The human body uses various kinds of food for energy and growth. To be used, however, food must be changed into a form that can be carried through the bloodstream. The body’s process of extracting useful nutrients from food is called digestion.
The digestive system of humans and other higher animals is the group of organs that changes food–carbohydrates, fats, and proteins–into soluble products that can be used by the body. Both mechanical action and chemical action are necessary to change food into products that are usable by the body.
Human digestion, or the change that food undergoes in the digestive system, takes place in a long tubelike canal called the alimentary canal, or the digestive tract. There is good reason why the passageway used by food to travel through the body is called the alimentary canal. Just as canals are constructed to guide ships through waterways to their destinations, the alimentary canal guides food as it travels through the human body. The whole canal is lined with a mucous membrane.
Digestion begins in the mouth. Here the food is cut and chopped by the teeth. The tongue helps mix the food particles with a digestive juice called saliva, which is secreted in the mouth. Saliva moistens the food so it can be swallowed easily. It also changes some starches into simple sugars.
It is important to chew food thoroughly to mix it well with saliva. Thorough chewing cuts food into small pieces that are more easily attacked by digestive juices. Food should not be washed down with quantities of liquid to avoid chewing.
From the mouth the food is swallowed into a transport tube, named the esophagus, or gullet. A flap called the epiglottis closes the windpipe while food is being swallowed. Peristalsis, a wavelike muscular movement of the esophagus walls, forces food down the tube to the stomach.
Peristalsis takes place throughout the digestive tract. It is an automatic, or involuntary, action, carried out in response to nerve impulses set up by the contents of the tube. When digestion is working normally, a person is unaware of the movements of the gullet, stomach, and most of the intestine. Swallowing is a voluntary muscular action.
At the end of the esophagus there is a muscular valve, or sphincter, through which food enters the stomach. This sphincter muscle keeps food in the stomach from being forced back into the esophagus. Peristalsis in the stomach churns the food and mixes it with mucus and with gastric juices, which contain enzymes and hydrochloric acid. These gastric juices are secreted from millions of small glands in the lining of the upper stomach walls. These glands pour about three quarts of fluid into the stomach daily. Similar glands in the small intestine also secrete enzymes and fluid. Hydrochloric acid secreted by the stomach sets up the sour or acid condition necessary for digestion. Certain remedies for indigestion are advertised as correcting this acid condition. If these remedies actually do get rid of the stomach acids it is not wise to take them. Acid is required for digestion to be properly undertaken in the stomach.
The stomach churns the food into a thick liquid, called chyme, before it is passed on by peristalsis into the small intestine. Another strong sphincter muscle further mashes the chyme and has some control over the rate at which it is passed out of the stomach into the duodenum, or upper small intestine. The sphincter also prevents the chyme from passing back into the stomach.
Little by little, as the digestive process in the stomach is completed, all the chyme is passed through the sphincter into the duodenum. This peristalsis is regulated by the autonomic nervous system. This process does not take place all at once. It continues over a period of time.
From the time a meal is eaten, it takes from 30 to 40 hours for food to travel the length of the alimentary canal. Different kinds of food, depending on their components, are held in the stomach for varying lengths of time. Starch and sugar are held in the stomach for a short time only, usually no more than one to two hours. Protein foods are there from three to five hours. Fat foods may remain in the stomach even longer than proteins. This is why eating a heavy dinner of meat, potatoes, and gravy satisfies our hunger longer than one made up entirely of sweets or greens. Furthermore, food made up of easily digested carbohydrates passes quickly from the stomach and into the small intestine.
The stomach, though important, is not considered by physicians to be essential to life. People who have had their stomachs completely or partially removed are frequently able to live by taking special foods in small quantities many times a day. The small intestine is then able to perform all necessary digestion.
The small intestine, which is from 22 to 25 feet (6.7 to 7.6 meters) long, is the longest part of the digestive tract of humans. The main parts of the small intestine are the duodenum, jejunum, and ileum. Food remains in the small intestine for several hours. Two large glands, the liver and the pancreas, connect with the small intestine by ducts, or tubes. Through these ducts the liver and pancreas pour secretions which further aid digestion.
Fluid from the pancreas is called pancreatic juice. Fluid from the liver is called bile. The pancreas is one of the most important glands in the body. It secretes up to a pint of pancreatic juice a day. This digestive fluid contains enzymes which help digest carbohydrates, proteins, and fats.
One of these enzymes is trypsin, which helps digest protein foods. Other enzymes are amylase and maltase, which help digest carbohydrates. The pancreatic enzyme lipase, along with bile from the liver, helps digest fat. Bile, however, does not contain important enzymes.
Bile is stored in the gall bladder, a small hollow organ located just under the liver. We could not live without the liver but the gall bladder can be removed by surgery without serious effect.
The liver stores glycogen for later use by the body and furnishes clotting material for the blood. When fully digested, proteins are changed into amino acids; fat foods are changed into fatty acids; and carbohydrates are changed into sugars. These soluble food products are dissolved and then absorbed into the bloodstream through the walls of the small intestine.
While food is in the small intestine it is further diluted by fluid secreted by the intestinal glands. In an adult the small intestine is about 21 feet long. By the time the diluted food products have traveled its length, most of their nutrients have been absorbed into the bloodstream.
The lining of the small intestine contains many folds. These folds increase the surface area that can be in contact with the food products. The lining surface of the intestinal folds is further increased by many microscopic fingerlike projections called villi. The digested food is passed through the cell membranes of the villi into the blood and lymph, which carry it to the cells. The body can then use the food for energy and growth.
Peristalsis moves material from the small intestine into the large intestine. Peristalsis continues in the large intestine but at a much slower rate. Although the large intestine is only about 6 feet long, waste material takes 10 to 20 hours to pass through it. Here most of the water that was mixed with the food is removed through the walls of the large intestine. The waste is turned into solids that are passed from the body by excretion.
In addition to the rectum, anus, and other parts, the large intestine is made up of the ascending colon, the transverse colon, and the descending colon. The contents of the small intestine enter the ascending colon through a sphincter muscle, which prevents their return into the small intestine
In the ascending colon, fluids and salts are absorbed. Water taken with meals is absorbed here. Water drunk between meals is mostly absorbed in the small intestine. In the transverse colon more water is removed from the waste materials until they are in solid form. The descending colon is a holding area for waste.
In the colon there are large numbers of bacteria. These bacteria aid in digesting the remaining food products. They also produce folic acid, which prevents anemia, and they aid in the absorption of several vitamins. Enzymes help plants and animals digest their food just as enzymes help humans digest theirs. An enzyme, amylase, helps break down starch into sugar. Another enzyme, maltase, acts upon malt sugar and changes it into glucose, which is more easily absorbed by the body. Lipase changes fats into more usable forms.