Anatomy
The liver is a multi-lobed organ that
is located at the most forward part of the abdomen. It is so far
forward that it lays up against the diaphragm, the muscle that aids in
breathing in mammals (birds and reptiles do not have a
diaphragm). This autopsy picture of a cat shows the gallbladder in
green, with several lobes of the liver laying right up against the
diaphragm (towards the top of the picture). On the other side of the
diaphragm is the chest cavity containing the lungs and heart.
This close-up of the gallbladder and one
of the lobes of the liver gives you a feel for what a normal liver lobe
looks like in a cat.
For fun comparison purposes, this is
the gallbladder from an Iguana. It is also surrounded by liver lobes.
Notice the difference in color from the cat liver above?
The liver is the largest organ that is
located in the body, a testament to its importance. It has 6 distinct
lobes organized into 3 regions. Like the kidneys, 25% of the blood
ejected with each beat of the heart goes to the liver. Of tremendous
significance is the fact that a healthy liver has great reserve power,
using only a small amount of its full potential at any one time.
Unfortunately, this great reserve power means that diseases that affect
the liver can be well entrenched and not show symptoms of disease
before a diagnosis is made. This obviously makes the prognosis worse.
The liver does have an advantage though. Liver cells (hepatocytes) can
regenerate themselves. This regenerative ability allows a diseased
liver to return to normal function in some cases. Very few organs in
the body have this ability.
The liver is supplied with nutrients
from the hepatic artery and the portal vein, which is different from
other internal organs. Blood from the heart goes through the aorta and
into the hepatic artery. The hepatic artery supplies a large amount of
the oxygen and nutrients the hepatocytes use during metabolism.
Approximately 1/3 of the blood that flows into the liver comes from
this artery.
The other source of blood for the liver
is the portal vein, supplying 2/3 of the blood that flows into the
liver. The nutrients in the portal vein originate from the digestive
tract, this time, not the heart, like the hepatic artery does. In
essence, raw food that is absorbed from the intestines flows directly
to the liver. This exposes the liver to toxins and bacteria, which are
metabolized and detoxified by a normal liver before they leave the
liver and enter the general circulation via the heart. This
detoxification process protects other organs, particularly the brain,
from bacteria and toxins that could injure brain cells. When this
system fails, which happens in liver shunts, excess ammonia will build
up in the bloodstream and affect the brain. You will learn more about
this when we discuss porto-systemic shunts later.
The extensive blood supply to the liver
is apparent in this picture. The large blue vessels are part of the
portal vein.
Functional Unit
At the cellular level the liver has a
basic functional unit call the acinus. This consists of hepatic
sinusoids (areas of blood pooling), liver cells (hepatocytes), central
vein and the portal triad. Blood from the portal triad flows into the
hepatic sinusoids that surround the hepatocytes. This blood then exits
through the central vein and back into the general circulation.
Biliary System
The biliary system consists of the
gallbladder, bile ductules, hepatic ducts, and the common bile duct.
The gallbladder is located between two
of the liver lobes. It stores bile that is made by the liver, and
secretes this bile through the common bile duct into the beginning of
the small intestine (duodenum). The bile that is secreted into the
duodenum aids in the digestion of many compounds, especially fat.
This is a picture of a greatly enlarged
gall bladder and common bile duct in a cat.
BD- Bile duct
GB- Gall Bladder
L- Liver
This "inside" lining view of the small
intestine gives you a great view of bile as it is being secreted from
the gall bladder into the duodenum.
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