2 0 0
chapter
12
Gastrointestinal Digestion and Absorption
FIGURE 12-1
Schematic representation of the radial architecture of the plates formed by
the liver cells. Blood from the portal vein and the hepatic artery flows into
sinusoids and eventually enters the central vein. The bile canaliculi are
located between the liver cells. Bile flows in the opposite direction and
empties into the bile duct in portal triads. [Reproduced with permission
from A. W. Ham
H isto lo g y,
8th ed., J. B. Lippincott, Philadelphia, 1979.]
the duodenum. This movement is accomplished by con-
traction of the gallbladder mediated by cholecystokinin, a
GI hormone.
The
liver
is the largest gland and second largest or-
gan in the body and weighs about 1.5 kg in an adult. It
functions in synthesis, storage, secretion, excretion, and
specific modification of endogenously and exogenously
derived substances. The liver takes up material absorbed
from the small intestine, since it is mainly supplied by
venous blood arriving from the GI tract. It is uniquely sus-
ceptible to changes in alimentation and to toxic substances.
The liver participates in numerous metabolic functions.
The liver is organized into
lobules,
functional aggre-
gates of hepatocytes arranged in radiating cords or plates
surrounding a central vein (Figure 12-1). In adults, the
branching plates are usually one-cell thick, and the vas-
cular channels between them are known as
sinusoids.
Blood in the sinusoids passes slowly between the rows
of cells, facilitating the exchange of substances between
cells and the plasma. The blood supply of the sinusoids is
furnished by the portal vein (75%) and by the right and
left hepatic arteries (25%). Blood flows from the periph-
ery of the lobules into the central vein. Sinusoids anasto-
mose to form central veins that join to form larger veins
which feed through the hepatic vein into the inferior vena
cava. Sinusoids are larger than capillaries and are lined by
reticuloendothelial cells, including the phagocytic
Kupffer
cells.
This layer contains numerous, small fenestrations
that provide access of sinusoidal plasma to the surface mi-
crovilli of hepatocytes via the intervening space, called
the
space o f Disse.
Fine collagen fibers (reticulin) within
this space provide the support for the liver cell plates
(Figure 12-2).
FIGURE 12-2
Schematic representation of structures within the hepatic lobule. H =
hepatocyte, BC = bile canaliculus, KC = Kupffer cell, EC = endothelial
cell, N —
nerve fiber, F = reticulin fibers, S = sinusoid, D —
space of
Disse, X = gap between sinusoid lining cells, and RBC = red blood cell.
Bile produced by hepatocytes is secreted into the bile
canaliculi between adjacent hepatic cells. The wall of the
canaliculus is formed by the plasma membrane of the
hepatocytes, which are held together by tight junctions.
Canaliculi arise near central veins and extend to the pe-
riphery of the lobules. The direction of bile flow in the
canaliculi is centrifugal, whereas that of the blood flow is
centripetal. Canaliculi coalesce to form ducts, which are
lined by epithelium, and the ducts coalesce to form the
right and left hepatic ducts. Outside the liver these ducts
form the common hepatic duct.
The volume of hepatic bile varies in a normal human
adult from 250 to 1100 mL, depending on the rate at
which bile acids recirculate in the enterohepatic cycle
(see below). Some GI hormones increase the volume and
bicarbonate content of bile through stimulation of the
duct epithelium. Hepatic bile has a pH of 7.0-7
.8
and
is isotonic with plasma. During the interdigestive period,
it is diverted into the gallbladder, since the
sphincter of
Oddi
is closed. Bile in the gall bladder is concentrated
1 2
- to
2 0
-fold by absorption of electrolytes and water.
Nonetheless, gallbladder bile is isotonic with plasma be-
cause cations (mainly Na+) associate with the osmoti-
cally inactive bile acid micellar aggregates. Gallbladder
capacity is 50-60 mL. Emptying requires a coordinated
