Parts
of the Digestive System
The
gastrointestinal tract or digestive tract, also
referred to as the GI tract or the alimentary canal or the
gut, is the system of organs within multicellular animals
which takes in food, digests it to extract energy and nutrients,
and expels the remaining waste.
Image credit: Body Atlas by Steve Parker,
illustration by Giuliano Fornari
Dorling Kindersley Book, New York, 1993.
The
Alimentary Canal In a normal human adult male,
the GI tract is approximately 7 and a half metres long (25
feet) and consists of the following components:
Upper gastrointestinal tract: mouth
(buccal cavity; includes salivary glands, mucosa, teeth
and tongue) pharynx; esophagus (gullet)
and stomach.
Lower gastrointestinal tract: bowel or
small intestine; (which has three parts: duodenum,
jejunum, ileum); large intestine, which has three parts:
colon; rectum and the
anus.
Mouth & Throat
The
mouth, also known as the buccal cavity or the oral
cavity, is the opening through which an animal takes in
food and water. Read more about the mouth here.
Image
credit:
Teeth
(singular, tooth) are structures found in the jaws of many
vertebrates. The primary function of teeth is to tear and
chew food. More about teeth
here.
Tongue
Image
credit: Eating a Meal: How you eat, drink and digest
by Steve Parker, Franklin Watts, NY, 1991
The
tongue is the large bundle of skeletal muscles
on the floor of the mouth that manipulates food for chewing
and swallowing, (deglutition). It is one of the organs of
taste. Much of the surface of the tongue is covered in taste
buds. More on the tongue here.
Saliva
A salivary gland made up of mucus glands (with the mucin
shown in pink.)
Image credit: Guts:
Our Digestive System by Seymour Simon, Harper Collins,
NY, 2005
Saliva
helps soften the food in the mouth so that it is easier
to swallow. Saliva is also the first of several chemicals
that start to break down foods into simpler forms. The salivary
gland in located underneath the back of our tongue. It creates
our saliva or spit.
Incredible Body, illustrated
by Stephen Biesty, DK Publishing, NY, 1998
The
esophagus (also spelled oesophagus/œsophagus),
or gullet is the muscular tube in vertebrates through which
ingested food passes from the mouth area to the stomach.
The
esophagus is simply a transportation tube from the mouth
to the stomach. When we swallow, what we are really doing
is closing a trap door in our throat called the epiglottis.
This sends food down the esophagus and prevents food from
going down the trachea (or windpipe) and into our lungs.
Food
is passed through the esophagus by using the process of
peristalsis.
Image credit: Guts: Our Digestive System
by Seymour Simon, Harper Collins, NY, 2005
The
esophagus is lined with mucous membrane, and is more deeply
lined with muscle that acts with peristaltic action to move
swallowed food down to the stomach.
Stomach
Image credit: Body Atlas by Steve Parker,
illustration by Giuliano Fornari
Dorling Kindersley Book, New York, 1993.
Stomach-
The first stop after the esophagus is the stomach. Once
the food gets to the stomach the stomach uses chemicals
to try to make the food tinier. These chemicals are called
gastric juices and the include hydrochloric acid and enzymes
(chemicals that break down food). The food is moved around
in the stomach and mixed with the chemicals for about 3
or 4 hours. When it is done in the stomach, the food is
now a cream-like liquid call chyme. The food is still not
small enough the get into our blood stream and it has not
provided the body with anything useful yet. Now a valve
at the end of the stomach opens sending the food past the
liver.
Image credit: Body Atlas by Steve Parker,
illustration by Giuliano Fornari
Dorling Kindersley Book, New York, 1993.
Stomach
lining
Image credit: Guts: Our Digestive System
by Seymour Simon, Harper Collins, NY, 2005
The
innermost layer is obliquely-oriented; this is not seen
in other parts of the digestive system; this layer is responsible
for creating the motion that churns and physically breaks
down the food.
Mucus
A layer of mucus along the inner walls
of the stomach is vital to protect the cell linings of that
organ from the highly acidic environment within it. In the
digestive system, mucus is used as a lubricant for materials
which must pass over membranes, e.g., food passing down
the esophagus.
Liver
and Pancreas
Image
credit
Liver/Gall
Bladder- At this point, our food is hit with more
chemicals. The liver makes a chemical called bile but bile
is not stored in the liver. Instead it is stored in the
gall bladder. When the gall bladder mixes bile with our
food, it does an important job: breaking down the fat (from
milk, butter, cheeses) into tiny droplets. This fat will
supply us with much energy later.
The
pancreas also adds a digestive chemical as the
food leaves the stomach. This digestive juice works on breaking
down the carbohydrates (from breads, potatoes, etc.) and
the proteins (from meats, cereals, peanut butter)
Small
Intestines
Image credit: Guts: Our Digestive System
by Seymour Simon, Harper Collins, NY, 2005
The Small Intestine is
the real hero of the digestive system. The small intestine
is a tube that is about 22 feet long! This is where the
real digestion takes place. As the food passes through,
it is mixed with the new chemicals and soon our "food"
is now digested small enough to be put to use by the body.
Small intestine cross-section published in
Body: an amazing tour of human anatomy
published by DK.com, 2005.
Villi
Along the walls of the intestine are thousands of tiny fingers
called villi.. Blood vessels (capillaries) in the villi
can absorb the tiny food molecules and send them off to
the rest of our body through the blood.
Section of human villi. Along the outer
edges are the epithelial cells, through which nutrient molecules
are absorbed. Inside are muscles that give the villi motion
and capillaries and lymph ducts that transport nutrients.
Image credit: Food & Nutrition by William
Sebrell, Jr., Time Life Books, 1967.
Kidneys
& Bladder: Urinary System
Urinary system diagram: red arrows show blood
carrying waste substances to kidneys; blue arrows show clean
blood leaving kidneys and returning to the heart; dotted
blue lines show route of urine from kidneys to bladder in
tubes called "ureters" from Body: an amazing
tour of human anatomy
published by DK.com, 2005.
Water from the food and
beverages we consume is absorbed in both the small and large
intestines. This liquid passes through and is filtered by
the kidneys. Humans are 2/3 water, so most of the liquid
continues on in the blood network, some is sent to the bladder
for elimination as urine.
The urinary system is the organ system
that produces, stores, and eliminates urine. In humans it
includes two kidneys, two ureters, the urinary bladder,
two sphincter muscles, and the urethra.
The kidney is one of the various organs
(together with the lungs, intestine and skin) that participates
in the elimination of the wastes of the organism. The kidneys
are bean-shaped organs about the size of a bar of soap.
They are near the middle of the spine, just below the ribcage.
A kidney consists of about 1 million filtering
units termed nephrons, each consisting of a glomerulus,
ball-shaped network of capillaries, and a network of tubules.
Blood plasma is filtered by the glomerulus, and the resultant
"prourine" passes through the tubular system where
water, electrolytes and nutrients are reabsorbed under the
supervision of hormone activity and the autonomic nervous
system.
Humans produce about 1.5 liters of urine
over 24 hours, although this amount may vary according to
circumstances. Increased fluid intake generally increases
urine production, while increased perspiration and respiration
may decrease the amount of fluid excreted through the kidneys.
A reduced intake of water will normally result in less urine
production as well.
The bladder stores urine; it swells into
a round shape when it is full and gets smaller when empty.
In the absence of bladder disease, it can hold up to 16
fluid ounces (500 ml) of urine comfortably for 2 to 5 hours.
Read more about the urinary system here.
Colon
Image credit: Guts: Our Digestive System
by Seymour Simon, Harper Collins, NY, 2005
Colon or Large Intestine-
Whatever the body cannot put to use is sent to the large
intestine. Many plants, for example, contain cellulose which
cannot be digested. The big job of the large intestine is
to remove water. Water has been necessary up until now but
it is no longer needed and in the large intestine water
is sent into the blood stream . Food spends about 12 hours
in the large intestine where it become feces and later leaves
the body through the anal opening when we go to the bathroom.
Photo credit: Look At Your Body: Digestion
by Steve Parker, Copper Beech Books, Brookfield, CT, 1996
Intestinal
bacteria.A healthy large intestine contains millions
of bacterial microbes, especially the type E. coli (above).
They live in a mutually helpful partnership with the body,
called symbiosis. The body provides a warm, moist, nutrient-rich
environment. In return, the bacteria break down leftovers
and wastes, produce certain vitamins (B6 and K) that the
body absorbs, and also make waste gases called "flatus"
that sometimes emerge through the anus.
Rectum
Image credit: Incredible
Body, illustrated by Stephen
Biesty, DK Publishing, NY, 1998
The rectum acts as a temporary
storage facility for feces. As the rectal walls expand due
to the materials filling it from within, stretch receptors
from the nervous system located in the rectal walls stimulate
the desire to defecate. If the urge is not acted upon, the
material in the rectum is often returned to the colon where
more water is absorbed. If defecation is delayed for a prolonged
period, constipation and hardened feces results. Learn more
about the rectum here.
The Anal Canal. When the
rectum is full the increase in intrarectal pressure forces
the walls of the anal canal apart allowing the fecal matter
to enter the canal. The rectum shortens as material is forced
into the anal canal and peristaltic waves propel the feces
out of the rectum. The internal and external sphincter allow
the feces to be passed by muscles pulling the anus up over
the exiting feces.
Feces. The average person gets rid of about
0.3 pounds of feces daily. About half of this weight is
water. The rest is rubbed off parts of the digestive tract
lining, mucus, millions of dead intestinal bacteria, plant
fiber and other undigested remnants from food. Feces are
brown in color which comes from pigments, such as bilirubin.
Learn more about feces here.
Image credits (from left): Digestive
System path; Vesalius
anatomy lesson;
woman with her alimentary canal ; The
Food Museum collection;
The Quest to Digest
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