Digestive System

Digestive System, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body’s cells, which use them for growth, repair, and energy.

All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals.

Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes break apart individual molecules of food to yield molecules that can be absorbed and distributed throughout the body. These enzymes are secreted (produced and released) by glands in the body.

The digestive system of most animals consists mainly of a long, continuous tube called the alimentary canal, or digestive tract. This canal has a mouth at one end, through which food is taken in, and an anus at the other end, through which digestive wastes are excreted. Muscles in the walls of the alimentary canal move the food along. Most digestive organs are part of the alimentary canal. However, two accessory digestive organs, the liver and pancreas, are located outside the alimentary canal. These organs contribute to chemical digestion by releasing digestive juices into the canal through tubes called ducts.

The simplest invertebrates (animals without backbones) do not have specialized digestive organs. Single-celled organisms, such as amoebas, rely on intracellular digestion (digestion within the cell). Some many-celled organisms, such as the sponge, also use intracellular digestion. The sponge obtains the tiny organic particles that make up its diet from water passing through its body. Water enters through the sponge’s pores and leaves through an opening called the osculum. As water flows through the interior canals of the sponge, specialized cells that line these canals, called collar cells, catch and engulf organic matter. Inside the collar cells, sacs called vacuoles form around the food and enzymes digest it. The digested food then passes to other cells in the sponge’s body.

Intracellular digestion meets the needs of simple animals, but more complex organisms require systems that are more specialized. Animals such as jellyfish and nonparasitic flatworms combine the intracellular process with some specialized digestive organs. These animals have a definite mouth and a saclike cavity, which is lined with digestive cells that secrete enzymes. Digestion begins when the enzymes break down food inside the cavity in an extracellular (outside the cell) process. Cells then engulf the partly digested food, and an intracellular process similar to that of sponges completes digestion. Wastes are excreted through the mouth.

Most of the more complex invertebrates and all vertebrates (animals with a backbone) digest food entirely through extracellular processes. Food moves in one direction, from mouth to anus, through the series of organs that make up the alimentary canal. Specialization of various parts of the alimentary canal improves the body’s ability to break down food and absorb various kinds of nutrients. The mouth of many animals contains teeth or other structures to break up large lumps of food. Behind the mouth, the pharynx and esophagus swallow the food and move it to the stomach. The stomach temporarily stores the food, mixes it with digestive juices, and carries out some digestion.

Digestion is completed in the intestine. The liver and pancreas pour their digestive juices into the anterior (front) end of this organ. After the anterior intestine absorbs the usable products of digestion, the walls of the posterior (rear) intestine absorb leftover water. In vertebrates the anterior intestine is called the small intestine; the posterior intestine is the large intestine. Feces, composed of unabsorbed and indigestible food residues, form in the posterior intestine, where they are stored until they are excreted through the anus.

Within this basic plan, the specific components of the digestive system vary enormously from one animal to another. For example, a fish’s pharynx contains gill slits for breathing but has no digestive function. An earthworm’s stomach consists of two organs: a crop, in which food is stored, and a muscular gizzard, which carries out mechanical digestion by grinding food against particles of sand. The stomachs of ruminant mammals, such as cattle and deer, consist of three or four compartments, each performing a specific function. Amphibians, reptiles, and birds have an organ called a cloaca, which serves as an exit for both digestive wastes and sex cells.

If a human adult’s digestive tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans, digestion begins in the mouth, where both mechanical and chemical digestion occur. The mouth quickly converts food into a soft, moist mass. The muscular tongue pushes the food against the teeth, which cut, chop, and grind the food. Glands in the cheek linings secrete mucus, which lubricates the food, making it easier to chew and swallow. Three pairs of glands empty saliva into the mouth through ducts to moisten the food. Saliva contains the enzyme ptyalin, which begins to hydrolyze (break down) starch—a carbohydrate manufactured by green plants.

Once food has been reduced to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called a bolus—to the back of the mouth and into the pharynx. This cavity between the mouth and windpipe serves as a passageway both for food on its way down the alimentary canal and for air passing into the windpipe. The epiglottis, a flap of cartilage, covers the trachea (windpipe) when a person swallows. This action of the epiglottis prevents choking by directing food from the windpipe and toward the stomach.

The presence of food in the pharynx stimulates swallowing, which squeezes the food into the esophagus. The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the trachea and heart and penetrates the diaphragm (muscular wall between the chest and abdomen) before reaching the stomach. Food advances through the alimentary canal by means of rhythmic muscle contractions (tightenings) known as peristalsis. The process begins when circular muscles in the esophagus wall contract and relax (widen) one after the other, squeezing food downward toward the stomach. Food travels the length of the esophagus in two to three seconds.

A circular muscle called the esophageal sphincter separates the esophagus and the stomach. As food is swallowed, this muscle relaxes, forming an opening through which the food can pass into the stomach. Then the muscle contracts, closing the opening to prevent food from moving back into the esophagus. The esophageal sphincter is the first of several such muscles along the alimentary canal. These muscles act as valves to regulate the passage of food and keep it from moving backward.

Most digestion, as well as absorption of digested food, occurs in the small intestine. This narrow, twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lower abdomen, extending about 6 m (20 ft) in length. Over a period of three to six hours, peristalsis moves chyme through the duodenum into the next portion of the small intestine, the jejunum, and finally into the ileum, the last section of the small intestine. During this time, the liver secretes bile into the small intestine through the bile duct. Bile breaks large fat globules into small droplets, which enzymes in the small intestine can act upon. Pancreatic juice, secreted by the pancreas, enters the small intestine through the pancreatic duct. Pancreatic juice contains enzymes that break down sugars and starches into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. Glands in the intestinal walls secrete additional enzymes that break down starches and complex sugars into nutrients that the intestine absorbs. Structures called Brunner’s glands secrete mucus to protect the intestinal walls from the acid effects of digestive juices.

The small intestine’s capacity for absorption is increased by millions of fingerlike projections called villi, which line the inner walls of the small intestine. Each villus is about 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a single layer of cells. Even tinier fingerlike projections called microvilli cover the cell surfaces. This combination of villi and microvilli increases the surface area of the small intestine’s lining by about 150 times, multiplying its capacity for absorption. Beneath the villi’s single layer of cells are capillaries (tiny vessels) of the bloodstream and the lymphatic system. These capillaries allow nutrients produced by digestion to travel to the cells of the body. Simple sugars and amino acids pass through the capillaries to enter the bloodstream. Fatty acids and glycerol pass through to the lymphatic system.

A watery residue of indigestible food and digestive juices remains unabsorbed. This residue leaves the ileum of the small intestine and moves by peristalsis into the large intestine, where it spends 12 to 24 hours. The large intestine forms an inverted U over the coils of the small intestine. It starts on the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is 1.5 to 1.8 m (5 to 6 ft) long and about 6 cm (2.5 in) in diameter.

The large intestine serves several important functions. It absorbs water—about 6 liters (1.6 gallons) daily—as well as dissolved salts from the residue passed on by the small intestine. In addition, bacteria in the large intestine promote the breakdown of undigested materials and make several vitamins, notably vitamin K, which the body needs for blood clotting. The large intestine moves its remaining contents toward the rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary canal. The rectum stores the feces—waste material that consists largely of undigested food, digestive juices, bacteria, and mucus—until elimination. Then, muscle contractions in the walls of the rectum push the feces toward the anus. When sphincters between the rectum and anus relax, the feces pass out of the body.

Infection of or damage to any part of the digestive system may affect digestion as well as other bodily functions. Common infectious agents that attack digestive organs include the mumps virus, which often infects the salivary glands; the bacterium Helicobacter pylori, which causes most stomach and duodenal ulcers; and viruses and bacteria that cause various forms of gastroenteritis, often called stomach flu or traveler’s diarrhea. Appendicitis is an inflammation of the appendix, a tube-like pouch about 9 cm (3.5 in) long that branches off the large intestine. It occurs most commonly among children and young adults. Diarrhea—frequent elimination of loose, watery feces—is a symptom of many disorders that occurs when the large intestine is irritated or inflamed. As a result, food residues move through it too quickly for it to absorb the excess water. The opposite condition, constipation, occurs when the large intestine absorbs too much water because food residues are moving slowly. As a result, the feces become hard and dry, which may make elimination difficult.

Cancerous tumors may develop in any part of the digestive system, though they most commonly occur in the large intestine, rectum, and anus (see Colorectal Cancer). Colitis, which has various causes, is a potentially life-threatening inflammation of the large intestine (see Ulcerative Colitis). Chronic conditions that cause at least intermittent distress include irritable bowel syndrome, caused by spasms of muscles in the lower intestine, and Crohn’s disease, an inflammation of the intestines. Abnormal sensitivity to proteins called glutens can damage the lining of the small intestine and hinder absorption of nutrients, leading to malnutrition and other problems. The eating disorders anorexia nervosa and bulimia disrupt the normal functioning of the digestive system and are potentially fatal.