8 The Digestive System
Follow food through the digestive tract to see how digestion, absorption, and elimination work together.
The and its supporting organs
The digestive system receives food, breaks it into absorbable molecules, transfers nutrients and water into the body, and removes undigested material. Its is a continuous passage from mouth to anus. assist by chewing food or adding secretions, but food does not pass through the liver, pancreas, or gallbladder.
How food moves through the tract
The organs of the act in sequence:
Mouth: Teeth mechanically break food into smaller pieces. The tongue mixes food with saliva and helps form a swallowable mass, or . Salivary enzymes begin carbohydrate .
Pharynx and esophagus: Swallowing moves the through the pharynx into the esophagus. Wave-like muscular contractions called propel it toward the stomach.
Stomach: Muscular contractions churn food and mix it with gastric juice. Acid and the enzyme pepsin begin protein . The resulting semi-liquid mixture is called .
Small intestine: The duodenum receives , , and pancreatic secretions. is completed and most nutrient occurs here. The jejunum and ileum continue .
Large intestine: The cecum and colon receive material remaining after most nutrients have been absorbed. The colon absorbs water and electrolytes, and gut microbes act on some undigested material. The rectum stores stool before it leaves through the anus. The appendix is a small pouch attached to the cecum.
Secretions from
The salivary glands release saliva into the mouth. The liver produces , which helps disperse large fat droplets into smaller ones; is not a digestive enzyme. The gallbladder stores and concentrates , then releases it into the small intestine. The pancreas sends digestive enzymes and bicarbonate-rich fluid into the small intestine. Bicarbonate helps neutralize acidic from the stomach.
Teeth and the tongue also assist , although food passes through the mouth rather than through them as separate organs.
and movement
includes both mechanical and chemical breakdown. Chewing, stomach churning, and intestinal mixing are forms of mechanical . Chemical uses enzymes and other substances to turn large food molecules into smaller units that can cross the intestinal lining.
Carbohydrates are broken down into simple sugars, such as glucose. begins in the mouth and continues in the small intestine.
Proteins are broken into amino acids and small peptides. Their begins in the stomach and continues in the small intestine.
Fats are dispersed by and broken down mainly by pancreatic enzymes in the small intestine into fatty acids and related smaller molecules.
moves contents forward, while mixing contractions expose them to digestive juices and the absorptive surface. Nervous signals and hormones help coordinate movements and secretions.
How nutrients are absorbed
moves digested nutrients, water, and electrolytes from the into blood or lymph. Folds, villi, and microscopic microvilli give the small intestine a large surface area, making it especially effective at .
Most sugars, amino acids, water-soluble vitamins, minerals, and other water-soluble nutrients enter blood vessels in the intestinal wall. Blood from much of the carries absorbed nutrients first to the liver, where they can be processed or stored. Many products of fat enter intestinal cells, are packaged for transport, and pass into lymphatic vessels called before reaching the bloodstream. The large intestine absorbs additional water and electrolytes as the remaining contents become stool.
For example, a piece of bread is chewed and mixed with saliva, and its starch is digested into sugars, chiefly in the small intestine. Those sugars cross the intestinal lining into blood and travel to the liver. Material that is not digested or absorbed continues into the large intestine.
of waste
, or defecation, removes stool from the body. Stool consists largely of water and material that was not digested or absorbed, along with bacteria and other intestinal contents.
As stool enters the rectum, stretching helps trigger the defecation reflex. Coordinated contractions move stool toward the anus, while sphincter muscles regulate its release. makes useful substances available to the body, while expels the remaining waste.