530
Chapter 15
Overview: Functions of the
Gastrointestinal Organs
Figure 15–3
presents an overview of the secretions and func-
tions of the gastrointestinal organs. The gastrointestinal tract
begins at the
mouth,
where digestion starts with chewing,
which breaks up large pieces of food into smaller particles
we can swallow.
Saliva,
secreted by three pairs of
salivary
glands
(see Figure 15–1) located in the head, drains into the
mouth through a series of short ducts. Saliva, which contains
mucus, moistens and lubricates the food particles before swal-
lowing. It also contains the enzyme
amylase,
which partially
digests polysaccharides (complex sugars). A third function of
saliva is to dissolve some of the food molecules. Only in the
dissolved state can these molecules react with chemoreceptors
in the mouth, giving rise to the sensation of taste (Chapter 7).
Finally, saliva has antibacterial properties. See
Table 15–1
for
the major functions of saliva.
The next segments of the tract, the
pharynx
and
esoph-
agus,
do not contribute to digestion but provide the pathway
for ingested materials to reach the stomach. The muscles in
the walls of these segments control swallowing.
The
stomach
is a saclike organ located between the
esophagus and the small intestine. Its functions are to store,
dissolve, and partially digest the macromolecules in food and
to regulate the rate at which the contents of the stomach empty
into the small intestine. The glands lining the stomach wall
secrete a strong acid,
hydrochloric acid,
and several protein-
digesting enzymes collectively known as
pepsin.
Actually, a
precursor of pepsin known as pepsinogen is secreted and con-
verted to pepsin in the lumen of the stomach.
The primary function of hydrochloric acid is to dis-
solve the particulate matter in food. The acidic environment
Mouth
Sublingual
salivary gland
Submandibular
salivary
gland
Trachea
Liver
Gallbladder
Parotid
salivary
gland
Pharynx
Esophagus
Stomach
Pancreas
Small
intestine
Large
intestine
Colon
Cecum
Rectum
Anus
Figure 15–1
Anatomy of the gastrointestinal system. The liver overlies the
gallbladder and a portion of the stomach, and the stomach overlies
part of the pancreas.
Esophagus
Stomach
Mouth
Food and
water
Digestion
Colon
Small intestine
Blood flow
Rectum
Anus
Feces
Hepatic
portal vein
Absorption
t
rt
rt
rt
rt
Heart
r
er
er
ver
iver
Liver
Liver
Liver
Liver
Liver
Liver
Liver
Liver
Live
Live
Live
Liv
Liv
Li
L
Secretion
Motility
&
Figure 15–2
Four processes the gastrointestinal tract carries out: digestion, secretion, absorption, and motility. Outward (black) arrows indicate absorption
of the products of digestion, water, minerals, and vitamins into the blood, most of which occurs in the small intestine. Inward-pointing (red)
arrows represent the secretion of enzymes and bile salts into the GI tract. The wavy confi guration of the small intestine represents muscular
contractions (motility) throughout the tract.
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