The Digestive System
for colon cancer. Currently recommendations are to have an oc-
cult blood examination once a year and a colonoscopy at 3- to
10-year intervals depending on prior ﬁndings.
Check Your Understanding
From which germ layer does the digestive system mucosa
How does cystic ﬁbrosis interfere with the digestive process?
Why are colon and stomach cancers so dangerous?
For answers, see Appendix H.
As summarized in
, the digestive system
keeps the blood well supplied with the nutrients needed by all body
tissues to fuel their energy needs and to synthesize new proteins for
growth and maintenance of health. Now we are ready to examine
how body cells use these nutrients, the topic of Chapter 24.
(making them inoperable) before a person seeks medical atten-
tion. Should metastasis occur, secondary cancer of the liver is al-
most guaranteed because of the “detour” the splanchnic venous
blood takes through the liver via the hepatic portal circulation.
However, when detected early, most GI tract cancers are treat-
able. Te best advice is to have regular dental and medical checkups.
Most oral cancers are detected during routine dental examinations,
50% of all rectal cancers can be felt digitally, and nearly 80% of co-
lon cancers can be seen and removed during a colonoscopy.
Until now it was believed that most colorectal cancers de-
rive from initially benign, knobby mucosal tumors called pol-
yps. However, ﬂat pancake-like growths which blend in with
the surrounding tissue are more common in Americans than
originally thought and are 10 times more likely to be cancerous
than polyps. Researchers believe that the ﬂat growths represent
a separate pathway to colon cancer. Tis information will likely
have a big impact on gastroenterology and scheduling of tests
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Overview of the Digestive System
Te digestive system includes organs of the alimentary canal
(mouth, pharynx, esophagus, stomach, small and large intestines)
and accessory digestive system organs (teeth, tongue, salivary
glands, liver, gallbladder, and pancreas).
Digestive system activities include six processes: ingestion (food
intake); propulsion (movement of food through the tract);
mechanical breakdown (processes that physically mix or break
foods down into smaller fragments); digestion (food breakdown
by enzymatic action); absorption (transport of products of
digestion through the intestinal mucosa into the blood); and
defecation (elimination of the undigested residues [feces] from
Basic Functional Concepts
Te digestive system controls the environment within its lumen to
ensure optimal conditions for digestion and absorption of foodstuﬀs.
Receptors and hormone-secreting cells in the alimentary canal
wall respond to stretch and chemical signals that result in
stimulation or inhibition of GI secretory activity or motility. Te
alimentary canal has an intrinsic nerve supply.
Digestive System; Topic: Control of the Digestive System,
pp. 1, 3, 4–5, 8.
Digestive System Organs: Relationships
Te parietal and visceral layers of the peritoneum are continuous
with one another via several extensions (mesenteries, falciform
ligament, lesser and greater omenta), and are separated by a
potential space containing serous ﬂuid, which decreases friction
during organ activity.
Te digestive viscera are served by the splanchnic circulation,
consisting of arterial branches of the celiac trunk and aorta and
the hepatic portal circulation.
All organs of the GI tract have the same basic pattern of tissue
layers in their walls; all have a mucosa, submucosa, muscularis,
and serosa (or adventitia). Intrinsic nerve plexuses (enteric
nervous system) are found within the wall.
of the Digestive System
The Mouth and Associated Organs
Food enters the GI tract via the mouth, which is continuous with
the oropharynx posteriorly. Te boundaries of the mouth are the
lips and cheeks, palate, and tongue.
Te oral mucosa is stratiﬁed squamous epithelium, an adaptation
seen where abrasion occurs.
Te tongue is mucosa-covered skeletal muscle. Its intrinsic
muscles allow it to change shape; its extrinsic muscles allow it to
Saliva is produced by many minor salivary glands and three
pairs of major salivary glands—parotid, submandibular, and
sublingual—that secrete their product into the mouth via ducts.
Largely water, saliva also contains ions, proteins, metabolic
wastes, lysozyme, defensins, IgA, salivary amylase, and mucin.
Saliva moistens and cleanses the mouth; moistens foods, aiding
their compaction; dissolves food chemicals to allow for taste;
and begins digestion of starch (salivary amylase). Saliva output is
increased by parasympathetic reﬂexes initiated by activation of
chemical and pressure receptors in the mouth and by conditioned
reﬂexes. Te sympathetic nervous system depresses salivation.