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Digestion is the process by which food and drink are broken down into

their smallest parts so the body can use them to build and nourish cells and to

provide energy.

ANATOMY OF THE DIGESTIVE SYSTEM

The human digestive system can be classified into two basic sections,

namely the upper gastrointestinal tract and the lower gastrointestinal tract. These

two tracts are made up of different organs of the digestive system. In addition to

the organs of the digestive system in the upper and lower gastrointestinal tracts,

there are several other different organs that act as auxiliaries to the process of

digestion. The auxiliary organs also help the human body to maintain good

digestive health.

A. Upper Gastrointestinal Tract

The function of the upper gastrointestinal tract, is simplification of food into

nutrients that can be assimilated by the lower gastrointestinal tract into the

human body. The funny fact about the human digestive system is that the

process of digestion actually starts, before the food has entered the digestive
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tract. The four primary organs ear, nose, eyes and tongue, sense the presence of

food and thereby alert other organs of the digestive system that begin the

production of different digestive juices. Following are the organs of the digestive

system that are involved in the breaking down of food.

Mouth: The first organ that directly contributes to the digestion process is the

mouth. The mouth, is further divided into three basics organs, namely the

salivary glands, tongue and teeth. The salivary glands produce saliva that exits

into the mouth. The saliva acts as a lubrication for the food products, when they

are being chewed. Some of the micro-organisms, like fungi and bacteria, are

killed by the saliva in the mouth itself, due to its disinfecting properties. Apart

from producing saliva, the salivary glands also produces amylase, which is an

enzyme that plays a very important role in breaking down starch into glucose.

The next important part of the mouth is the tongue. The tongue is not just

used for tasting delicious food. It also helps in chewing and swallowing, which is

also known as deglutition. This process is basically, the physical simplification of

food.

Just like the tongue, the teeth are also responsible for physical breakage

of food. The action of chewing breaks the food into smaller pieces and helps the

rest of the digestive system to break down the food. Though, the teeth do not

play an active role in the chemical simplification of food, chewing makes

digestion easier for the other organs of the digestive system to break down the

food and its nutrients.

Pharynx and Esophagus: The next digestive organ is the pharynx, which lies

behind the mouth or the buccal cavity. The pharynx prevents the food from

entering the voice box orlarynx. Instead, the pharynx diverts the food to the

esophagus, a muscular tube that connects the mouth to the stomach. Though,

the pharynx and esophagus are not directly related to the actual simplification of


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food, their function of conveyance of food is extremely important among the

organs of the digestive system in the order of the gastrointestinal tract.

Stomach: The organ of the human body that conducts the mammoth task of

breaking food is the stomach. The stomach is divided into four parts. The cardia

is the receiver of food from the esophagus. This food is handed over to the

curvature of the organ that is known as the fundus. The food is later transported

to the corpus body, the central part of the stomach which contributes to the

breakage of food. After the food has been simplified, it is transferred to the

antrum which conveys it to the smaller intestine. Within the stomach, the food is

broken down into simpler nutrients, like vitamins, carbohydrates, proteins etc.

B. Lower Gastrointestinal Tract

The lower gastrointestinal tract compromises two primary organs, namely,

the intestines or the bowels and the anus. Among the digestive system organs

and functions, the organs of the lower gastrointestinal tract help the body to

assimilate the nutrients, which have been simplified by the upper gastrointestinal

tract.

Small Intestine: There are two primary intestines, namely the small intestine and

the large intestine. The small intestine is further divided into three parts. The

duodenum is the receiver of simplified food and is connected to the lower section

of the stomach. Though, the duodenum is the shortest part of the intestines, a lot

of chemical digestion takes place in it. The duodenum is also the place where the

digestive juices that are generated by the pancreas and liver mix for further

digestion. The second part of the small intestine is thejejunum. One of the most

important functions of this organ is abrogation of nutrients. The jejunum is the

mid-section of the smaller intestine and coveys the remainder of the food to the

ileum. The ileum absorbs the nutrients that have been missed by the jejunum.

Most of the vitamins are absorbed by the ileum.
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Large Intestine: Like the small intestine, the large intestine is divided into three

parts. The first organ of the large intestine is the cecum. The cecum is attached

to the appendix and is also the connecting pouch between the small and the

large intestines. The colon comes after the cecum and is responsible for the

abrogation of water and salts from the digested foods. This is considered to be

the last stage of digestion. The third part of the large intestine is the rectum,

which acts as the connection between the intestines and anus.

Anus: The anus is among the organs of the digestive system that does the job of

ejecting the waste matter from the body.

C. Auxiliary Organs

The human body has some very important organs in the digestive system

which can be classified as the auxiliary organs. Without these auxiliary organs,

the process of digestion would become almost impossible.

Gallbladder: The gallbladder is a very important organ that is responsible for the

storage of bile that has been produced by the liver. Though, in spite of being an

important part of the digestive process, the gallbladder is a non-vital organ,meaning that it is removed in case it gets infected.

Liver: The liver is one of the most important organs of the human body, as it is

also necessary for survival. The liver basically performs the task of producing

digestive juices, biochemicals and also helps in protein synthesis. The liver is

also responsible for detoxification of the food that comes in.

The whole process of digestion requires about 24 to 72 hours and an average

person's digestive tract, along with all the organs of the digestive system

measures about 9 meters long.


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SUMMARY OF THE ANATOMY OF THE DIGESTIVE SYSTEM

Digestive

System

Organ

Secretions Function

Mouth/Oral

Cavity

Salivary Amylase

secreted by the 3

salivary glands -

parotid,

submandibular and

sublingual.

We chew food to break them down into smaller

pieces. This provides greater surface area for the

enzymes released by the salivary glands to break

down carbohydrates in the food.

Oesophagus None

The esophagus does not have any digestive role. It

just helps the food to be pushed down from the

mouth down to the next organ, which is the stomach.

This it does by the process ofperistalsis.

Stomach Gastric acid, pepsin

(enzyme) and other

digestive enzymes

The muscles of the stomach churn the food and mix it

with gastric juice and enzymes released into the

stomach. The gastric juice maintains the acidic pH,

so that digestion in the stomach can take place.

Pepsin breaks down proteins (into amino acids). The

other enzymes, that is, gastric amylase and gastric


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lipase, further breakdown carbohydrates and fats

respectively.

Small

Intestine

Bile and pancreatic

enzymes secreted by

the pancreas,

intestinal enzymes.

It is in the small intestine that digestion is completed

and all the nutrients (present in their simplest form)

are absorbed by blood through the walls of the small

intestine. Bile secreted by the pancreas emulsifies

fat, so that it can be absorbed. The pancreatic and

intestinal enzymes complete the final stage of

digestion of proteins, fats and carbohydrates.

Large

Intestine/Colo

n

None

The large intestine is mainly involved with the

absorption of water and electrolytes from the

undigested substance, that reaches it from the small

intestine. It also stores this undigested material till it is

ready to be excreted from the body through the

rectum and the anus.

Digestive System Facts

Here are some facts about the digestive system:

We produce almost one quart of saliva every day.

Saliva is 98% water and only 2% enzymes.

Length of the digestive tract is about 30 feet.

Food stays in the stomach for almost 2 to 3 hours.

The small intestine is almost 20 feet long.

Food takes almost 4 hours to travel through the small intestine.

The large intestine measures up to 5 feet in length.

We excrete solid waste material through the anus, when the sphincter

muscles lining it, relax.

Liver and pancreas are not a part of the digestive tract but they help in

digestion by secreting powerful enzymes.
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PHYSIOLOGY OF DIGESTIVE SYSTEM

Digestion involves mixing food with digestive juices, moving it through the

digestive tract, and breaking down large molecules of food into smaller

molecules. Digestion begins in the mouth, when you chew and swallow, and is

completed in the small intestine.

Movement of Food through the System


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The large, hollow organs of the digestive tract contain a layer of muscle that

enables their walls to move. The movement of organ walls can propel food and

liquid through the system and also can mix the contents within each organ. Food

moves from one organ to the next through muscle action called peristalsis.

Peristalsis looks like an ocean wave traveling through the muscle. The muscle of

the organ contracts to create 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 you are able to start swallowing by choice, once the swallow begins, it

becomes involuntary and proceeds under the control of the nerves.


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Swallowed food is pushed into the esophagus, which connects the throat above

with the stomach below. At the junction of the esophagus and stomach, there is a

ringlike muscle, called the lower esophageal sphincter, closing the passage

between the two organs. As food approaches the closed sphincter, the sphincter

relaxes and allows the food to pass through to the stomach.

The stomach has three mechanical tasks. First, it stores the swallowed food and

liquid. To do this, the muscle of the upper part of the stomach relaxes to 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 kind of food and the

degree of muscle action of the emptying stomach and the small intestine.

Carbohydrates, for example, spend the least amount of time in the stomach,

while protein stays in the stomach longer, and fats the longest. As the food

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

the intestine are mixed and pushed forward to allow further digestion.


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Finally, the digested nutrients are absorbed through the intestinal walls and

transported throughout the body. 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 pushed into the colon, where they remain

until the feces are expelled by a bowel movement.

Production of Digestive Juices

The digestive glands that act first are in the mouththe salivary glands.

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

starch from food into smaller molecules. An enzyme is a substance that speeds

up chemical reactions in the body.

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

stomach acid and an enzyme that digests protein. A thick mucus layer coats the

mucosa and helps keep the acidic digestive juice from dissolving 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 smallintestine, the juices of two other digestive organs mix with the food. One of these

organs, the pancreas, 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.

The second organ, the liver, produces yet another digestive juicebile.

Bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of

the gallbladder, through the bile ducts, and into the intestine to mix with the fat in

food. The bile acids dissolve fat into the watery contents of the intestine, much

like detergents that dissolve grease from a frying pan. After fat is dissolved, it is

digested by enzymes from the pancreas and the lining of the intestine.


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Absorption and Transport of Nutrients

Most digested molecules of food, as well as water and minerals, are

absorbed through the small intestine. The mucosa of the small intestine contains

many folds that are covered with tiny fingerlike projections called villi. In turn, the

villi are covered with microscopic projections called microvilli. These structures

create a vast surface area through which nutrients can be absorbed. Specialized

cells allow absorbed materials to cross the mucosa into the blood, where they

are carried off in the bloodstream to other parts of the body for storage or further

chemical change. This part of the process varies with different types of nutrients.

Carbohydrates. The Dietary Guidelines for Americans 2005recommend that 45

to 65 percent of total daily calories be from carbohydrates. Foods rich in

carbohydrates include bread, potatoes, dried peas and beans, rice, pasta, fruits,

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

The digestible carbohydratesstarch and sugarare 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 calledmaltose. Then an enzyme in the lining of the small intestine 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.

Sugars are digested in one step. An enzyme in the lining of the small

intestine digests sucrose, also known as table sugar, into glucose and fructose,

which are absorbed through the intestine into the blood. Milk contains another

type of sugar, lactose, which is changed into absorbable molecules by another

enzyme in the intestinal lining.


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Fiber is undigestible and moves through the digestive tract without being

broken down by enzymes. Many foods contain both soluble and insoluble fiber.

Soluble fiber dissolves easily in water and takes on a soft, gel-like texture in the

intestines. Insoluble fiber, on the other hand, passes essentially unchanged

through the intestines.

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. Then in the small intestine, several enzymes from the

pancreatic juice and the lining of the intestine complete the breakdown of huge

protein molecules into small molecules called amino acids. These small

molecules can be absorbed through 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

intestine. The bile acids produced by the liver dissolve fat into tiny droplets and

allow pancreatic and intestinal enzymes to break the large fat molecules intosmaller ones. Some of these small molecules are fatty acids and cholesterol. The

bile acids combine with the fatty acids and cholesterol and help these molecules

move into the cells of the mucosa. In these cells the small molecules are formed

back into large ones, 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 food that is absorbed through the small intestine

are vitamins. The two 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, E, and K). Fat-soluble vitamins are stored in


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the liver and fatty tissue of the body, whereas water-soluble vitamins are not

easily stored and excess amounts are flushed out in the urine.

Water and salt. Most of the material absorbed through the small intestine is

water in which salt is dissolved. The salt and water come from the food and liquid

you swallow and the juices secreted by the many digestive glands.

How is the digestive process controlled?

Hormone 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 main hormones that control digestion are gastrin, secretin, and

cholecystokinin (CCK):

Gastrin causes the stomach to produce an acid for dissolving and

digesting some foods. Gastrin is also necessary for normal cell growth inthe lining of the stomach, small intestine, and colon.

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

bicarbonate. The bicarbonate helps neutralize the acidic stomach contents

as they enter the small intestine. Secretin also stimulates the stomach to

produce pepsin, an enzyme that digests protein, and stimulates the liver to

produce bile.

CCK causes the pancreas to produce the enzymes of pancreatic juice,

and causes the gallbladder to empty. It also promotes normal cell growth

of the pancreas.


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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 digestive 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. Researchers are studying other hormones that may play a part in

inhibiting appetite, including glucagon-like peptide-1 (GPL-1), oxyntomodulin (+ ),

and pancreatic polypeptide.

Nerve Regulators

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

Extrinsic, or outside, nerves come to the digestive organs from the brain or

the spinal cord. They release two chemicals, acetylcholine and adrenaline.

Acetylcholine causes the muscle layer of the digestive organs to squeeze with

more force and increase the push of food and juice through the digestive tract.

It also causes the stomach and pancreas to produce more digestive juice.Adrenaline has the opposite effect. It relaxes the muscle of the stomach and

intestine and decreases the flow of blood to these organs, slowing or stopping

digestion.

The intrinsic, or inside, nerves 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.


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Together, nerves, hormones, the blood, and the organs of the digestive

system conduct the complex tasks of digesting and absorbing nutrients from the

foods and liquids you consume each day.

SUMMARY OF THE PHYSIOLOGY OF DIGESTIVE SYSTEM

The Digestive Process:

The start of the process - the mouth: The digestive process begins in the

mouth. Food is partly broken down by the process of chewing and by thechemical action of salivary enzymes (these enzymes are produced by the

salivary glands and break down starches into smaller molecules).

On the way to the stomach: the esophagus - After being chewed and

swallowed, the food enters the esophagus. The esophagus is a long tube that

runs from the mouth to the stomach. It uses rhythmic, wave-like muscle

movements (called peristalsis) to force food from the throat into the stomach.

This muscle movement gives us the ability to eat or drink even when we're

upside-down.


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In the stomach - The stomach is a large, sack-like organ that churns the food

and bathes it in a very strong acid (gastric acid). Food in the stomach that is

partly digested and mixed with stomach acids is called chyme.

In the small intestine - After being in the stomach, food enters the duodenum,

the first part of the small intestine. It then enters the jejunum and then the ileum

(the final part of the small intestine). In the small intestine, bile (produced in the

liver and stored in the gall bladder), pancreatic enzymes, and other digestive

enzymes produced by the inner wall of the small intestine help in the breakdown

of food.

In the large intestine - After passing through the small intestine, food passes

into the large intestine. In the large intestine, some of the water and electrolytes

(chemicals like sodium) are removed from the food. Many microbes (bacteria like

Bacteroides, Lactobacillus acidophilus, Escherichia coli, and Klebsiella) in the

large intestine help in the digestion process. The first part of the large intestine is

called the cecum (the appendix is connected to the cecum). Food then travels

upward in the ascending colon. The food travels across the abdomen in the

transverse colon, goes back down the other side of the body in the descendingcolon, and then through the sigmoid colon.

The end of the process - Solid waste is then stored in the rectum until it is

excreted via the anus.

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