A summary of the digestive system and digestive processes
Digestion is the process by which the body breaks down carbohydrates, proteins, and fats into substances that cells can absorb and use. Solids cannot enter most body cells, so it is the job of the digestive system to change the solid foods into some dissolved form that protoplasm can use. Enzymes change all carbohydrates into simple sugars. Fats are then broken down into fatty acids and glycerol. Proteins are made of amino acids. During digestion, the bonds between the amino acids are broken, and they are separated from each other. It is these amino acid molecules that we absorb into our cells. There, they are used to build our body proteins. Digesting enzymes are in juices produced by several of the digestive organs. When your digestive system is working properly, you won't even be aware of it.
There are two kinds of digestion: mechanical and chemical. Mechanical digestion is the process of chewing, mashing, and breaking food into smaller pieces. Chemical digestion is the process of enzymes changing food into simpler substances. Enzymes are proteins that speed up the chemical breakdown of complex substances. The chief system movements are peristalsis--a process that involves carrying food through the tube--and the dividing movement, which breaks up the food particles.
Your digestive tract is a long tube running from the mouth to the anus. This tube is divided into certain specialized compartments, each with a more or less different function.
The best method of defining the elaborate digestive system is to trace a meal as soon as mechanical digestion begins. As the food enters the mouth, the teeth will tear, grind, and chop it into smaller pieces. The tongue mashes soft food and mixes it with saliva. Saliva comes from three pairs of glands located in the sides of the face and under the jaw. These glands are masses of special cells. The saliva flows into the mouth through little tubes or ducts. One duct enters the mouth on the inside of each cheek. The other four ducts pour saliva under the tongue. Saliva breaks down starches into sugars.
Once the food is chewed and softened in the mouth, the tongue rolls it into a ball or bolus. The tongue pushes the bolus to the back of the throat to be swallowed. During swallowing, a small flap of tissue called the epiglottis prevents food from entering the windpipe. The food then passes into the esophagus, which is a muscular tube that connects the mouth to the stomach. Food is pushed along as circular fibers of muscle tissue in the wall of the esophagus contract. Your food will then be passed into the stomach slowly. At the opening of the stomach there is a circular muscle, or sphincter, that opens and closes to keep food from backing up into the esophagus.
The main digestive organs are located in the abdominal cavity below the diaphragm. In mammals, there are two parts of the body cavity: one is the chest cavity and the other is the abdominal cavity. These two parts of the body are separated by a tough, muscular partition called the diaphragm.
The stomach is a pear-shaped, muscular sac. Its main function is to store food. The stomach walls are made of three layers of muscle, each arranged on a different angle. As the food enters the stomach, muscle contractions begin to twist, turn, and churn the food, which is part of mechanical digestion. The stomach lining contains a great many glands, which produce a clear, yellow liquid that mixes with the food called gastric juice. Gastric juices are secreted into the stomach to help liquefy food and break it into simpler forms. Gastric juices contain water, enzymes, and acid. This acid softens fibers in the food and kills many of the bacteria. The juice also contains an enzyme that acts on proteins. Muscles in the stomach wall continue to contract and relax, and this mixes the food with the gastric juices. Gastric juices begin the digestion of proteins and fats and turn them into amino acids. Food will usually remain in the stomach for around four hours and while it is there, it becomes partially digested food that changes into a thick liquid called chyme. The sphincter at the end of the stomach allows this chyme to move into the small intestine a little bit at a time.
As food passes through the specialized compartments, it is gradually made ready for absorption into the blood. Most of this absorption occurs in the small intestine with the exception of drugs and alcohol, which are absorbed chiefly in the stomach. The small intestine is a long, coiled organ about one inch in diameter and about two centimeters across and seven meters long. The digesting food takes several hours to pass through it. The stomach connects with the duodenum, the first part of the small intestine. The duodenum is about a foot long and is shaped like the letter C. The major digestion of food occurs here.
While the food is in the small intestine, there are three juices that act upon it. One is the intestinal juice produced by the intestine lining; another is pancreatic juice that flows through a duct from an important gland lying near the stomach. This gland is the pancreas, which has a duct that enters the small intestine near its upper end. The pancreas is a long, soft gland lying behind the stomach, and it secretes digestive enzymes into the duodenum. The intestinal glands are tiny organs found in the lining of the small intestines. These tiny organs release digestive enzymes and mucus. There is also a juice called bile, which also enters the intestine at this same place and is produced by the liver.
In the small intestine are villi, very tiny fingerlike projections that line the walls. These many projections of villi increase the surface area of the small intestine allowing the small intestine to absorb most of the nutrients that enter the body. A fluid called lymph is located inside the villi blood vessels. Fat soluble vitamins and fatty acids are absorbed into the lymph system. These blood vessels also absorb glucose, amino acids, and water-soluble vitamins and minerals; then the blood and lymph carry the completely digested food throughout the body.
The liver is a large, glandlike organ in the upper right side of the abdomen that has several functions in addition to producing bile. In the liver, some digested food is removed from the blood and changed into other forms or stored. Fats are prepared so that they can be combined with proteins. Amino acids are either used to make proteins or stored in the liver until they can be carried by the blood to other parts of the body. Carbohydrates stored in the liver are changed from glucose. Glucose is then released into the blood and carried to all body cells. Excess glucose is stored in the liver as glycogen, a starchlike substance used by the body to maintain blood sugar levels between meals. The bile that is formed in the liver is stored in a sac called the gall bladder. After food leaves the stomach, the gall bladder empties its bile through a bile duct. This bile duct and pancreatic duct come together when they reach the small intestine and their juices enter together.
The small intestine is the main organ of absorption. Ridges in the intestine lining add extra surface area for absorption. Food that has been dissolved into molecules passes through the membrane linings of the intestine and enters the blood stream. Blood carries the food away to other parts of the body.
Water, fiber, and foods that the body is unable to break down pass out of the small intestine into the large intestine. The large intestine is the part of the digestive system that extends from the small intestine to the anus. The large intestine is about five feet long. Its major function is to reabsorb the water that was used during digestion; as the water is removed, the wastes become solid. These solid wastes contain undigested foods, bacteria, and cholesterol. These materials are waste materials, fibers, and cell wall materials that cannot be digested. The small intestine empties into the large intestine on the lower right side of the abdomen.
The large intestine goes up the right side of the abdomen, across the front, and down the left side. It is about five centimeters wide and l.5 meters long. Many types of waste line the large intestine. Some of the wastes are held several hours in the large intestine while water is absorbed back into the bloodstream. This leaves the wastes in a more solid form, called feces. Feces must pass out of the body through the anus. The last five to six inches of the large intestine make up the rectum, which stores solid wastes until the body is ready to expel them. During the process of elimination, solid wastes leave the body through a muscular opening at the end of the rectum called the anus. This process completes the digestion of food.