Digestive System

I

Introduction

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.

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II

Digestive Systems of Animals

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.

III

The Human Digestive System

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.

A

The Esophagus

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.

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