Digestive System Digestion Overview Without

Digestive System

Digestion Overview

Without the digestive system, the human body (or the body of any organism, for that matter) would not be able to access the nutrients that exists in the form of various food substances produced by nature and increasingly by man. There are six major processes that occur in the digestive system that allow for the necessary nutrients from food to be delivered to the places in the body where it is needed: ingestion, secretion, propulsion or movement, digestion, absorption, and excretion or defecation (MNSU 2010; Advameg 2010). These are carried out by organs that can be categorized into two major divisions, the alimentary canal and the accessory organs (Smith & Morton 2001). The alimentary canal consists of the pathway of organs that take the food substances from ingestion to defecation, with the absorption of nutrients and the influence of accessory organs occurring along the way.

It's Alimentary, Dear Watson

The alimentary canal begins at the mouth, where several different digestive processes occur. Mechanical digestion occurs with the breaking up of food by the teeth through chewing and grinding, and act that is assisted by the secretion of saliva into the mouth (MNSU 2010). Saliva contains salivary amylase, the first digestive enzyme to contact food, and this breaks down starches; the tongue moves the food around while the secretion of mucin lubricates the process and helps to hold food in a bolus, easing swallowing (SUNY 2010). The food is then swallowed, pushed down the esophagus by muscles contractions to the stomach.

The stomach is the site of further chemical digestion as the acids in the stomach break down food particles into a substance called chyme (Smith & Morton 2001). Proteins also begin to break down in the stomach, with the enzyme pepsinogen, which is converted to pepsin, making up a part of the gastric juices that are secreted (GIllson 2005; SUNY 2010). Mucous continues to lubricate the digestive process, and the chyme is passed into the small intestine where the majority of digestion occurs. Specifically, it is in the duodenum -- the first part of the small intestine that the chyme reaches -- that the majority of nutrients are broken down and absorbed by the body's transport systems (Smith & Morton 2001; Advameg 2010). A mixture of enzymes produced by accessory organs break down fats and continue the digestion of proteins and carbohydrates (SUNY 2010; Gilsson 2005).

Below the duodenum are the jejunum and finally the ileum, the last section of the small intestine where approximately five percent of undigested food particles are broken down (Advameg 2010). From here, the remaining material passes to the large intestine or colon, where a great deal of reabsorption of water and other specific nutrients and minerals secreted into the alimentary canal during digestion or being drawn out of the food occurs (SUNY 2010; Smith & Morton 2001). Friendly bacteria in the colon also produce necessary nutrients, such as vitamin K, which is absorbed into the body through the colon wall (Gillsson 2005). The last twenty centimeters of the colon are comprise the rectum, where feces are formed and eventually expelled, ending the digestive process (SUNY 2010).

Accessories Are Key

The basic path of digestion, as can be seen above, follows along the alimentary canal. The organs of this canal could not properly perform their functions without the work of the accessory organs, however, particularly the liver and the pancreas. The blood and lymph systems are also vital to digestion's efficacy, transporting nutrients to the cells of the body where they are needed (Smith & Morton 2001). Again, most of the "action" of the accessory organs and their products takes place in the small intestine where the major work of both breaking down and absorbing nutrients takes place (SUNY 2010). It is here that the enzymes secreted by the pancreas and liver arrive to do their work, and it is through the small intestine -- and primarily through the duodenum -- that carbohydrates, proteins, and lipids (fats) as well as other vitamins and mineral nutrients reach the lymph and circulatory systems that carry them throughout the body where they are useful (MNSU 2010).

Pancreatic amylase, one of the enzymes secreted by the pancreas, continues the breakdown of starches that begins in the mouth, converting these carbohydrates into maltose (SUNY 2010). Trypsin and chymotrypsin both continue the breakdown of proteins that began in the stomach, converting specific amino acids into peptides, while lipase breaks fat down into glycerol and fatty acids; all of these are enzymes secreted by the pancreas into the small intestine (SUNY 2010; Smith & Morton 2001). Bile produced in the liver emulsifies fats, and the liver also regulates the absorption and creation of certain nutrients (SUNY 2010).