The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus (see figure). Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food.

Food follows the path: mouth, esophagus, stomach, small intestine, large intestine, rectum, anus.

Two solid organs, the liver and the pancreas, produce digestive juices that reach the intestine through small tubes. In addition, parts of other organ systems (for instance, nerves and blood) play a major role in the digestive system.

Why is digestion important?

When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy.

How is food digested?

Digestion involves the mixing of food, its movement through the digestive tract, and the chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine. The chemical process varies somewhat for different kinds of food.

Movement of Food Through the System

The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.

The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves.


Mouth: Mechanical and chemical digestion begin in the mouth where food is chewed. The glands that act first are . Saliva produced by these glands contains an enzyme called ptyalin that begins to digest the starch from food into smaller molecules (maltose).

Esophagus: no digestion occurs here. The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass.

Stomach: The next set of digestive glands is in the stomach lining. This is where protein begins it digestion. The stomach lining produce stomach acid (HCl) and an enzyme called pepsin that digests protein . One of the unsolved puzzles of the digestive system is why the acid juice of the stomach does not dissolve the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.

The stomach has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.

Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and the degree of muscle action of the emptying stomach and the next organ to receive the contents (the small intestine).

Small Intestine: The small intestine is where most chemical digestion occurs. After the stomach empties the food and juice mixture into the small intestine, the juices of two other digestive organs mix with the food to continue the process of digestion. One of these organs is the pancreas. It produces a juice that contains a wide array of enzymes to break down the carbohydrate, fat, and protein in food. Other enzymes that are active in the process come from glands in the wall of the intestine or even a part of that wall.

The liver produces yet another digestive juice—bile. The bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in our food. The bile acids dissolve the fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

The three major classes of nutrients that undergo digestion in the small intestine are: proteins, lipids (fats) and carbohydrates

starting nutrient end product enzyme(s) responsible
proteins and peptides amino acids trypsin and chymotrypsin
lipids fatty acids and gylcerol Pancreatic lipase with help from bile (not an enzyme)
carbohydrates simple sugars (monsacharides) Pancreatic amylase

Digested food broken down in the small intestine is the size of molecules and can now pass through the villi into the blood stream through the process of diffusion.

Digested molecules of food, as well as water and minerals from the diet, are absorbed from the cavity of the upper small intestine. Most absorbed materials cross the mucosa into the blood and are carried off in the bloodstream to other parts of the body for storage or further chemical change. As already noted, this part of the process varies with different types of nutrients. Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food, known as fiber, and older cells that have been shed from the mucosa.

Large Intestine - Colon -- These materials are propelled into the colon, where they remain, usually for a day or two. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body. It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation. Dietary fiber, or simply called fiber, refers to plant cell wall components that are not digestible.

Note: The large intestine houses over 700 species of bacteria that perform a variety of functions. The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. Cellulose is not digested at all in the human.

Assessment Questions:

M ultiple Choice Questions

1. Where does digestion begin?
a) Digestion begins in the mouth where only mechanical digestion works on the food breaking it down.
b) Digestion begins in the mouth where mechanical digestion breaks down food, and chemical digestion begin to break down starch
c) Digestion of food does not begin till food enters the stomach
d) Food digestion only occurs in the small intestine

2. Where is food broken down to molecule size allowing it to pass through villi into the blood stream?
a) stomach
b) large intestine

c) small intestine
d) all of the above

3. Peristalsis is
a) the digestion of starch to sugar in the mouth
b) the passing of food nutrients through the villi into the blood
c) The rhythmic, wave-like contraction of the digestive tract that forces food through it
d) the breaking down of fat by bile

4. Which statement is not true.
a) Dietry fiber, or simply clled fiber, refers to plnt cell wll components tht re not digestible
b) The stomach contains Hydrochloric acid (HCl)
c) The large intestine removes excess water
d) An example of a complex sugar is glucose

5. Cellulose is digested by human enzymes into starch in which organ?
a) mouth
b) stomach
c) small intestine
Humans do not have an enzyme that can digest cellulose.






Source: National Digestive Diseases Information Clearinghouse

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