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Digestive System

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Digestive System

 

 

 

 

 

The Digestive System is responsible for the break-down of food into substances that the body can absorb and use for energy, growth, and repair.

The digestive system is controlled by enzymes that help break down the food that we eat. These enzymes are      produced by the organs attached to the alimentary canal, and they are responsible for many of the chemical reactions involved in digestion (Jacoby and Youngson, 2005).

The digestive process begins in the mouth. When the teeth chew food, the salivary glands beneath the tongue increase their various secretions which produce a number of enzymes. Ptyalin is one such enzyme which is responsible for the break-down of carbohydrates into smaller molecules, known as maltose and glucose. Lingual Lipase is another enzyme that helps break down lipids and fats. When food has been chewed and the enzymes from the mouth have began breaking down the the nutrients in the food it then travels down the esophagus and into the stomach, where a mixture of chemicals—including mucus, hydrochloric acid, and the enzyme pepsin—pours onto the grinded food. Ptyalin and other enzymes in the mouth stop working at this point, but a new series of chemical reactions begins, triggered by a set of nerve impulses (Jacoby and Youngson, 2005). Nerve impulses along with the presence of food and the secretion of hormones govern the quantity of juices produced in the stomach and small intestine. The hormone gastrin stimulates the stomach cells to release hydrochloric acid and pepsin once food is in the stomach, so that it can be broken down into substances called peptones. Mucus secretion at this point is important as it prevents the acid from damaging the stomach lining. When the acidity reaches a certain point, gastrin production ceases (Kong and Singh, 2008).

The food then leaves the stomach into the duodenum (first part of the small intestine) where it is met with a thickish, acidic liquid called chyme. The duodenum makes and releases large quantities of mucus, which protects it from damage by the acid in the chyme and by other enzymes. The duodenum also receives digestive juices from the pancreas and large amounts of bile, which is made in the liver, stored in the gallbladder, and released into the duodenum to break down fat globules (Maton et al, 1993).

Two hormones are triggered at this point which releases pancreatic juices consisting of enzymes and other chemicals necessary in the digestive process. The first hormone triggered is secretin, which is responsible for stimulating the production of large quantities of alkaline juices, which neutralize the acidic levels in the partially digested chyme (Silk, 1974). A second hormone called pancreozymin, causes pancreatic enzymes to be produced, which helps digest carbohydrates, proteins and fats. These enzymes include trypsin, which breaks the peptones into smaller units known as peptides. A second enzyme is lipase, which breaks fat down into smaller molecules of glycerol and fatty acids. A third enzyme is amylase, which further breaks down carbohydrates into maltose and glucose (Jacoby and Youngson, 2005).

The digested food then enters the jejunum and ileum which sits next to the duodenum in the small intestine. Most food absorption occurs in the ileum, which contains millions of tiny projections called villi on its inner wall. Each villus contains a small blood vessel (capillary) and a tiny, blind-end branch of the lymphatic system called a lacteal. When digested food comes into contact with the villi, the glycerol, fatty acids, and dissolved vitamins enter the lacteals (Sherwood, 2006). From here they are carried into the lymphatic system where they then pour into the bloodstream. Amino acids from protein digestion and the sugars from carbohydrates (plus vitamins and important minerals, such as iron, calcium, and iodine) are absorbed directly into the capillaries and transported into the hepatic portal vein, which takes food directly to the liver. The liver filters out substances for its own use and storage, and the remainder pass into general circulation (Jacoby and Youngson, 2005).

Food that is not absorbed in the small intestine then enters the large intestine which is approximately 1.5m long. The large intestine is made up of the cecum, appendix, colon, rectum, and bottom canal. The large intestines primary function is to absorb water from the remaining food matter as well as absorb any remaining nutrients before sending the indigestible matter to the rectum to be expelled. The colon is responsible for absorbing nutrients like vitamin K, vitamin B12, riboflavin and thiamine (Kapoor, July 2011). The cecum which sits at the beginning of the large intestine is responsible for collecting faecal material from the ileum. This process in the large intestine takes approximately 16 hours to complete (Kapoor, July 2011).

 

 

 

References

 

Jacoby, D.B. and Youngson, R.M. (2005) Encyclopedia of Family Health. Volume: 4: New York.

 

Kapoor, V.K. ( Jul 2011). Large Intestine Anatomy. Medscape. WebMD LLC.

 

Kong, F. and Singh, R.P. June 2008. Disintegration of solid foods in human stomach. Journal. Food Science. 73 (5): R67–80.

 

Maton, A and Hopkins, J. And McLaughlin, C.W. and Johnson, S. And Warner, M.Q. and LaHart, D. and Wright, J.D. (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall.

 

Silk, D.B. (1974). Progress report. Peptide absorption in man. Gut 15 (6): 494–501.

 

Sherwood, L. (2006). Fundamentals of physiology: a human perspective (Third Ed.) Florence, KY: Cengage Learning. p. 768.

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