The Gastrointestinal Tract The process of breaking down food into molecules the body can use is called digestion. Digestion occurs in the gastrointestinal tract, or digestive tract, which is a long tube which begins at the mouth and winds through the body to the anus. Organs next to the digestive tract also aid in the digestion of food through the secretions that they produce.

The Mouth and Esophagus Mechanical digestion begins when the teeth cut and grind food, and the tongue keeps it there

The Mouth Mouth Mechanical digestion begins when the teeth cut and grind food, and the tongue keeps it there Saliva is produced by the salivary glands to begin chemical digestion of food. Salivary amylase is the chemical in saliva that begins the chemical digestion of carbohydrates.

The Mouth and Esophagus The esophagus has two muscle layers and by alternating contractions between these muscle layers, the ball of food can be pushed into the stomach. This series of alternating rhythmic muscular contractions and relaxation is called peristalsis.

Stomach The stomach is an organ involved in both mechanical and chemical digestion. The stomach has three layers of smooth muscle which churn the food. The inner lining of the stomach contains openings called gastric pits which release acidic secretions into the stomach. Hydrochloric acid helps dissolve food and minerals and keep bacteria at bay.

Stomach Formation of Chyme Peristalsis forces the newly formed chyme into the small intestine from the stomach. The pyloric sphincter controls the amount of chyme that enters the small intestine from the stomach. Digestion in the stomach is also aided by other organs near the stomach.

The Liver, Gallbladder, and Pancreas The other organs that help digestion are the liver, gallbladder, and pancreas.

The Liver, Gallbladder, and Pancreas The liver stores glucose (in the form of glycogen), makes proteins, and breaks down toxic substances. The liver produces bile, which is vital to digesting fats.

The Liver, Gallbladder, and Pancreas After the liver produces bile it travels through a Y-shaped duct to the gallbladder, where the bile is stored. The gallbladder releases the bile through a common bile duct into the small intestine.

The Liver, Gallbladder, and Pancreas increases the pH of the chyme by producing sodium bicarbonate. secretes enzymes that help to further break down macromolecules

Small Intestine Absorption Absorption occurs mostly in the small intestine. Absorption occurs when the end products of digestion are transferred into the circulatory system.

Small Intestine Absorption The surface area of the small intestine is large due to the presence of many folds within the lining of the small intestine and the millions of villi. The surface area is further increased by the presence of microvilli on each villus

Cross Section of the Small Intestine Section 2 Digestive System Chapter 48 Cross Section of the Small Intestine

Large Intestine Peristalsis will move the remaining contents into the colon or large intestine. The colon finishes the absorption of some nutrients, but mostly water.

Large Intestine As nutrients and water are absorbed from the matter in the colon, the matter solidifies into feces. Feces leave the body by passing through the rectum and the anal canal.