The Process of Human Digestion

digestive system. These organs put in chemical digestion by releasing digestive juices into the digestion system through tubes called ducts. If an adult’s digestive tract were stretched out, it would be 20 to 30 ft long. In humans, digestion begins in the mouth, where both mechanical and chemical digestion occur. The mouth quickly turns food into a soft, moist mass. The muscular tongue pushes the food against the teeth, which cut, chop, and grind the food. Glands in the cheek linings secrete mucus, which lubricates the food, making it easier to chew and swallow. Three pairs of glands empty saliva into the mouth through ducts to moisten the food. Saliva contains the enzyme ptyalin, which begins to break down starch a carbohydrate manufactured by green plants. Once food has been reduced to a soft mass, it is ready to be swallowed. The tongue pushes this mass called a bolus to the back of the mouth and into the pharynx. This cavity between the mouth and windpipe serves as a passageway both for food on its way down the digestive system canal and for air passing into the windpipe. The epiglottis, a flap of cartilage which is soft tissue, covers the windpipe when a person swallows. This action of the epiglottis prevents choking by directing food from the windpipe and toward the stomach. The presence of food in the pharynx stimulates swallowing, which squeezes the food into the esophagus. The esophagus, a muscular tube about 10 in long, passes behind the trachea and heart and penetrates the muscular wall between the chest and abdomen before reaching the stomach. Food advances through the digestive system by means of rhythmic muscle contractions known as peristalsis. The process begins when circular muscles in the esophagus wall contract and relax (widen) one after the other, squeezing food downward toward the stomach. Food travels the length of the esophagus in two to three seconds. A circular muscle called the cardiac sphincter separates the esophagus and the stomach. As food is swallowed, this muscle relaxes, forming an opening through which the food can pass into the stomach. Then the muscle contracts, closing the opening to prevent food from moving back into the esophagus. The cardiac sphincter is the first of several such muscles along the digestive system. These muscles act as valves to prevent the passage of food and keep it from moving backward. The stomach is a J shaped organ, located in the upper abdomen just below the diaphragm, is a saclike structure with strong, muscular walls. The stomach can make itself bigger to store all the food from a meal for both mechanical and chemical processing. The stomach contracts about three times per minute, churning the food and mixing it with gastric juice. This fluid, secreted by thousands of gastric glands in the lining of the stomach, consists of water, hydrochloric acid, an enzyme called pepsin, and mucin . Hydrochloric acid creates the acidic environment that pepsin needs to begin breaking down proteins. It also kills small organism that may have been ingested in the food. Mucin covers the stomach, protecting it from the effects of the acid and pepsin. About four hours or less after a meal, food processed by the stomach, called chyme, begins passing a little at a time through the sphincter into the duodenum, the first portion of the small intestine. Most digestion, as well as absorption of digested food, occurs in the small intestine. This narrow, twisting tube, about 1 in in diameter, fills most of the lower abdomen, extending about 20 ft in length. Over a period of three to six hours, peristalsis moves chyme through the duodenum into the next portion of the small intestine, the jejunum, and finally into the ileum, the last section of the small intestine. During this time, the liver secretes bile into the small intestine through the bile duct. Bile breaks large fat globules into small droplets, which enzymes in the small intestine can act upon. Pancreatic juice, secreted by the pancreas, enters the small intestine through the pancreatic duct. Pancreatic juice contains enzymes that break down sugars and starches into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. Glands in the intestinal walls secrete additional enzymes that break down starches and complex sugars into nutrients that the intestine absorbs. Structures called Brunner’s glands secrete mucus to protect the intestinal walls from the acid effects of digestive juices. The small intestine’s capacity for absorption is increased by millions of fingerlike projections called villi, which line the inner walls of the small intestine. Each villus is about 0.02 to 0.06 in long and covered with a single layer of cells. Even tinier fingerlike projections called microvilli cover the cell surfaces. This combination of villi and microvilli increases the surface area of the small intestine’s lining by about 150 times, multiplying its capacity for absorption. Beneath the villi’s single layer of cells are capillaries (tiny vessels) of the bloodstream and the lymphatic system. These capillaries allow nutrients produced by digestion to travel to the cells of the body. Simple sugars and amino acids pass through the capillaries to enter the bloodstream. Fatty acids and glycerol pass through to the lymphatic system. A watery residue of indigestible food and digestive juices remains unabsorbed. This residue leaves the ileum of the small intestine and moves by peristalsis into the large intestine, where it spends 12 to 24 hours. The large intestine forms an inverted U over the coils of the small intestine. It starts on the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is 5 to 6 ft long and about 2.5 in in diameter. The large intestine serves several important functions. It absorbs about 1.6 gallons of water daily—as well as dissolved salts from the residue passed on by the small intestine. In addition, bacteria in the large intestine promote the breakdown of undigested materials and make several vitamins, notably vitamin K, which the body needs for blood clotting. The large intestine moves its remaining contents toward the rectum, which makes up the final 6 to 8 in of the alimentary canal. The rectum stores the feces—waste material that consists largely of undigested food, digestive juices, bacteria, and mucus—until elimination. Then, muscle contractions in the walls of the rectum push the feces toward the anus. When sphincters between the rectum and anus relax, the feces pass out of the body. The various steps of digestion so that the process proceeds smoothly and cells obtain a steady supply of nutrients and energy. The central nervous system and various glands control activities that regulate the digestive process, such as the secretion of enzymes and fluids. The Digestive system also have its ups and downs. some of the downs we don't like and wish we can never get them. One of the downs is diarrhea, Diarrhea is when the large intestine don't absorb a lot of water and the pup is left in a watery form. The other down is constipation. constipation is when the large intestine absorbs too much water and the pup is hard and its very hard to get it out the body.