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How the Digestive System Works

The Functions of the Gastrointestinal Tract

Sep 10, 2009 Noreen Kassem

Digestion of food is a complex process that is controlled by the brain & involves several organs & body systems. Understand how this finely tuned system works.

The digestive system is a portal for food nutrients to get to the circulatory system, which relays it to each of the body’s billions of cells. Before this can happen, food must be reduced to very simple molecules by a combination of mechanical and chemical degradations.

The resulting molecules are the building blocks of life: sugars, amino acids, fatty acids, which are then transported across the epithelium lining the intestine into the blood. This complex process is digestion.

The Anatomy of Food Digestion

The digestive system, also called the gastrointestinal (GI) system is a series of tubes that begins at the mouth and ends at the rectum. The term “gastro” refers to the stomach. The GI tube is enveloped by a wall composed of four basic layers: the mucosa, submucosa and 2 outer muscular layers.

The layers vary in muscularity, thickness and permeability to molecules depending on the function of varying areas of the digestive tract. Circular sphincter muscles pinch close openings to the stomach, intestines, and body exterior as necessary. The salivary glands, liver, gall bladder and the pancreas are accessory organs essential for digestion.

Digestion begins even before the first bite is taken as the brain signals the GI system to prepare for a downpour of food. In the mouth, the initial breakdown of carbohydrates begins as the food is chewed into a round bolus. The esophagus is a conduit to the stomach where strong acid further degrades the swallowed material, beginning the digestion of proteins. The muscular walls of the stomach powerfully churn the food into liquefied chyme.

This is then sent to the small intestine where it is mixed with bile, a detergent-like emulsifier of fat molecules. Bile is produced in the liver and stored in the gall bladder from where it is delivered to the small intestine through the bile duct.

The pancreas secretes pancreatic fluid, important for the digestion of carbohydrates in the small intestine. Additonally, the lining of the stomach exudes a substance called intrinsic factor, which plays an important role in the absorption of vitamin B12 from the diet and in the development of red blood cells.

The Function of the Intestines

Compactly folded beneath the stomach, the small intestine is 2 inches around and about 22 feet long. It is ‘small’ compared to the diameter of the large intestine which is about 4 inches around and 5 feet long. Food matter can stay in the small intestine for up to 4 hours and in the large intestine for up to several days. The small intestine is divided into the duodenum, jejunum and ileum and the large intestine into the cecum, colon and rectum.

The small intestine is the most dynamic area of the GI tract; this is where almost all of the digestion of the food and the absorption of nutrients occurs. The inner walls of the small intestine have small projections called villi, designed to increase the absorption area for nutrients.

The long length of the small intestine allows the slow progression of food so that thorough digestion and absorption can take place. From here the nutrient rich blood goes directly to the liver for processing. The liver filters out harmful substances, stores some nutrients and sends the rest to the body.

After the food is squeezed for every possible nutrient, the waste matter is passed to the large intestine, where the last bit of water and minerals are absorbed back into the body, resulting in the dry waster matter ready for a bowel movement.

For further information on ailments of the digestive system, read: The Causes of Indigestion and Irritable Bowel Syndrome.

Reference:

Guyton and Hall Textbook of Medical Physiology Eleventh Edition

The copyright of the article How the Digestive System Works in General Medicine is owned by Noreen Kassem. Permission to republish How the Digestive System Works in print or online must be granted by the author in writing.

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