Liver (An Overview)
The liver has two major lobes and two minor lobes. Anteriorly, the
right lobe is separated from the smaller left lobe by the falciform
ligament. Inferiorly, the caudate lobe is near the inferior vena cava,
and the quadrate lobe is adjacent to the gallbladder. The falciform
ligament is responsible for attaching the liver to the anterior
abdominal wall and the diaphragm by way of the coronary ligament, the
upper layer of which is exposed as if the liver were to be pulled away
from the diaphragm. A ligamentum teres is continuous along the free
border of the falciform ligament and is a remnant of the umbilical vein
of the fetus.
The porta of the liver is where the hepatic artery, portal vein
lymphatics, and nerves enter the liver and where the hepatic ducts
exit. Although the liver is the largest internal organ of the body, it
is only one to two cells thick. This is due to the fact that hepatocytes,
or liver cells, are only one to two cells thick and separated from each
other by large capillary spaces called sinusoids. The plate structure
of the liver and high permeability of the sinusoids allow each hepatocyte
to be in close contact with the blood.
The hepatic plates are arranged into functional units called liver
lobules. In the middle of each lobule is a central vein and at the
periphery of each lobule are branches of the hepatic portal vein and
hepatic artery, opening into spaces between hepatic plates. Arterial
blood and portal venous blood, containing nutrient molecules absorbed
in the gastrointestinal tract mix as the blood flows from the periphery
of the lobule to the central vein. The central veins of the lobules
will converge to form two hepatic veins which will carry blood from
the liver to the inferior vena cava.
Bile is produced in the liver by the hepatocytes and secreted into
thin channels called bile canaliculi located within each hepatic
plate. The canaliculi are drained peripherally by bile ducts which
in turn drain into hepatic ducts that carry bile away from the liver.
As a result, blood travels in the sinusoids and bile travels in the
opposite direction so blood and bile never mix in the lobules of the
liver under normal conditions.
Cirrhosis, an irreversible liver disease destroys large numbers of
liver lobules and replaces them with a permanent type of connective
tissue from hepatocytes called regenerative nodules. These nodules
don't have the plate-like structure of normal liver tissue and are
consequently, less functional. Cirrhosis is often accompanied by
the presence of ammonia from the hepatic portal vein on into systemic
circulation. Any disease that attacks liver cells such as viral
hepatitis or chemicals affecting the liver such as seen in chronic
alcohol abuse may bring about sclerosis.
