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Basic concepts / Gastrointestinal Tract
Gastrointestinal Tract
The small intestine constitutes the main site for food enzymatic digestion and nutrient absorption. The lining of the small intestine consists of tiny folds, called villi, which, in turn, are covered by microvilli. The villi contain capillaries and lymphatic vessels for nutrient absorption.
Within the small intestine, there is an area for nutrient exchange. This area is quite extensive (over 200 m2) because of the intestinal folds that increase the surface area more than 600-fold relative to a simple cylinder of comparable length.
The colon absorbs large quantities of water and electrolytes and allows evacuation of waste and toxic substances, but also constitutes a reservoir for a large number of bacterial species.
In humans, the GI tract from mouth to anus is about 6.5 meters long ]
According to the traditional view of the large intestine, no significant digestion of organic molecules would take place therein. However, in recent years this view has been challenged, and new, more complex theories on the colon's role have been now put forward.
In the process of food digestion, the intestinal flora is known to play an important role, namely, undigested complex carbohydrate and protein degradation through fermentation. The end products include lactate and short-chain fatty acids(1), such as butyric acid. Fatty acids are used by colonocytes since they are their favourite energy substrate.
The gastrointestinal (GI) tract is a long tube whose function is to pass nutrients, water, and electrolytes from the external to the internal environment. The food we eat is mostly in the form of polymers or macromolecules, such as proteins and complex carbohydrates, so our digestive system must secrete powerful enzymes to convert food into smaller molecules or nutrients to be absorbed and transported from the intestine to the body tissues, wherein they are utilised in the form of monomers or oligomers.
The primary function of the gastrointestinal tract is to permit digestion and absorption of nutrients.]
Digestion consists of the chemical and mechanical breakdown of food into smaller units that can pass across the intestinal epithelium into the body. Absorption is the both active and passive transfer of simple nutrients produced by digestion from the lumen of the GI tract to the extracellular fluid, so that they are used by the body for growth and energy supply.
The GI tract begins with the oral cavity (mouth and pharynx), which serves as a receptacle for food. The oral cavity is the space where the first stages of digestion take place, with chewing and secretion of saliva. Once swallowed, food moves into a long tube, called the oesophagus, lined with muscular walls and a transporting epithelium, lining. In the stomach, food is mixed with gastric juices, containing digestive enzymes and hydrochloric acid. This mixture, referred to as chyme, is then actively pumped out of the stomach into the small intestine, where further enzymes and bile are mixed with the chyme. Then, breakdown of dietary proteins, fats and carbohydrates is completed.
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