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Human Digestive System
human digestive system, the system used in the human body for the process
of digestion. The human digestive system consists primarily of the digestive
tract, or the series of structures and organs through which food and liquids
pass during their processing into forms absorbable into the bloodstream.
The human digestive system is a complex series of organs and glands that
processes food. In order to use the food we eat, our body has to break the food
down into smaller molecules that it can process; it also has to excrete waste.
Organs that make up the digestive tract are the mouth, esophagus, stomach,
small intestine, large intestine—also called the colon—rectum, and anus.
Inside these hollow organs 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. The digestive tract also contains a
layer of smooth muscle that helps break down food and move it along the
tract.
Two “solid” digestive organs, the liver and the pancreas, produce digestive
juices that reach the intestine through small tubes called ducts. The
gallbladder stores the liver's digestive juices until they are needed in the
intestine. Parts of the nervous and circulatory systems also play major roles in
the digestive system.
 How is food digested
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.
 Production of Digestive Juices
The digestive 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. 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
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Mr. Ali H. Alhamdany
Msc . physiology
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 small intestine,
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 juice—bile. 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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Mr. Ali H. Alhamdany
Msc . physiology
The Digestive Process:
The start of the process : mouth: The digestive process begins in the mouth. Food is partly broken
down by the process of chewing and by the chemical action of salivary
enzymes (these enzymes are produced by the salivary glands and break
down starches into smaller molecules)
 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.
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.
 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.
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Mr. Ali H. Alhamdany
Msc . physiology
 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 descending colon, 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.
 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
in the 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.
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.
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Mr. Ali H. Alhamdany
Msc . physiology

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.
 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
Digestive System Glossary:
abdomen - the part of the body that contains the digestive organs. In human
beings, this is between the diaphragm and the pelvis
alimentary canal - the passage through which food passes, including the
mouth, esophagus, stomach, intestines, and anus.
mouth - the first part of the digestive system, where food enters the body.
Chewing and salivary enzymes in the mouth are the beginning of the digestive
process (breaking down the food).
epiglottis - the flap at the back of the tongue that keeps chewed food from
going down the windpipe to the lungs. When you swallow, the epiglottis
automatically closes. When you breathe, the epiglottis opens so that air can go
in and out of the windpipe.
salivary glands - glands located in the mouth that produce
saliva. Saliva contains enzymes that break down carbohydrates (starch) into
smaller molecules.
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Mr. Ali H. Alhamdany
Msc . physiology
esophagus - the long tube between the mouth and the stomach. It uses
rhythmic muscle movements (called peristalsis) to force food from the throat
into the stomach
stomach - a sack-like, muscular organ that is attached to the esophagus. Both
chemical and mechanical digestion takes place in the stomach. When food
enters the stomach, it is churned in a bath of acids and enzymes.
pancreas - an enzyme-producing gland located below the stomach and above
the intestines. Enzymes from the pancreas help in the digestion of
carbohydrates, fats and proteins in the small intestine.
gall bladder - a small, sac-like organ located by the duodenum. It stores and
releases bile (a digestive chemical which is produced in the liver) into the
small intestine.
liver - a large organ located above and in front of the stomach. It filters toxins
from the blood, and makes bile (which breaks down fats) and some blood
proteins.
bile - a digestive chemical that is produced in the liver, stored in the gall
bladder, and secreted into the small intestine.
intestines - the part of the alimentary canal located between the stomach and
the anus.
duodenum - the first part of the small intestine; it is C-shaped and runs from
the stomach to the jejunum
jejunum - the long, coiled mid-section of the small intestine; it is between the
duodenum and the ileum.
ileum - the last part of the small intestine before the large intestine begin
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Mr. Ali H. Alhamdany
Msc . physiology
gastrointestinal tract - (also called the GI tract or digestive system) the
system of the body that processes food and gets rid of waste.
cecum - the first part of the large intestine; the appendix is connected to the
cecum
appendix - a small sac located on the cecum.
ascending colon - the part of the large intestine that run upwards; it is located
after the cecum.
transverse colon - the part of the large intestine that runs horizontally across
the abdomen
descending colon - the part of the large intestine that run downwards after the
transverse colon and before the sigmoid colon.
sigmoid colon - the part of the large intestine between the descending colon
and the rectum.
rectum - the lower part of the large intestine, where feces are stored before
they are excreted
.
anus - the opening at the end of the digestive system from which feces (waste)
exits the body.
chyme - food in the stomach that is partly digested and mixed with stomach
acids. Chyme goes on to the small intestine for further digestion.
peristalsis - rhythmic muscle movements that force food in the esophagus
from the throat into the stomach Peristalsis is involuntary - you cannot
control it.
.
It is also what allows you to eat and drink while upside-down.
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Mr. Ali H. Alhamdany
Msc . physiology