3. Liver Function Tests
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Today we will look at liver function the tests used to diagnose abnormal liver function.
The liver is an organ about the size of a football, and it just sits under your rib cage on the
right side of your abdomen.
It is essential for digesting food, ridding your body of toxic substances and therefore has an
integral role in day-to-day physiology.
It is prone to disease and damage and therefore liver disease can be inherited which means
you could have a genetic predisposition to having a dysfunctional liver or it could be caused
by a variety of factors that damage the liver itself. These can be things like viruses for
instance, excessive alcohol abuse or being severely overweight or obese, this is associated
with ectopic fat disposition within the liver.
Liver disease is an umbrella term which encompasses numerous diseases, and we will be
focusing on just a select few.
Here is a diagram of some cells are involved in the liver tissue, it has its own blood system,
so it has a unique blood supply and delivery system which consists of the portal vein and the
hepatic artery.
The portal vein and hepatic artery form the livers dual blood supply, approximately 70%-75%
of hepatic blood flow is derived from the portal vein while the remainder is from the hepatic
arteries, unlike most veins the portal vein does not drain into the heart itself rather it is part
of a portal venous system that delivers venous blood into another capillary system the
hepatic sinusoids. In carrying venous blood from the GI tract to the liver, the portal vein
accomplishes a number of tasks, rather than the blood going straight back to the heart
which veins normally do the blood coming towards the heart goes through the liver first.
By supplying the liver first with metabolic substrates, it ensures that the substance ingested
are processed by the liver before reaching the systemic circulation.
Furthermore, this accomplishes itself; if you have ingested any possible toxins or bacteria,
these can be detoxified by the hepatocytes (cells of the liver). Secondly, it is the first organ
to absorb any nutrients taken in by the intestines and you'll know from physiology in
previous years that the liver is a great storer of glucose and you have a large glycogen store
there as well.
It is also a site of bile production and your ability to produce and release bile is an indicator
of liver function.
In the exam, introduce the organ, what it does in normal conditions and then you can delve
into what happens when it starts to deteriorate.
Urea nitrogen is a normal waste product that your body creates after you eat, and it
generates it from breaking down proteins in the food that you consume.
BUN test – blood, urea, nitrogen.
Liver converted nitrogen to urea, and it excretes it.
The liver also produces albumin which is fairly large protein which serves many functions
and one of its most important ones is to maintain osmotic pressure within the organs.
Elephantiasis is a condition in which people have particularly leaky blood vessels and you get
the leakage of large proteins into surrounding tissues, water travels into those tissues via its
osmotic potential, this causes add swelling of your feet hands and other extremities.
, 3. Liver Function Tests
The liver is involved in the metabolism of drugs and of course alcohol, it also has a greater
role in energy homeostasis specifically gluconeogenesis so that's a pathway which results in
the generation of glucose from non-carbohydrate substrates such as lactic acid so
gluconeogenesis works particularly well when you're completing an extreme bought of
exercise and you produce lactic acid and your body needs a bit more ATP, it can metabolise
lactic acid to an extent or at times when you are going through a longer fasting period.
The liver contains bile and bile acids which are critical for the digestion and absorption of
fats and if the liver is so instrumental in digesting fats, it is also involved in the absorption of
fat-soluble substances as well and these tend to be your vitamins particularly A, D, E and K.
Now we will be looking at conditions in which liver function is compromised.
One of the most visually common conditions observe and you can indicate is a liver issue is
jaundice.
This is diagnosed by abnormally high concentration of a pigment known as bilirubin. Most
bilirubin is produced when haemoglobin (a pigment in RBCs which carries oxygen) is broken
down into unconjugated bilirubin, unconjugated bilirubin binds to albumin in the blood for
transport to the liver. Therefore, if your liver function is compromised or you're suffering
from a disease which is increasingly rupturing your blood cells, the levels of bilirubin in the
blood will increase and the effects of jaundice will manifest. Bilirubin itself is a yellowish
pigment and if you have high amounts of it in your blood, your skin starts to turn slightly
pale and as you can see in the picture here this patients’ eyeballs are turning yellow as well.
This is just one condition associated with liver dysfunction.
Liver dysfunction manifests in three different ways.
It could be pre-hepatic which means that there is a condition which is not directly involved
with the liver but it's something before the liver gets involved but as a result is affecting the
liver.
Hepatocellular damage which is damage of the liver itself.
Cholestatic damage – cholestasis is defined as a decrease in bile flow due to impaired
secretion by the hepatocytes so could even be an obstruction or a blockage of bile flow
within the bile ducts.
This is how bilirubin is normally dealt with in the body.
In the spleen the RBCs go through a turnover process, the spleen is involved in breaking
dead red blood cells, and it's also involved with producing new ones to certain extent.
Haemoglobin is broken down into two individual compounds (haem and globin) and if we
break haem down further it contains iron (which is the component which is involved in
carrying oxygen within the haem group) and bilirubin.
Bilirubin does not like water or liquid and therefore is known as a hydrophobic pigment so in
order for it to be transferred around the body it has to be bound to albumin, this is one of
the roles of albumin as well, once it binds to albumin it moves over through the liver and the
albumin goes back and waits for more bilirubin to pick up.
Now that bilirubin is in the liver, it becomes conjugated, and it's known as bilirubin
diglucuronide. Then it is actively transported from the liver into the large intestine.
The large intestine contains numerous bacteria, and it breaks down the bilirubin into
urobilinogen and urobilin. Urobilin is a dark brownish pigment and is also responsible for the
colour of your stools. In normal conditions you'll get the bacteria fermenting and breaking
down this bilirubin into urobilin and it's excreted.
