The Human Digestive System

<h1>Text Preview</h1> <p>When you chew a morsel of food, both physical digestion and chemical digestion begin. Your teeth cut and grind the food into smaller pieces, and saliva moistens the food. Saliva is a complex mixture of chemicals, including a glycoprotein that lubricates the food and protects the mouth from abrasion. Saliva also contains a digestive enzyme that breaks down complex carbohydrates into simpler carbohydrates and sugars.</p> <p>The tongue rolls the food into a ball, which makes it easier to swallow. </p> <p>Swallowing is the process by which food passes from the mouth past the pharynx—the intersection of the digestive system and the respiratory system. The act of swallowing causes the epiglottis to block the glottis, the entrance to the trachea. Once past the pharynx, the ball of food reaches the muscular esophagus —the tube leading from the pharynx to the stomach. </p> <p>Swallowing food is a voluntary muscle movement, but involuntary contractions of the smooth muscles surrounding the esophagus move the food toward the stomach. These waves of contractions are called peristalsis.</p> <p>The human stomach is a large baglike structure. Its wall is folded and can expand to accommodate up to two liters of food. This storage capacity means you don’t have to eat continuously in order to obtain nutrients. </p> <p>The stomach joins to the esophagus at one end and the duodenum at the other through a ringlike valve called the pyloric sphincter. The duodenum is the region in which secretions from the liver, pancreas, and gallbladder enter the intestines. </p> <p>The stomach is surrounded by bands of muscles that contract it in a churning motion, like a slow-motion version of your washing machine at home. Without the churning process, the mixing of food with stomach enzymes and stomach acid—a fluid called gastric juice—wouldn’t be so effective.</p> <p>The mixture of partially digested food and gastric juices is called chyme. The pyloric sphincter prevents the chyme from emptying all at once into the small intestine.</p> <p>The components of gastric juice are made by three types of cells in the stomach wall: mucous cells, chief cells, and parietal cells. Mucous cells secrete a mucous coating that protects the stomach lining and a hormone that stimulates the further production of gastric juice. Chief cells secrete an inactive enzyme called pepsinogen. Parietal cells secrete hydrochloric acid, which converts pepsinogen to pepsin, an enzyme that breaks down proteins by splitting their peptide bonds. Hydrochloric acid also kills bacteria and denatures proteins, aiding their digestion by pepsin.</p> <p>You might expect that pepsin, which breaks down proteins, would damage the cells lining the intestine. However, the inactive pepsinogen doesn’t damage the cells that secrete it. Mucus also protects the lining from being digested. Even with both levels of protection, the intestinal epithelial cells are constantly eroded and replaced every three days by a new layer of cells.</p> <p>The small intestine’s wall is folded into projections called villi. Each villus is made up of many epithelial cells with surfaces that are folded into microvilli. The effect of these structures is to increase the surface area of the intestine, thereby making the absorption of nutrients more efficient. Inside each villus are capillaries and a tiny lymph vessel. </p> <p>The veins that carry blood from these intestinal capillaries go directly to the liver.The small intestine is where most of the digestion and absorption of nutrients takes place. Before we continue our journey to the large intestine, let’s look at how fats, proteins, and carbohydrates are digested in the small intestine.</p> <p>When the partially digested mass of food leaves the stomach, any fat it contains still has to be digested. The problem with fat molecules is that they don’t dissolve in water. The liver produces bile salts that suspend the fat, producing a cloud of droplets that are small enough for the enzyme lipase to break down. Lipase hydrolyzes the fats into glycerol and fatty acids, which are absorbed by the epithelium of the small intestine and then mixed with cholesterol to form small droplets. The lymph vessels in each villus carry these droplets into the body’s lymphatic system and eventually into the blood supply. About an hour after eating a fatty hamburger, your lymph is full of these droplets!</p> <p>Protein digestion starts in the stomach, and three enzymes produced in the pancreas complete the process by breaking down the proteins into amino acids. Like pepsin, these enzymes— trypsin, carboxypeptidase, and chymotrypsin—are secreted in an inactive form that protects the cells that secrete them. The enzymes travel along the pancreatic duct, entering the duodenum where the enzyme enteropeptidase, which activates the pancreatic enzymes, is produced.</p> <p>A number of enzymes break down carbohydrates in the small intestine. The pancreas produces enzymes that break down starch and glycogen into disaccharides such as maltose, sucrose, and lactose. Each disaccharide has a specific enzyme that breaks it down into simple sugars: maltose is hydrolyzed by maltase, sucrose by sucrase, and lactose by lactase.</p> <p>Now that we’ve looked at the main site of digestion – the small intestine – we’ll move on to the region in which water reabsorption and waste compaction occurs – the large intestine.</p> <p>The large intestine’s main job is to store and concentrate undigested and indigestible residues into feces and to dispose of the material by way of the rectum and anus. The rectum is the region in which feces are stored until they can be eliminated. The large intestine also conserves water by reabsorbing it, along with salts and vitamins synthesized by intestinal bacteria.</p> <p>Now that we’ve seen the various components of the human digestive system, see if you can label the components on this diagram correctly.</p> <p>Drag the labels into place. Then click Submit to see if you are correct.</p> <p>Yes, that’s right!</p> <p>Sorry that’s not quite right.</p> <p>The epiglottis is the flap of tissue that prevents food from going down your windpipe when you swallow. </p> <p>The esophagus is the tube that connects your pharyns with your stomach.</p> <p>The pyloric sphincter is the valve at the junction of the stomach and duodenum. </p> <p>The duodenum is the first part of the small intestine, where the ducts from the pancreas and liver join to the intestine.</p> <p>The cecum is the part of the small intestine that aids breakdown of plant material in herbivores. It is not thought to paly a role in human digestion.</p> <p>As we’ll see next, the muscular and secretory activities of the digestive system are regulated by the nervous system and the endocrine system.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>