Digestive System Health Article

Absorption and elimination in the large intestine

The large intestine, or colon, is wider and heavier then the small intestine, but much shorter—only about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.

Liver

The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the broken-down products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.

Gallbladder

The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gallbladder is a small hollow organ; its main function is to store bile until it is concentrated enough to be used by the small intestine. The gallbladder can store about 2 oz of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids.

Appendix

The appendix is a hollow finger-like projection that hangs from the cecum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood, which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.

Pancreas

When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices.

Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system, rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells, and allows them to take excess sugar from the blood and store it in the form of glycogen.

KEY TERMS

Amylase—A digestive enzyme found in saliva and the pancreas that breaks down carbohydrates to simple sugars.

Bile—Liquid produced in the liver and stored in the gallbladder that emulsifies fats.

Gastric juice—Digestive juice produced by the stomach wall that contains hydrochloric acid and the enzyme pepsin.

Gastrin—A hormone produced by the stomach lining in response to protein in the stomach that produces increased gastric juice.

Helicobacter pylori—Recently discovered bacteria that live in gastric acids and are believed to be a major cause of most stomach ulcers.

Lower esophageal sphincter—A strong muscle ring between the esophagus and the stomach that keeps gastric juice and even duodenal bile from flowing upwards out of the stomach.

Lymphatic system—The system that produces, transports, and filters lymph throughout the body. It also transports fats, proteins, and some vitamins to the blood system.

Mucosa—The digestive lining of the intestines.

Nutrients—Vitamins, minerals, proteins, lipids, and carbohydrates needed by the body.

Peristalsis—The wavelike motion of the digestive system that moves food through the digestive system.

Villi—Fingerlike projections found in the small intestine that add to the absorptive area for the passage of digested food to the bloodstream and lymphatic system.