Chapter 23
The Digestive System
851
23
Te
accessory digestive organs
are the
teeth
,
tongue
,
gall-
bladder
, and a number of large digestive glands—the
salivary
glands
,
liver
, and
pancreas
. Te teeth and tongue are in the
mouth, or oral cavity, while the digestive glands and gallbladder
lie outside the GI tract and connect to it by ducts. Te accessory
digestive glands produce a variety of secretions that help break
down foodstuffs.
Digestive Processes
List and define the major processes occurring during
digestive system activity.
We can view the digestive tract as a “disassembly line” in which
food becomes less complex at each step of processing and its nu-
trients become available to the body. Te processing of food by
the digestive system involves six essential activities
(Figure 23.2)
:
1.
Ingestion
is simply taking food into the digestive tract,
usually via the mouth.
2. Propulsion
, which moves food through the alimentary ca-
nal, includes
swallowing
, which is initiated voluntarily, and
peristalsis
(per
0
ĭ-stal
9
sis), an involuntary process.
Peristalsis
(
peri
5
around;
stalsis
5
constriction), the major means of
propulsion, involves alternating waves of contraction and
relaxation of muscles in the organ walls
(Figure 23.3a)
. Its
main effect is to squeeze food along the tract, but some mix-
ing occurs as well. In fact, peristaltic waves are so powerful
that, once swallowed, food and fluids will reach your stom-
ach even if you stand on your head.
3. Mechanical breakdown
increases the surface area of
ingested food, physically preparing it for digestion by
enzymes. Mechanical processes include chewing, mix-
ing food with saliva by the tongue, churning food in the
Food
Ingestion
Propulsion
Esophagus
Stomach
Pharynx
Mechanical
breakdown
Digestion
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Small
int
estine
Lar
ge
int
estine
Defecation
Anus
F
eces
Mainly
H
2
O
Blood
v
essel
Lymph
v
essel
Absorption
Sw
allo
wing
(or
ophar
ynx)
Pe
r
istalsis
(esophagus
,
stomach,
small intestine
,
large intestine)
Figure 23.2
Gastrointestinal tract activities.
Note that sites of
digestion produce enzymes or receive enzymes or other secretions
made by accessory organs outside the alimentary canal.
(a) Peristalsis:
Adjacent segments of alimentary
tract organs alternately contract and relax,
moving food along the tract distally.
From
mouth
(b) Segmentation:
Nonadjacent segments of
alimentary tract organs alternately contract and
relax, moving food forward then backward.
Food mixing and slow food propulsion occur.
Figure 23.3
Peristalsis and segmentation.
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