Digestive System Summary

Digestion starts with the mouth, which begins the mechanical breakdown of food by the grinding and chomping action of the teeth. Chemical digestion (breakdown) of starches, such as crackers, into glucose because of enzymes such as amylase that are secreted into the mouth from the salivary glands. The tongue mashes the food into a soft ball called a bolus, now moistened from saliva. Saliva contains water, enzymes and mucus. The softened ball of food is swallowed where it travels down the esophagus (a tube that connects the mouth to the stomach) because of wavelike involuntary muscular contractions called peristalsis. No digestion occurs in this structure. Food next enters the stomach through a valve called the cardiac valve. The stomach is responsible for storing food while its thick, muscular walls contract and relax, churning and mixing food with gastric juices including hydrochloric acid, mucus, enzymes and hormones. Mucus lines the stomach walls, protecting the lining from acid it produces. The lining is also replaced every 3 days to replace damaged tissues. The churning action of the stomach continues mechanical digestion, but some chemical digestion also happens. Proteins are clumped together by HCl. Enzymes then continue chemical digestion by breaking them into smaller molecules called amino acids. The food is now a liquid called chyme as it leaves the stomach through another valve called the pyloric valve and enters the small intestine. If something harmful, such as bacteria, is consumed and causes irritation (an internal stimulus) then the response of the stomach will likely be vomiting which removes harmful or disease-causing organisms from the body. Most nutrients are digested and absorbed in the small intestine. The small intestine is responsible for most of the chemical digestion in the digestive system. It digests fats, sugars, starches and proteins. Most digestion in the small intestine occurs in the first 10 inches. The remainder of the small intestine absorbs nutrients. Villi, tiny finger-like projections, increase the surface area making absorption more efficient, but the microvilli add even more surface area, allowing the small intestine to absorb even larger amounts of nutrients. There are 3 main accessory organs that assist the small intestine to chemically digest food. Food never enters these organs. Instead, the liver, gall bladder and pancreas, make and secrete fluids into the small intestine. The liver produces bile, which helps break down fats in the small intestine. The gall bladder stores the bile until it is needed. The pancreas produces hormones and enzymes, such as insulin, that are released into the small intestine to further break down sugars and other nutrients. From the small intestine the remaining material, mostly plant fiber and water, moves into the large intestine. Only about 10% of the original material and secretions makes it to this structure. The primary function of this organ is to reabsorb water from undigested food. This is very important to prevent dehydration. The large intestine prepares the indigested materials, now called feces, for removal from the body. Foods that are high in fiber such as beans are necessary for a healthy colon, the final part of the large intestine because they allow the feces to move through the colon efficiently. The large intestine does not have any digestive enzymes to get energy and instead have to depend on bacteria. Feces spend 12 to 24 hours in the large intestine being absorbed by bacteria. These bacteria create large amounts of intestinal gas as a waste product as they digest fruit and vegetable fiber. These bacteria are necessary for survival because in addition to digesting fiber, they produce vitamins B and K. If bacteria are killed through illness or antibiotics they cannot produce vitamins or allow the absorption of water, when illness strikes and the large intestine is unable to absorb the excess water, diarrhea occurs which can lead to dehydration. Eating yogurt, which is full of healthy bacteria, can help replace the bacteria lost. From here, feces waits in the final section of the large intestine, the rectum, before being expelled from the body through the final valve, or opening, called the anus.