306
UNIT 3
Organ Systems
contact with
enterokinase
in the
glycocalyx
. The
glycocalyx is
an
extracellular polymeric
material (glycoprotein), which coats
the surface of microvilli in the small intestines (Figs. 16-9A to
16-10B).
Liver
The
liver
also plays both
endocrine
and
exocrine
roles. Its
endocrine role
is in synthesizing and releasing plasma proteins,
such as
f brinogen
,
prothrombin
,
lipoproteins
, and
albumins
,
into the bloodstream. Its
exocrine role
is the production of
bile
.
Bile is important in emulsifying and degrading fat into smaller
molecules and in carrying wastes out of the body. Bile contains
water
,
bile salts
,
bilirubin
,
cholesterol
,
Fatty acids
,
lecithin
, and
electrolytes
. Bile is produced by
hepatocytes
and is collected
by
bile canaliculi
; it drains into the hepatic duct, then into the
cystic duct, and ±
nally enters the gallbladder (Figs. 16-11 and
16-15A). Other functions of the liver include detoxi± cation;
involvement in
lipid
,
carbohydrate
, and
protein metabolism
;
and storage of
iron
,
blood
,
glycogen
,
triglycerides
(
lipid drop-
lets
), and
vitamins
A
,
D
,
and
B12
. The liver is supplied by the
portal veins
and the
hepatic arteries
at the
portal triads
. The
hepatic arteries carry oxygen-rich blood, and the portal veins
carry nutrient-rich blood to the
hepatic sinusoids
through their
branches (hepatic arterioles and portal venules). The hepatocytes
are not in direct contact with the blood stream. A small space
between the hepatocyte and the endothelium of the sinusoids
is called the
perisinusoidal space
or
space oF Disse
. The discon-
tinuous endothelium of the sinusoids allows proteins, nutrients,
wastes, and plasma components (but not blood cells) from the
hepatic
sinusoids to enter the space of Disse.
Hepatocytes take
up
nutrients and transport wastes, such as
bilirubin
, into the
bile. The central veins collect the exchanged blood from sinu-
soids and drain into the sublobular veins and then into the large
hepatic veins. The basic structure of the liver includes the
classic
lobule
, the
portal lobule
, and the
liver
acinus
(Figs. 16-11 to
16-12A).
Gallbladder
The
gallbladder
is a pear-shaped, saclike organ closely associ-
ated with the liver. It stores, concentrates, and releases bile.
When the smooth muscle (muscularis) of the gallbladder
contracts, bile is released from the gallbladder through the
cystic duct into the bile duct. This joins the
hepatopancreatic
ampulla
and drains into the duodenum at the
major duode-
nal papilla
. The wall of the gallbladder is composed of three
layers: (1)
mucosa
, which includes epithelium (simple colum-
nar epithelium) and lamina propria; (2)
muscularis
, which
contains interlacing smooth muscle bundles and contracts in
response to
cholecystokinin
, which is released by
enteroen-
docrine cells
in the small intestine (the smooth muscle of the
gallbladder extends from the body into the neck where it is
known as the
spiral valve oF Heister
and is vital in controlling
gallbladder fl
ow); and (3)
serosa/adventitia
, a connective layer
with a lining of mesothelium (serosa) or without mesothelium
(adventitia). Most of the outer surface of the gallbladder is
covered with
serosa
, which is continuous with the peritoneum
that covers the liver. A small region of the gallbladder is cov-
ered by
adventitia
, which attaches the gallbladder to the liver
(Fig. 16-15A,B).
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