Chapter 35 Notes, The Digestive System

The Digestive System The function of the digestive system is to ingest food, break it down so that nutrients can be absorbed, and eliminate the waste. The first part of digestion is mechanical digestion. Mechanical digestion is when food is chewed and broken down into smaller pieces by the teeth, tongue, and by smooth muscles of the stomach and small intestine. The second part, called chemical digestion is accomplished by action of enzymes like amylase found in the mouth.

The Digestive System After food is chewed in the mouth, it moves past the pharynx, into the esophagus. The esophagus is a long tube made of smooth muscle that leads to the stomach. Food moves through the esophagus by peristalsis, the muscular contractions made by the smooth muscles of the esophagus. After food goes through the esophagus, it passes through a circular muscle called the cardiac sphincter into the stomach.

Peristalsis, The Esophagus and the Cardiac Sphincter

The Digestive System When the cardiac sphincter muscles relax, stomach acid can leak into the esophagus causing heartburn. The acidic environment of the stomach is favorable to an enzyme in the stomach called pepsin. Pepsin is an enzyme in the stomach that breaks proteins down into amino acids. Cells in the stomach secrete a protective mucus that prevents acid and pepsin from digesting the walls of the stomach.

The Digestive System After the food is broken down by chemical digestion in the stomach, it is released by the pyloric sphincter into the small intestine. The small intestine is the longest part of the digestive system. The human small intestine is about 7 meters (over 20 ft.) long! The small intestine is divided into three different regions; the duodenum, jejunum, and ileum. The smooth muscles of the small intestine move the food by peristalsis.

Anatomy of the Stomach

Small Intestine

Digestive System Once food is in the small intestine, some other organs assist chemical digestion. The pancreas produces enzymes that digest carbohydrates, proteins, and fats. The pancreas also secretes hormones like insulin and helps regulate pH. Liver produces bile, which breaks down fats, and the bile is stored in the gall bladder. The liver is the largest internal organ and it performs a variety of functions including removing toxic substances from the body.

The Liver, The Gall bladder, and The Pancreas

Digestive System After chemical digestion is complete, absorption of the nutrients into the bloodstream takes place in the small intestine. Small fingerlike structures called villi line the walls of the small intestine. Villi increase the surface area. The villi are filled with capillaries. The villi increase the surface area of the small intestine to give it about the same amount of surface area as a tennis court!

Small Intestine

Digestive System Once digestion is complete, the food, now called chyme, moves into the large intestine. The large intestine includes the colon, the rectum, and the appendix. The function of the appendix is not fully understood, although animals that eat vegetation, have a similar organ called a cecum that is used for digesting vegetable matter. The function of the colon is to absorb water from the chyme. After water is absorbed from the chyme, feces is formed and removed through the rectum.

Anatomy of the Large Intestine

Comparative Anatomy of Digestive Systems in Mammals

Complete Overview of the Digestive System