal tract is lined by two layers of smooth muscle; contraction of this muscle mixes the contents of the lumen and moves them through the tract. Motility in the digestive system is under neural and hormonal control.

Exocrine glands secrete enzymes, ions, water, mucins and other substances into the digestive tract. The glands are situated within the gastrointestinal tract, in the walls of the stomach, small intestine and large intestine, or outside it in the case of salivary glands, pancreas and liver. Secretion in all regions of the gastrointestinal tract is controlled by nerves and hormones.

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