852
UNIT 4
Maintenance of the Body
23
stomach, and
segmentation
, or rhythmic local constric-
tions of the small intestine (Figure 23.3b). Segmentation
mixes food with digestive juices and makes absorption
more efficient by repeatedly moving different parts of the
food mass over the intestinal wall. Although mechanical
breakdown is sometimes referred to as
mechanical diges-
tion
, historically the term digestion applies only to chemi-
cal breakdown processes that utilize enzymes.
4. Digestion
involves a series of catabolic steps in which en-
zymes secreted into the lumen (cavity) of the alimentary
canal break down complex food molecules to their chemi-
cal building blocks.
5. Absorption
is the passage of digested end products (plus
vitamins, minerals, and water) from the lumen of the GI
tract through the mucosal cells by active or passive trans-
port into the blood or lymph.
6. Defecation
eliminates indigestible substances from the
body via the anus in the form of feces.
Some of these processes are the job of a single organ. For
example, only the mouth ingests and only the large intestine
defecates. But most digestive system activities require the co-
operation of several organs and occur bit by bit as food moves
along the tract as described later.
Check Your Understanding
1.
Name one organ of the alimentary canal found in the thorax.
Name three organs located in the abdominal cavity.
2.
What is the usual site of ingestion?
3.
Which digestive system activity actually moves nutrients from
the outside to the inside of the body?
For answers, see Appendix H.
Basic Functional Concepts
Describe stimuli and controls of digestive activity.
A theme we have stressed in this book is the body’s efforts to
maintain a constant internal environment. Most organ systems
respond to changes in that environment either by attempting to
restore some plasma variable to its former levels or by changing
their own function.
Te digestive system, however, creates an optimal environ-
ment for its functioning in the lumen of the GI tract, an area that
is actually
outside
the body. Essentially all digestive tract regula-
tory mechanisms control luminal conditions so that food break-
down and absorption can occur there as effectively as possible.
±wo facts apply to these regulatory mechanisms:
Digestive activity is provoked by a range of mechanical and
chemical stimuli.
Sensors involved in controlling GI tract
activity are located in the walls of the tract organs. Tese
sensors respond to several stimuli, most importantly stretch-
ing of the organ by food in the lumen, osmolarity (solute
concentration) and pH of the contents, and the presence
of substrates and end products of digestion. When stimu-
lated, these receptors initiate reflexes that (1) activate or in-
hibit glands that secrete digestive juices into the lumen or
hormones into the blood or (2) stimulate smooth muscle of
the GI tract walls to mix lumen contents and move them
along the tract.
Controls of digestive activity are both intrinsic and extrin-
sic.
Many of the controlling systems of the digestive tract are
intrinsic
—“in-house” nerve plexuses and hormone-producing
cells. Te so-called
gut brain
consisting of enteric nerve plex-
uses spreads like chicken wire along the entire length of the GI
tract and regulates digestive activity all along the tract.
±wo kinds of reflex activity occur, short and long.
Short
reflexes
are mediated entirely by the local
enteric
or “gut”
plexuses in response to stimuli arising in the GI tract.
Long
reflexes
are initiated by stimuli arising inside or outside the GI
tract and involve CNS centers and extrinsic autonomic nerves
(Figure 23.4)
.
Te stomach and small intestine also contain hormone-
producing cells. When stimulated, these cells release their
products to the interstitial fluid in the extracellular space.
Blood and interstitial fluid distribute these hormones to their
target cells in the same or different digestive tract organs,
which they induce to secrete or contract.
Check Your Understanding
4.
When sensors in the GI tract are stimulated, they respond
via reflexes. What types of digestive activity may be put into
motion via those reflexes?
5.
The term “gut brain” does not really mean there is a brain in
the digestive system. What does it refer to?
For answers, see Appendix H.
Digestive System Organs:
Relationships
Relationship of the Digestive Organs
to the Peritoneum
Describe the location and function of the peritoneum.
Define retroperitoneal and name the retroperitoneal
organs of the digestive system.
Most digestive system organs reside in the abdominopel-
vic cavity. Recall from Chapter 1 that all ventral body cavities
contain slippery
serous membranes
. Te
peritoneum
of the ab-
dominopelvic cavity is the most extensive of these membranes
(Figure 23.5a)
. Te
visceral peritoneum
covers the external
surfaces of most digestive organs and is continuous with the
pa-
rietal peritoneum
that lines the body wall (see Figure 23.30d,
p. 889). Between the two peritoneums is the
peritoneal cavity
,
a slitlike potential space containing a slippery fluid secreted by
the serous membranes. Te serous fluid lubricates the mobile
digestive organs, allowing them to glide easily across one an-
other and along the body wall as they carry out their activities.
A
mesentery
(mes
9
en-ter
0
e) is a double layer of peritoneum—
a sheet of two serous membranes fused back to back—that ex-
tends to the digestive organs from the body wall. Mesenteries
provide routes for blood vessels, lymphatics, and nerves to reach
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