Physiology, Digestion

Excerpt

Digestion is the process of mechanically and enzymatically breaking down food into substances for absorption into the bloodstream. The food contains three macronutrients that require digestion before they can be absorbed: fats, carbohydrates, and proteins. Through the process of digestion, these macronutrients are broken down into molecules that can traverse the intestinal epithelium and enter the bloodstream for use in the body. Digestion is a form of catabolism or breaking down of substances that involves two separate processes: mechanical digestion and chemical digestion. Mechanical digestion involves physically breaking down food substances into smaller particles to more efficiently undergo chemical digestion. The role of chemical digestion is to further degrade the molecular structure of the ingested compounds by digestive enzymes into a form that is absorbable into the bloodstream. Effective digestion involves both of these processes, and defects in either mechanical digestion or chemical digestion can lead to nutritional deficiencies and gastrointestinal pathologies.

Through the gastrointestinal system, the nutritional substances, minerals, vitamins, and fluids, enter the body. Lipids, proteins, and complex carbohydrates are broken down into small and absorbable units (digested), principally in the small intestine. The products of digestion, including vitamins, minerals, and water, which cross the mucosa and enter the lymph or the blood (Absorption).

Digestion of the major food macronutrients is an orderly process involving the action of a large number of digestive enzymes. Enzymes from the salivary and the lingual glands digest carbohydrates and fats, enzymes from the stomach digest proteins, and enzymes from the exocrine glands of the pancreas digest carbohydrates, proteins, lipids, RNA, and DNA. Other enzymes that help in the digestive process are found in the luminal membranes and the cytoplasm of the cells that lines the small intestine. The action of the enzymes is promoted by the hydrochloric acid (HCl), which is secreted by the stomach, and bile from the liver.

The mucosal cells in the small intestines are called enterocytes. In the small intestines, they have a brush border made up of numerous microvilli lining their apical surface. This border is rich in enzymes. It is lined on its luminal side by a layer that is rich in neutral and amino sugars, the glycocalyx. The membranes of the mucosal cells contain the glycoprotein enzymes that hydrolyze carbohydrates and peptides, and glycocalyx is made up in part of the carbohydrate portion of these glycoproteins that extend into the lumen of the intestine. Following the brush border and the glycocalyx is an unstirred layer similar to the layer adjacent to the biologic membrane. Solutes must diffuse across this layer to reach the mucosal cells. The mucous coat overlying the cells also continues a significant barrier to diffusion. Most substances pass from the lumen if the intestines into the enterocytes and then out of the enterocytes to the interstitial fluids.