Unit7

Unit7

Stomach and small intestine

We

humans do amazing things with food. Somewhere in the

world,one person might eat only raw whale blubber in a given day,another might eat only rice,and still another miht eat only pizza and bananas,or drink dandelion wine.Through its metabolic magic,the human body converts these and a dizzying variety of other substances into usable energy and tissues of its own. 我们人类对食物做了许多神奇的事情。在世界上的某些地方，那里的人在特定的一天可能只吃生的鲸的脂肪，另外地方的人可能只吃米饭，仍然存在另一种人可能只吃披萨和香蕉，或是喝蒲公英酒。通过他（人体）的神奇的代谢过程，人体将这些物质以及其它多得令人眼花缭乱的东西转化成可用的能量和自身的组织。

Overview of the Digestive System(消化系统的概观）

A digestive system is a body cavity or tube in which food is reduced to particles, then to molecules small enough to be absorbed into the internal environment.

消化系统是一个体腔或管，在那里食物被分解成颗粒，然后又变成小得可以吸入内环境的分子。

Some invertebrates have an incomplete digestive system

– a saclike gut with one opening. What goes in but cannot be digested goes out the same way.Most animals have a complete digestive system.This is a tube with an opening at one end for food intake and an opening at the other end for elimination of unabsorbed residues.The tube also has specialized regions for food transport, processing, and storage.

有些无脊椎动物有一个不完整的消化系统—一个有一个开口的囊状肠。进入系统但无法消化的东西从原路出来。大多数动物都有完全的消化系统。这是一条有一端开口摄取食物和另一端开口排出未被吸收的残留物的管。这条管也有专门的区域用作运输，处理，和储存粮食。

Stretched out, the digestive system of a human would extend 6.5 to 9 meters (21 to 30 feet).Its main regions are the mouth, pharynx, esophagus, and gut (gastrointestinal tract).The gut is subdivided into a stomach, small intestine, large intestine (colon), rectum, and anus.Salivary glands and a gallbladder, liver, and pancreas secrete enzymes and other substances into different parts of the system.The overall functions of the system may be summarized this way:

1. Motility. Muscular movement of the system’s tubular walls. The movement leads to the mechanical reduction , mixing, and passage

of food material, then elimination of undigested and unabsorbed residues.

2. Secretion. Release of digestive enzymes, fluids, and other substances into the lumen (the space inside the digestive tube). 3. Digestion. Breakdown of food into particles, then into nutrient molecules small enough to be absorbed.

4. Absorption. Passage of digested nutrients, fluid, and ions across the tube wall and into the blood or lymph, which will distribute them through the body.

如果拉伸出来，人的消化系统会达到6.5米至9米长（21到30英尺）。它的主要区域是口腔，咽，食道，肠（消化道）。肠道再细分为一个胃，小肠，大肠（结肠），直肠，肛门。唾液腺和胆囊，肝脏和胰腺分泌的酶和其他物质进入系统的不同部分。系统的整体功能可以用以下的方式概括：

1、蠕动。该系统的管壁的肌肉运动。（肌肉）运动导致食物的机械性分解、混合和通过，然后促成未消化和未吸收的残留物的排泄。

2、分泌。释放消化酶，液体，和其他物质进入管腔（消化管内部空间）。

3、消化。把食物分解成颗粒，然后分解成小得可 以被吸收的营养分子。

4、吸收。被吸收的营养物质，液体，和离子穿过管壁进入血液或淋巴，血液和淋巴把它们分发给全身。

The stomach

The stomach, a muscular, stretchable sac, has three main

functions. First, it stores and mixes food. Second, its secretions help dissolve and degrade food. Third, it helps control passage of food into the small intestine.

胃是一个肌肉发达的、有弹性的囊，主要有三个功能。首先，它存储（容纳）和混合食物。其次，它的分泌物可以帮助溶解，降解食物。第三，它有助于控制食物进入小肠。

Stomach Acidity. Epithelium loaded with glands lines the inside of the stomach wall. Each day, cells in that lining secrete about two liters of hydrochloric acid (HCl), pepsinogens, mucus, and other substances.The substances make up gastric fluid (that is, the stomach's own fluid）. stomach acidity increases when the HCL separates into H+ and Cl-，and this helps dissolve bits of food to form a liquid mixture called chyme.The acidity kills many microorganisms present in food. It may cause heartburn when gastric fluid from the stomach backs up into the esophagus. 胃酸。布满了腺体的上皮位于胃壁内侧。每天，细胞在那内层分泌大约两公升的盐酸（HCl）、胃蛋白酶原、粘液、和其他物质。这些物质组成了胃液（就是胃自己的液体）。当盐酸分离成H+ 和 Cl-

时，胃酸增多，这有助于溶解小块食物，以形成称为食糜的液体混合物。胃酸杀死许多存在于食物中的微生物。当胃液从胃回流到食道时，可能会导致胃灼热。

In response to the sight, aroma, and taste of food, the brain signals the secretory cells to step up activity. Also, when food enters the stomach, it distends the wall and activates receptors in the lining. This also leads to increased secretory activity.

大脑对食物的视觉，香味和滋味做出反应 ，向分泌细胞发出指令加快活动。此外，当食物进入到胃后，它使胃壁扩张并且激活胃粘膜上的受体。这也导致分泌活动的增强。

Protein digestion starts in the stomach. The high acidity resulting from HCl secretions alters protein structure and exposes peptide bonds.It also converts pepsinogens to active enzymes（pepsins）that break the peptide bonds.Protein fragments start accumulating,and this triggers secretion of gastrin—a hormone that stimulates the pepsinogen-and HCl-secreting cells even more. 蛋白质的消化起始于胃。由盐酸分泌物导致的高酸度会改变蛋白质的结构，并使肽键暴露。它也会将胃蛋白酶原转化成使肽键断裂的活性酶（胃蛋白酶）。蛋白质（分解形成的）片段开始积累， 这又触发胃分泌素的分泌，胃分泌素是一种进一步刺激胃蛋白酶原和盐酸分

泌细胞的酶。

Mucus and buffers (bicarbonate ions especially）keep HCl and pepsin from acting on proteins in the stomach wall.When something interferes with controls over their secretion,HCl is free to diffuse into the stomach's lining.There, it triggers the release of a chemical（histamine）from

tissue

cells.Among

other

things,histamine stimulates more HCl secretion.A positive feedback loop is set up, and stomach tissues become damaged. If the tissues damage leads to bleeding in the stomach,the condition is called a peptic ulcer. Think about this if you are sensitive to aspirin.Aspirin promotes inflammation of the stomach一largely by suppressing mucus and bicarbonate secretions.

粘液和缓冲液（尤其是碳酸氢根离子）会阻止盐酸和胃蛋白酶作用于胃壁的蛋白质.当某些物质干扰他们的分泌所受到的控制时，盐酸会自由扩散进入胃粘膜。在胃的粘膜上，盐酸会引起组织细胞产生的化学物质（组胺）的释放.除了别的事物以外，组胺会刺激更多的盐酸分泌。正反馈循环被建立导致胃组织被损坏。如果组织损伤导致胃出血,这种情况被称为消化性溃疡（胃溃疡）。想一想这个，如果你是对阿司匹林敏感。阿司匹林会造成胃的炎症——主要是通过抑制粘液和碳酸氢盐分泌而导致的。

Stomach Emptying. In the stomach, waves of contractions mix

the chyme and build up force as they approach a sphincter at the start of the small intestine.The arrival of a strong contraction closes the sphincter, so most of the chyme is squeezed back. But a small amount moves into the small intestine.

胃排空。在胃里，一阵阵的收缩混合食糜并在他们接近位于小肠起始部位的括约肌时形成力量.强烈收缩的到来会使括约肌关闭，所以大部分的食糜被挤压回来。但有少量食糜进入小肠。

Chyme's volume and composition affect how fast the stomach empties. For example, large meals activate more stomach receptors, these call for more forceful contraction, and emptying speeds up.As another example, increased acidity (or fat content) in the small intestine stimulates hormone secretions that cause a slowdown in stomach emptying一so food isn't moved along faster than it can be processed. Fear, depression, and other emotional upsets also trigger slowdowns.

食糜的体积和成分影响胃清空的速度。例如，大量进食会激活更多的胃受体，受体引来更强有力的收缩， 排空的速度加快了.另一个例子是，在小肠增加酸度（或脂肪含量）刺激荷尔蒙分泌，这会引起胃排空速度减慢——所以食物向前移动的速度不比它被加工时快。恐惧，抑郁和其他情绪低落也会导致（胃排空）减速。

Only alcohol and a few other substances are absorbed across the stomach wall. Most substances move onward, to the small intestine.

只有酒精和一些其他物质通过胃壁被吸收了。大多数物质向上移动至小肠。

The Small Intestine(小肠）

Carbohydrates, lipids(including fats),proteins, and nucleic acids are digested by specific enzymes, then absorbed into the internal environment. Digestion is completed and most nutrients are absorbed in the small intestine. Secretions from the pancreas and liver assist in these tasks. Each day, the small intestine receives about nine liters of fluid from the stomach, liver, and pancreas.At least 95 percent is absorbed across an epithelial lining that is exposed to the intestinal lumen.

碳水化合物 ，脂质（包括脂肪），蛋白质，和核酸都被具体的酶消化，然后被吸收进入内环境。在小肠里，消化被完成并且许多营养物质被吸收。来自胰腺和肝脏的分泌物协助这些任务。每一天，小肠接受来自胃，肝脏和胰腺大概九公升的液体。至少95%的液体经过暴露于小肠管腔的上皮层被吸收。

Processes of Digestion.Pancreatic enzymes are essential for

digestion. They break down carbohydrates, proteins, nucleic acids, and fats. The pancreas also secretes bicarbonate, a buffer that helps neutralize HCL arriving from the stomach.(You may have heard of two other pancreatic secretions, insulin and glucagon, but these have roles in nutrition, not digestion)

消化过程.胰酶对消化是必需的。它们分解碳水化合物、蛋白质、核酸和脂肪。胰腺也分泌碳酸氢盐，一种帮助中和来自胃的盐酸的缓冲剂。（你可能已经听说过两种其他的胰腺分泌物，胰岛素和胰高血糖素，但是他们是在营养中起作用而不是在消化中。）

Fat digestion depends on more than enzyme action. It depends also on bile,which is secreted continually by the liver.Bile contains bile salts，bile pigments,cholesterol,and lecithin (a phospholipid).It flows through ducts from liver to the small intestine.When food isn’t moving through the gut,a sphincter closes off the main duct.Bile backs up into the saclike gallbladder,where it is stored and concentrated.

脂肪消化不仅仅依赖酶的作用。它还依赖由肝脏持续分泌的胆汁。胆汁包括胆汁盐、胆色素、胆固醇和卵磷脂（一种磷脂）。它通过导管从肝脏流到小肠。当食物不穿过肠的时候，括约肌会隔绝主要的导管。(这样)胆汁将回流到储存和浓缩胆汁的囊状胆囊里。

By a process called emulsification,bile salts speed up fat digestion.Most fats in our diet are triglycerides.These are insoluble in water and tend to aggregate into large fat globules in the chyme.When movements of the intestinal wall agitate chyme,fat globules break up into small droplets,which become coated with biles salts.Because bile salts carry negative charges,the coated droplets repel each other and stay separated. This suspension of fat droplets is the ―emulsion‖.

胆汁盐通过一个叫做乳化的过程加速了脂肪消化。在我们饮食中的大多数脂肪都是甘油三酯。这些（脂肪）不溶于水，而且会在食糜中聚集成大颗的脂肪小球。当肠壁的蠕动推动食糜（移动）的时候，脂肪小球分解为被胆汁盐覆盖的小液滴。因为胆汁盐带负电荷，被覆盖着（胆汁盐）的小滴就会相互排斥并且保持分离状态。这种脂肪滴的悬浮液就叫―乳液‖。

Compared to fat globules,emulsion droplets give fat-digesting enzymes a much greater surface area to act upon.Thus triglycerides can be broken down much more rapidly to fatty acids and monoglycerides.

与脂肪小球相比，乳液小滴给予消化脂肪的酶以更大得多的作用面.因此，甘油三酯能够更快速地被分解为脂肪酸和单酸甘油酯。

Processes of absorption(吸收过程）

The intestinal lining is one of the body’s marvels. It has dense folds, which are absorptive structures called villi(singular, villus).Villi increase the surface area available for interactions with chyme. Epithelial cells line these structures, and each cell has a crown of microvilli. Each microvillus is a threadlike projection of the plasma membrane.Collectively, they greatly increase the surface area available for absorption.

肠的内层是身体奇异的结构之一。它有叫做小肠绒毛的吸收结构组成的浓密的襞。小肠绒毛为食糜的相互作用增加了可利用的表面积。上皮细胞排成一行组成这些结构，每一个细胞都有冠状的微绒毛。每一个微绒毛是细胞膜的细长的突出部分。总之，它们很大程度上为吸收增加了可利用的表面积.

By the time carbohydrate, protein, and lipid molecules are halfway through this elaborate tube, most have been broken down and digested. Some of the products, including monosaccharides and amino acids, are absorbed in straightforward fashion. So are water and mineral ions.Transport proteins in the plasma membrane of epithelial cells actively shunt them across the intestinal lining.By contrast, bile salts assist the absorption of fatty acids and monoglycerides.These products of fat digestion can diffuse across

the lipid bilayer of the plasma membrane of epithelial cells. 在碳水化合物，蛋白质和脂质分子在通过消化管的途中时，大部分已经被分解和消化。一部分产物，包括单糖和氨基酸以直接的方式被吸收。水和矿物质离子也是以简单方式吸收。肠上皮细胞质膜上的运输蛋白主动地把营养物质穿过肠壁，从肠腔移向肠壁另一边。与之相比，胆汁协助脂肪酸和单酸甘油酯的吸收。这些脂肪消化得到的产物可以分散穿过小肠上皮细胞膜的磷脂双分子层。

By a process called micelle formation, bile salts combine with the

products

of

fat

digestion,

forming

tiny

droplets

(micelles).Product molecules in the micelles continuously exchange places with product molecules that are dissolved in the chyme.When concentration gradients favor it, molecules diffuse out of micelles, out of the chyme, and into epithelial cells.There, fatty acids and monoglycerides recombine as triglycerides. Then the triglycerides exocytosis and enter the internal environment. 通过一个叫胶束形成的过程，胆汁与脂肪的消化产物结合在一起，形成微小液滴（胶束）。胶束里的产物分子不断地和溶解在食糜里的产物分子交换着位置。当浓度梯度适当时，这些分子就会从胶束，食糜扩散出去，并进入上皮细胞。在那里，脂肪酸和单酸甘油酯重新组合成甘油三酯。然后甘油三酯与蛋白质结合成微粒（乳糜微粒），这些微粒通过胞吐作用离开细胞而进入内环境。

Once absorbed into the internal environment, glucose and amino acids enter blood vessels. The triglycerides enter lymph vessels, which drain into blood vessels of the circulatory system. 一旦被吸收到内部环境中，葡萄糖和氨基酸就会进入血管。甘油三酯进入淋巴管，在这里排水到循环系统的血管中.

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