Your Digestive System and How It Works

[Guest guest]

Your Digestive System and How It Works

_http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/)

On this page:

* _Why is digestion important?_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#1)

* _How is food digested?_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#2)

* _How is the digestive process controlled?_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#3)

The digestive system is a series of hollow organs joined in a long, twisting

tube from the mouth to the anus (_see figure_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#fig) ). 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.

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.[_Top_ (http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#top) ]

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.[_Top_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#top) ]

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.

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.

The food then enters the stomach, which 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). As the food is digested in the small intestine and dissolved into

the juices from the pancreas, liver, and intestine, the contents of the

intestine are mixed and pushed forward to allow further digestion.

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.

These

materials are propelled into the colon, where they remain, usually for a day

or two, until the feces are expelled by a bowel movement.

Production of Digestive Juices

The glands that act first are in the mouth—the salivary glands. Saliva

produced by these glands contains an enzyme that begins to digest the starch

from

food into smaller molecules.

The next set of digestive glands is in the stomach lining. They produce

stomach acid and an enzyme 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.

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.

Absorption and Transport of Nutrients

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.

Carbohydrates. It is recommended that about 55 to 60 percent of total daily

calories be from carbohydrates. Some of our most common foods contain mostly

carbohydrates. Examples are bread, potatoes, legumes, rice, spaghetti,

fruits, and vegetables. Many of these foods contain both starch and fiber.

The digestible carbohydrates are broken into simpler molecules by enzymes in

the saliva, in juice produced by the pancreas, and in the lining of the

small intestine. Starch is digested in two steps: First, an enzyme in the

saliva

and pancreatic juice breaks the starch into molecules called maltose; then an

enzyme in the lining of the small intestine (maltase) splits the maltose

into glucose molecules that can be absorbed into the blood. Glucose is carried

through the bloodstream to the liver, where it is stored or used to provide

energy for the work of the body.

Table sugar is another carbohydrate that must be digested to be useful. An

enzyme in the lining of the small intestine digests table sugar into glucose

and fructose, each of which can be absorbed from the intestinal cavity into

the blood. Milk contains yet another type of sugar, lactose, which is changed

into absorbable molecules by an enzyme called lactase, also found in the

intestinal lining.

Protein. Foods such as meat, eggs, and beans consist of giant molecules of

protein that must be digested by enzymes before they can be used to build and

repair body tissues. An enzyme in the juice of the stomach starts the

digestion of swallowed protein. Further digestion of the protein is completed

in the

small intestine. Here, several enzymes from the pancreatic juice and the

lining of the intestine carry out the breakdown of huge protein molecules into

small molecules called amino acids. These small molecules can be absorbed from

the hollow of the small intestine into the blood and then be carried to all

parts of the body to build the walls and other parts of cells.

Fats. Fat molecules are a rich source of energy for the body. The first step

in digestion of a fat such as butter is to dissolve it into the watery

content of the intestinal cavity. The bile acids produced by the liver act as

natural detergents to dissolve fat in water and allow the enzymes to break the

large fat molecules into smaller molecules, some of which are fatty acids and

cholesterol. The bile acids combine with the fatty acids and cholesterol and

help these molecules to move into the cells of the mucosa. In these cells the

small molecules are formed back into large molecules, most of which pass into

vessels (called lymphatics) near the intestine. These small vessels carry

the reformed fat to the veins of the chest, and the blood carries the fat to

storage depots in different parts of the body.

Vitamins. Another vital part of our food that is absorbed from the small

intestine is the class of chemicals we call vitamins. The two different types

of

vitamins are classified by the fluid in which they can be dissolved:

water-soluble vitamins (all the B vitamins and vitamin C) and fat-soluble

vitamins

(vitamins A, D, and K).

Water and salt. Most of the material absorbed from the cavity of the small

intestine is water in which salt is dissolved. The salt and water come from

the food and liquid we swallow and the juices secreted by the many digestive

glands.[_Top_ (http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#top) ]

How is the digestive process controlled?

Hormone Regulators

A fascinating feature of the digestive system is that it contains its own

regulators. The major hormones that control the functions of the digestive

system are produced and released by cells in the mucosa of the stomach and

small

intestine. These hormones are released into the blood of the digestive tract,

travel back to the heart and through the arteries, and return to the

digestive system, where they stimulate digestive juices and cause organ

movement.

The hormones that control digestion are gastrin, secretin, and

cholecystokinin (CCK):

* Gastrin causes the stomach to produce an acid for dissolving and

digesting some foods. It is also necessary for the normal growth of the lining

of the stomach, small intestine, and colon.

* Secretin causes the pancreas to send out a digestive juice that is

rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme

that digests protein, and it also stimulates the liver to produce bile.

* CCK causes the pancreas to grow and to produce the enzymes of

pancreatic juice, and it causes the gallbladder to empty.

Additional hormones in the digestive system regulate appetite:

* Ghrelin is produced in the stomach and upper intestine in the

absence of food in the digestive system and stimulates appetite.

* Peptide YY is produced in the GI tract in response to a meal in the

system and inhibits appetite.

Both of these hormones work on the brain to help regulate the intake of food

for energy.

Nerve Regulators

Two types of nerves help to control the action of the digestive system.

Extrinsic (outside) nerves come to the digestive organs from the unconscious

part

of the brain or from the spinal cord. They release a chemical called

acetylcholine and another called adrenaline. Acetylcholine causes the muscle of

the

digestive organs to squeeze with more force and increase the " push " of food

and juice through the digestive tract. Acetylcholine also causes the stomach

and pancreas to produce more digestive juice. Adrenaline relaxes the muscle of

the stomach and intestine and decreases the flow of blood to these organs.

Even more important, though, are the intrinsic (inside) nerves, which make

up a very dense network embedded in the walls of the esophagus, stomach, small

intestine, and colon. The intrinsic nerves are triggered to act when the

walls of the hollow organs are stretched by food. They release many different

substances that speed up or delay the movement of food and the production of

juices by the digestive organs.[_Top_

(http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/#top) ]

____________________________________

National Digestive Diseases Information Clearinghouse

2 Information Way

Bethesda, MD 20892–3570

Email: _nddic@..._

(http://digestive.niddk.nih.gov/about/contact.htm)

No information in this letter should be construed as medical advice.

This information is for educational purposes only.

Jeff el

10360 Pine Lakes Blvd

North Fort Myers, Fl 33903

http://www.msprotocols.com/