The food we eat cannot be absorbed directly into the body. It must first undergo a process of conversion into substances which can be directly conveyed into the bloodstream - the carrier and distributor of our foods throughout the body. This process is called digestion. (For a full understanding ofthe following text, refer to the diagram of the digestive system included overleaf.)
Digestion begins when food enters the mouth. The role the mouth plays in digestion is twofold: it physically breaks down the food into smaller fragments through the act of chewing while simultaneously secreting saliva through the salivary glands. The quantity of saliva secreted depends on the taste and look of the food, as well as on appetite. However, normally during the day one to two litres of saliva are secreted. Saliva contains the enzyme ptyalin, which starts to break down the starches present in the food into simpler forms of carbohydrates known as sugars. Saliva also contains bicarbonate, which neutralizes acids within the stomach. Many people take bicarbonate of soda to relieve hyperacidity, yet if enough time was given to allow a sufficient secretion of saliva there would be no need for such remedies. Therefore, for a good digestion, slow eating and adequate chewing are important to allow the saliva to complete its function properly as well as increasing one's enjoyment of food.
After the food has been chewed sufficiently it passes through the throat (pharynx) to the food pipe (oesophagus). From there it is pushed down to the stomach by a series of rhythmic waves known as peristalsis. At the entrance to the stomach lies the cardiac sphincter, which allows the food to pass into the stomach.
The stomach when empty is about the size of your hands held palm to palm. Its walls are much thicker than any other part of the digestive tract. The stomach is designed to knead and churn the food with the gastric digestive juices, which are secreted by the millions of glands lining the stomach wall. These gastric juices are mainly comprised of the enzymes pepsin and hydrochloric acid, which are responsible for the breakdown of proteins. Rennin, which is secreted in less quantity, coagulates or curdles certain types of foods (such as the caseinogen of milk solids) so that they can be exposed for a longer period of time to the action of the digestive juices. Another important enzyme present in the gastric juices is pepsinogen, which terminates the action of the saliva. Together with hydrochloric acid, it also destroys germs which might be present in the food. The amount of gastric juices secreted depends on the amount of food consumed as well as the person's appetite. Tasteless, monotonous food produces little gastric juices, whereas pleasant, tasteful food
The Digestive System
The Digestive System
encourages abundant secretions of these juices. However, on an average several litres a day are utilized. An average meal requires about 800 cc of gastric juices to be secreted from the stomach walls. This is not all released during meals; about 200 cc are secreted while eating and the remainder during the subsequent time that the food remains in the stomach.
The length of time that solid food stays in the stomach varies from two to six hours, depending on the nature of the food. Fats are more difficult than proteins and carbohydrates, and therefore remain in the stomach longer. Water and other liquids do not stay in the stomach for more than a few minutes. They pass almost immediately into the small intestine and are quickly absorbed into the bloodstream.
From the stomach the food gradually passes into the small intestine through the pyloric valve in a semi-liquid form called chyme. The first section of the small intestine is called the duodenum. Within the duodenum further digestive juices are mixed with the food from various glands within the gastrointestinal system. The pancreas is the most important of these glands. The pancreatic secretions contain powerful enzymes including amylase, lipase and trypsin which are capable of digesting all types of foodstuffs - proteins, fats and carbohydrates. The pancreas does not function properly unless the food has already been sufficiently mixed with hydrochloric acid from the stomach. This is why some people who secrete insufficient hydrochloric acid fail to digest their food properly. About 600 cc of pancreatic juices are used every day.
Another important gland in the digestive system is the liver, which is the biggest single gland in the body. It is primarily concerned with the storage of food after it has been absorbed by the blood. It changes and stores the food in the form of glycogen. When energy or nutrition is required in any part of the body, the glycogen is converted into glucose (blood sugar) and discharged into the bloodstream for distribution. It also aids the pancreatic juice, lipase, in breaking down the fats. It performs this function by producing a clear, golden-coloured liquid called bile which is stored in the gallbladder, where it becomes more concentrated. This liquid not only aids the pancreatic juices but also helps to keep the food moving in the small intestine by stimulating the peristalsis.
The walls of the small intestine look and feel like velvet. It is lined with hundreds of thousands of hair-like nodules called villi which contain tiny blood vessels. T hey increase the surface area of the small intestine so that the nutrients in the chyme can be easily absorbed by the bloodstream and carried to the liver for storage. A further feature of the villi is their constant motion, which progressively moves the food along the intestinal tract. In this manner, food is allowed to come into contact with the different types of enzymes secreted by small glands which are also embedded in the intestinal walls. These enzymes include lactose, maltose, enterokose and sucrose, all having various functions to perform. Minerals and vitamins are also absorbed into the body from the small intestine.
In the intestinal walls are various muscles which relax and contract when stimulated by special nerves during what is termed intestinal peristalsis. During the digestion process, the small intestine is in constant motion under the action of this peristalsis and the food is progressively moved along the intestinal tract and brought into contact with the enzymes and villi.
The term small intestine is confusing for the total length is over six meters. The word small refers to the smaller diameter when compared to the large intestine, which is only one and a half meters long. Food must traverse the whole length of the small intestine undergoing great changes and having the bulk of its nutrients absorbed into the bloodstream.
The chyme completes its journey through the small intestine and passes into the large intestine by way of the ileocecal valve. This valve regulates the flow of the chyme to the large intestine by preventing the small intestine emptying too quickly. At the same time it prevents any chyme from going back into the small intestine. At the point of entering the large intestine, the chyme contains mainly waste products (undigested food) and water. The water is largely absorbed in the large intestine to prevent dehydration of the body. The remaining waste matter proceeds to the rectum where it is eliminated from the body in the form of stool. This completes the process of digestion.
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