Patricia Justin J., Dhamoon Amit S.
Upstate Medical University
SUNY Upstate Medical University
Digestion is the process of mechanically and enzymatically breaking down food into substances for absorption into the bloodstream. The food contains 3 macronutrients that require digestion before they can be absorbed: fats, carbohydrates, and proteins. These macronutrients are broken down through digestion 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 2 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 processes and mechanical or chemical digestion defects can lead to nutritional deficiencies and gastrointestinal pathologies. The nutritional substances, minerals, vitamins, and fluids enter the body through the gastrointestinal system. 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, 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 line 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. The small intestines have a brush border made up of numerous microvilli lining their apical surface. This border is rich in enzymes. The glycocalyx is lined on its luminal side by a layer rich in neutral and amino sugars. The membranes of the mucosal cells contain the glycoprotein enzymes that hydrolyze carbohydrates and peptides, and glycocalyx is 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 of the intestines into the enterocytes and then out of the enterocytes to the interstitial fluids.
消化是一个机械和酶促过程，将食物分解成可被吸收进入血液循环的物质。食物含有三种宏量营养素，在被吸收之前需要进行消化：脂肪、碳水化合物和蛋白质。这些宏量营养素通过消化分解成能够穿过肠上皮细胞并进入血液循环以供身体利用的分子。消化是一种分解代谢形式，即分解物质，它涉及两个独立的过程：机械消化和化学消化。机械消化是将食物物质物理分解成更小的颗粒，以便更有效地进行化学消化。化学消化的作用是通过消化酶进一步降解摄入化合物的分子结构，使其变成可被吸收进入血液循环的形式。有效的消化需要这两个过程，机械或化学消化缺陷可能导致营养缺乏和胃肠道疾病。营养物质、矿物质、维生素和液体通过胃肠道系统进入人体。脂质、蛋白质和复合碳水化合物主要在小肠内被分解成小的、可吸收的单位（被消化）。消化产物，包括维生素、矿物质和水，穿过黏膜进入淋巴或血液（吸收）。主要食物宏量营养素的消化是一个有序的过程，涉及大量消化酶的作用。唾液腺和舌腺分泌的酶消化碳水化合物和脂肪，胃分泌的酶消化蛋白质，胰腺外分泌腺分泌的酶消化碳水化合物、蛋白质、脂质、RNA和DNA。其他有助于消化过程的酶存在于小肠内衬细胞的腔膜和细胞质中。胃分泌的盐酸（HCl）和肝脏分泌的胆汁促进了酶的作用。小肠中的黏膜细胞称为肠上皮细胞。小肠有一个由许多微绒毛组成的刷状缘，位于其顶端表面。这个边缘富含酶。糖萼在其腔侧衬有一层富含中性糖和氨基糖的物质。黏膜细胞的膜含有水解碳水化合物和肽的糖蛋白酶，糖萼是这些糖蛋白碳水化合物部分的一部分，延伸到肠腔内。在刷状缘和糖萼之后是一层未搅动层，类似于与生物膜相邻的层。溶质必须扩散穿过这层才能到达黏膜细胞。覆盖在细胞上的黏液层也对扩散构成了显著的屏障。大多数物质从肠腔进入肠上皮细胞，然后从肠上皮细胞排出到组织液中。
