The Human Digestive System

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Diagram of the Human Digestive System
Diagram of the Human Digestive System
*Accessory Organs of Digestion


The Human Digestive System is basically made up of two parts: the digestive tract and the accessory organs of digestion. Without the organs of digestion that secrete important chemicals and enzymes which act upon the ingested food, the digestive tract, measuring about 25 - 30 Feet would probably be unable to perform the functions of digestion and absorption.


The mouth is the first organ of digestion. It contains structures such as the teeth, tongue and palate which help to chew the food and break it down into smaller pieces that are easy to swallow. Also, the smaller pieces have an increased surface area which makes the action of digestive juices/enzymes faster. In addition to the structures that aid in food mastication, the mouth also contains salivary glands.


The major salivary glands in the mouth are the parotid glands, the submandibular glands, and the sublingual glands. These glands secrete a substance called saliva that helps in mixing the food that is being chewed, thus forming a bolus that can be easily swallowed. The saliva contains two enzymes know as the amylase and the lipase. These enzymes act on certain carbohydrates and fats respectively, converting them into simple sugars and lipids.


Food (bolus) passes from the mouth into the esophagus through the pharynx by deglutition reflex. The esophagus is a muscular tube measuring 25cm in length. The muscles in the lower region of the tube are stronger and they give rise to the lower esophageal sphincter which allows a one way passage of food from the esophagus to the stomach. The sphincter remains constricted to prevent the stomach contents from moving into the esophagus. When the muscles in this region lose their tone (possibly due to excessive smoking and alcohol), it leads to a condition known as Gastro Esophageal Reflux Disease (GERD). Food travels from the esophagus to the stomach by peristalsis.


The stomach is a large "J" and shaped organ that is made up of four layers: the mucosa, the submucosa, the muscularis layer, and the serosa. The stomach secretes Hydrochloric acid (HCl) and Sodium Chloride (NaCl). Because these substances can damage the walls of the stomach, the gastric glands secrete mucous to coat the stomach walls. Pepsinogen, an enzyme that is converted to pepsin which acts on proteins, is also secreted in the stomach. Also, bicarbonate ions (HCO3-) are secreted to provide a tight cell juncture with HCl.
The stomach is maintained at a low pH and hence the semi-liquid food substances (know as Chyme) is usually very acidic. The acidic Chyme is moved from the stomach into the small intestine after an hour by muscular contractions know as peristalsis. The lower end of the stomach has a sphincter know as the pylori sphincter which opens to allow the passage of Chyme into the small intestine.


The small intestine which is about 5 meters in length and it is the major part of the digestive system where digestion and absorption take place. It is divided into three parts: the duodenum, the jejunum, and the ileum. The duodenum is the first part of the small intestine which is 25cm in length and receives the acidic Chyme that is passed from the stomach through the pylori sphincter. In order to prevent the damage of the duodenum, the walls secrete bicarbonate to dilute the acidic Chyme. Digestive substances and enzymes are secreted into the duodenum from the liver, the gall bladder, and the exocrine part of the pancreas.


Biles, which is secreted by the liver and is stored in the gall bladder, helps to emulsify fats into micelles that can be absorbed by the intestinal walls. The pancreas secretes chymotrypsinogen (which is converted to chymotrypsin), trypsinogen (which is converted to trypsin), amylase, and lipase. Chymotrypsin and trypsin act on proteins and convert them to peptides and amino acids. Lipase acts on fats and convert them to fatty acids and glycerol. Amylase act on carbohydrates and convert them to glucose. As the Chyme is moved along the small intestine by segmental contractions, these breakdown processes take place over a period of several hours.


The walls of the small intestine secrete several brush border enzymes such as dextrinase, glucoamylase, maltase, sucrase, and lactase, all of which assist in the breakdown of carbohydrates. Also, the small intestinal walls are made up of villi and microvilli, which increase their surface area for absorption. Following the absorption of nutrients by the walls of the small intestine into the blood vessels, the semi-solid Chyme is passed from the small intestine through the ileum into the cecum, the first part of the large intestine.


The large intestine, measuring about 1.5m long, is divided into the ascending colon, the transverse colon, and the descending colon. The descending colon extends into the sigmoid colon which further extends into the rectum and finally the anal canal. In the large intestine, absorption of water and other nutrients take place, leaving the undigested materials solid which is compacted into stool and stored in the rectum until it is excreted out of the body through the anus.


The large intestine contains over 700 species of gut bacteria (weighing about 200g) and are capable of making important substances such propionate, butyrate, etc., for the nourishment of the cells of the large intestine. Vitamin K, and other blood clothing factors are also produced in the large intestine.

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