Introduction: In higher organisms, food is taken in and then transported through a series of organs that make up the digestive system. It is here that the food is broken down into its component parts to release its energy and other materials for the organism to use. Most digestive systems have a series of specialized parts that each do something different to the food, but all work together for a common goal.

Starting in the mouth (A), food is broken down both mechanically by the teeth and chemically by saliva. Saliva has an enzyme in it called salivary amylase that breaks down start into smaller molecules. The tongue (B) then positions the food (now called a bolus) for swallowing. The bolus then travels through the pharynx down the esophagus (D) by a series of muscular contractions called peristalsis until it reaches the stomach (F).

In the stomach, the bolus is further broken down mechanically by the three layers of strong muscle and chemically by acids released from the stomach walls. Hydrochloric acid and an enzyme called pepsin are present in the stomach. Pepsin works to break the proteins in the food into amino acids. From the stomach, the mostly digested food leaves through an opening called the pyloric sphincter into the small intestine (J). Here, the food is further chemically digested by enzymes that are released from the pancreas and liver (E). Trypsin from the pancreas helps to break down the proteins even further and bile made in the liver but stored in the gall bladder (G) break down lipids. Additionally, the small intestine is the primary site of nutrient absorption.

From the small intestine the remainder of the food enters the large intestine (I) where water is reabsorbed. Attached to the large intestine is a structure called the appendix. While its function is somewhat of a mystery, some evidence does suggest that it is responsible for helping to clean the blood and store beneficial bacteria. Finally, after all the water has been reabsorbed, the waste products are forces by peristalsis into the last portion of the large intestine called the rectum (L) until the urge to eliminate arises. The last step is for the waste to be pushed out of the anus (M).

This is a diagram of the human digestive system. Most vertebrates (animals with backbones) have identical, or at least similar, structures.

Directions for this Lesson: Answer the practice questions and then watch this video to learn more about how digestion occurs.