section
12.1
Anatomy and Physiology of the Gl Tract
199
Food and its digestion products stimulate receptors in
the stomach that increase gastric secretion during the
g a stric p h a se ,
mediated primarily by secretion of gastrin.
G astrin
is a gastrointestinal hormone synthesized and
stored in G cells of the pyloric glands and mucosa of the
proximal part of the small intestine. When food moves
into the small intestine, the intestinal phase of gastric se-
cretion involves complex interactions of stimulatory and
inhibitory factors. Thus,
ch ym e
(the semifluid, homoge-
neous material produced by gastric digestion) in the upper
jejunum inhibits gastric secretion, whereas amino acids in
the middle and lower jejunum stimulate gastric secretion.
These activities are primarily mediated through GI hor-
mones (see below). The three phases of gastric secretion
(cephalic, gastric, and intestinal) are interdependent and
proceed more or less simultaneously.
The rate of passage of food into the duodenum depends
on the type of food and on the osmotic pressure it exerts in
the duodenum. Food rich in fat moves most slowly, food
rich in carbohydrate moves most rapidly, and food rich
in protein moves at an intermediate rate. Hyperosmolal-
ity decreases gastric emptying via duodenal osmorecep-
tors. Some GI hormones also inhibit gastric motility and
secretion.
Small Intestine
Most digestion and absorption of food constituents oc-
curs in the small intestine. The small intestine is also the
major site for absorption of water and electrolytes. It is
a convoluted tube about
6
m long, beginning at the py-
lorus and ending at the ileocecal valve. It is divided into a
duodenum (a C-shaped, short, fixed segment), the mo-
bile jejunum, and ileum. Its wall consists of the same
four concentric layers as in other parts of the GI tract:
m u cosa, su b m u co sa , m u scu la ris extern a,
and
serosa.
Finger-like projections known as
villi
that arise from the
luminal side of the mucosal membrane provide a large
surface area in the small intestine. Each villus consists
of a single layer of tall, columnar epithelial absorptive
cells (
e n tero cytes
) that contain
m icro villi
which further
increase the absorptive surface and give it a brush-like
appearance (brush border). The surface membrane of the
microvillus contains digestive enzymes and transport sys-
tems. The core of the microvillus consists of a bundle of
filamentous structures that contain actin and associated
proteins (e.g., myosin, tropomyosin, villin, fimbrin; see
Chapter 21), which may have a structural role. Enterocytes
are abundantly supplied with mitochondria, endoplasmic
reticulum, and other organelles. The outermost surface of
the microvillus is the carbohydrate-rich
g lyco ca lyx ,
which
is anchored to the underlying membrane. The glycocalyx
is rich in neutral and amino sugars and may protect cells
against digestive enzymes. Villi also contain
g o b le t cells,
which secrete mucus. The absorptive cells are held to-
gether by junctional complexes that include “tight junc-
tions.” However, the tight junctions contain pores that
permit the transport of water and small solutes into the
intercellular spaces according to the osmotic pressure
gradient.
Endocrine cells are distributed throughout the small in-
testine and other sections of the GI tract. About 20 different
cell types have been identified. Some, easily identifiable
by their staining characteristics, are argentaffin or ente-
rochromaffin cells containing 5-hydroxytryptophan, the
precursor of serotonin (Chapter 17). Neoplastic transfor-
mation of these cells leads to excessive production and se-
cretion of serotonin, which causes diarrhea, flushing, and
bronchoconstriction
(carcinoid syndrome).
Other types of
cells lack 5-hydroxytryptophan but take up amine precur-
sors (e.g., amino acids) to synthesize and store biologically
active peptides (e.g., hormones).
The mucosal cells of the small intestine and stomach
are among the most rapidly replaced cells of the human
body, being renewed every 3-5 days. Millions of cells
are sloughed off (or exfoliated) from the tips of villi
every minute; about 17 billion such cells are shed per day.
These mucosal cells amount to about 20-30 g of protein,
most of which is reclaimed as amino acids after hydro-
lysis in the lumen. These cells are replaced through di-
vision of undifferentiated cells in the crypts of the villi
(crypts of Lieberkiihn)and subsequent migration to the
top of the villus, followed by maturation. Mitotic poi-
sons (e.g., antineoplastic agents), injury, and infection
can adversely affect mucosal cell proliferation, produc-
ing a flat mucosa devoid of villi and leading to severe
malnutrition.
Digestion in the small intestine requires the biliary and
pancreatic secretions that are emptied into the duodenum
(usually at a common site).
Formation, Secretion, and Composition of Bile
Bile is formed and secreted continuously by polygonally
shaped liver parenchymal cells called
h epa to cytes.
An
aqueous buffer component (e.g., HCOj") is added to the
bile by the hepatic bile duct cells that carry the secretion to-
ward the common bile duct. The membrane of the hepato-
cytes in contact with the blood has microvilli that facilitate
the exchange of substances between plasma and the cells.
Hepatocytes are rich in mitochondria and endoplasmic
reticulum. Hepatic bile flows into the gallbladder, where
it is concentrated, stored, and emptied into the duodenum
when the partially digested contents of the stomach enter
