The Digestive System -Describe the organs of the digestive system and explain their functions. -Explain what happens during digestion. -Describe how nutrients are absorbed into the bloodstream and wastes are eliminated from the body.

The four steps of Digestion: Ingestion Digestion Absorption Elimination

How Digestion Proceeds through the Digestive System During digestion, food travels through the mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus.

The Mouth Chewing begins the process of mechanical digestion.   Chemical digestion begins as digestive enzymes, such as amylase, in saliva, start the breakdown of carbohydrates. Saliva also has lysozymes, enzymes that fights infection by digesting the cell walls of many bacteria that enter the mouth with food.

The Esophagus The bolus of food passes through the pharynx into a tube called the esophagus that leads to the stomach.   Contractions of smooth muscles, known as peristalsis, provide the force that moves food through the esophagus toward the stomach.

Chemical Digestion in the Stomach The stomach is a large muscular sac that continues chemical and mechanical digestion of food. Pepsin is the enzyme in the stomach that breaks down proteins into amino acids.

Digestion in the Small Intestine The small intestine secretes many enzymes, one of which is lipase, to break down lipids. Absorption of nutrients takes place in the small intestine.  

Absorption From the Small Intestine The folded surfaces of the small intestine are covered with fingerlike projections called villi.   The surfaces of the cells of the villi are covered with thousands of tiny projections known as microvilli, for absorption of nutrients. This dramatically increases the surface area of the small intestine allowing for maximum absorption.

The Large Intestine The large intestine’s function is to reabsorb water before the remaining food material is eliminated as feces through the anus.

Elimination If not enough water is absorbed by the large intestine, a condition known as diarrhea occurs.   If too much water is absorbed from the undigested materials, a condition known as constipation occurs.