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Digestive System
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Digestive System, organs for
changing food chemically for
absorption by body tissues.
Digestion involves breaking organic
compounds into simple soluble
substances absorbable by tissues.
Digestive System
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This process involves
catalytic reactions (reactions
that break things down)
between ingested food and
enzymes secreted into the
intestinal tract .
Digestive System
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Other nutrients such as iron and
vitamin B12 are absorbed by specific
"carrier proteins" that make them
transferable by the intestinal cells.
Digestion includes both mechanical
and chemical processes.
Digestive System: mechanical
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The mechanical processes
include chewing to reduce food
to small particles, the churning
action of the stomach, and
intestinal peristaltic action.
These forces move the food
through the digestive tract and mix
it with various secretions.
Digestive System: Chemical
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Three chemical reactions take place:
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1. conversion of carbohydrates into such simple
sugars as glucose
2. breaking down of protein into such amino acids
which are the building blocks of proteins.
3. conversion of fats into fatty acids and glycerol.
These processes are accomplished by
specific enzymes.
Digestive System: Saliva
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When food is eaten, the six salivary glands
produce secretions that are mixed with the
food.
The saliva breaks down carbohydrates
(with the enzyme amylase) into maltose,
dissolves solid food to make it susceptible
to the action of later intestinal secretions,
stimulates secretion of digestive enzymes,
and lubricates the mouth and esophagus
for the passage of solids.
Salivary Glands
Holt, Modern Biology
Digestive System
Holt, Modern Biology
Stomach
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Gastric juice in the stomach contains agents
such as hydrochloric acid and some enzymes,
including pepsin, rennin, and lipase.
The surface of the stomach itself is thought to be
protected from acid and pepsin by its mucous
coating.
 Pepsin breaks proteins into peptones and
proteoses.
 Rennin separates milk into liquid and solid
portions;
 lipase acts on fat.
Stomach
Digestive System: Stomach
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Another function of stomach digestion is
gradually to release materials into the
upper small intestine, where digestion is
completed.
Some constituents of gastric juice become
active only when exposed to the alkalinity of
the small intestine; secretion is stimulated by
chewing and swallowing and even by emotion
precipitated by seeing or thinking of food.
Stomach and Small Intestine
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The presence of food in the stomach also
stimulates production of gastric secretions;
in turn, these stimulate the production of
digestive substances in the small intestine.
The most extensive part of digestion
occurs in the small intestine;
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here most food products are further
hydrolyzed (broken down with the addition of
water) and absorbed.
Small Intestine
Small Intestine: Digestive Fluids
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Predigested material supplied by the stomach
is subjected to the action of three powerful
digestive fluids:
 pancreatic fluid,
 intestinal juice, and
 bile.
These fluids neutralize the gastric acid,
ending the gastric phase of digestion.
Small Intestine: Pancreatic Fluid
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Pancreatic fluid is introduced into the
small intestine through several ducts.
Secretion of pancreatic juice is
stimulated by the ingestion (eating)
of proteins and fats.
Pancreatic Fluid
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Pancreatic fluid contains trypsin and
chymotrypsin, enzymes that split
complex proteins into simpler
components that can be absorbed
and used in reconstructing body
proteins.
Lipase breaks down fat;
amylase hydrolyzes polysaccharides.
Pancreas
Digestive System
Holt, Modern Biology
Small Intestine: Intestinal Juices
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Intestinal juice is secreted by the small
intestine.
It contains a number of enzymes
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peptidase: for proteins
maltase, lactase, and sucrase; its function is to
complete the process begun by the pancreatic
juice.
The flow of intestinal juice is stimulated by the
mechanical pressure of food partly digested in
the intestine.
Bile
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The role of bile in digestion is to aid in
absorption of fats by combining with them to
form structures called micelles, which are
soluble in the blood and more accessible to
lipases.
Secreted by the liver and stored in the
gallbladder, bile flows in response to fat in the
stomach and upper intestine.
Liver, Gall Bladder, Pancreas
Digestive System
Holt, Modern Biology
Small Intestine: Active Transport
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Transport of the products of digestion
through the wall of the small intestine may
be either passive or active.
Sodium, glucose, and many amino acids
are actively transported.
The products of digestion are thus
assimilated into the body through the
intestinal wall, which is able to absorb
nutritive substances selectively.
Digestive System
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The water-soluble substances, including
minerals, amino acids, and carbohydrates,
are transferred into the venous drainage of the
intestine and through the portal blood channels
directly to the liver and then on to the heart.
Many of the fats, however, are resynthesized
in the wall of the intestine and are picked up
by the lymphatic system, which carries
them into the systemic blood flow as it
returns to the heart.
Small Intestine
Holt, Modern Biology
Small Intestine
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Intestinal absorption has another unique feature.
Many nutrients are more efficiently absorbed when
the body need is greater.
The absorptive, extensively convoluted surface of
the intestine amounts to 140 sq m (1500 sq ft) in an
adult.
Absorption is also aided by the length of the small
intestine, 6.7 to 7.6 m (22 to 25 ft).
Digestive System: Large Intestine
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The stomach and the colon or
large intestine also have the
ability to absorb water, certain
salts, alcohol, and some drugs.
Certain whole proteins are also
believed to pass through the
intestinal barrier.
Digestive System
Holt, Modern Biology
Large Intestine
Excretion and Water Reabsorption
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Undigested material is formed into a solid
mass in the colon by reabsorption of water
into the body.
If colonic muscles propel the excretory mass
through the colon too quickly, it remains
semiliquid.
 The result is diarrhea.
Insufficient activity of the colonic musculature,
on the other hand, produces constipation.
The stool is held in the rectum until excreted
through the anus.
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"Digestive System," Microsoft® Encarta®
Encyclopedia 99. © 1993-1998 Microsoft
Corporation. All rights reserved.
Enzyme Review
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Mouth
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Stomach
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Amylase-breaks down starch
Pepsin-breaks peptide bonds, making
proteins smaller
Small Intestine
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peptidase-breaks down proteins
lactase- lactose
sucrase- sucrose
maltase- maltose
Enzyme Review
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Pancreas
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Trypsin- Breaks down proteins
Amylase-breaks down starch
Lipase- breaks down fat
Liver
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Bile- breaks fat down into droplets which
are easier for the enzymes to work with