PHYSIOLOGY
The various functions of the liver are carried out by the
liver cells or hepatocytes. Currently, there is no artificial
organ or
device capable of emulating all the functions of the liver. Some functions can
be emulated by liver dialysis, an experimental treatment for liver
failure.
Metabolism of Carbohydrates
The
liver breaks down complex
carbohydrates (example: fructose, lactose) into simpler
carbohydrates like glucose,
which can the be utilized by the body for energy production. Excess glucose
which is not needed immediately by the body is stored in the liver as glycogen.
In
the event of low blood sugar levels, the liver then releases the stored
glucose to regulate the sugar levels.
This is known as the glucose
buffer function. In the absence of carbohydrates, especially with high
protein, no carbohydrate diets, the liver will use protein and fats to produce glucose.
§ The liver performs several roles
in carbohydrate metabolism:
§ Gluconeogenesis (the synthesis of glucose from certain amino
acids, lactate or
or glycerol)
§ Glycogenolysis (the breakdown of glycogen into glucose)
§ Glycogenesis (the formation of glycogen from glucose) (muscle
tissues can also do this)
Metabolism of Protein
The
liver breaks down amino acids by
removing the functional group (NH2) which allows it to be used for energy
production or converted into carbohydrates and/or fat. This process is known
as deamination. During the
deamination process, ammoniais
formed. The liver is able to remove ammonia by converting it into urea and excreting it mainly via
the kidneys as urine.
Apart
from breaking down proteins, the liver can also produce amino acids. It does this by converting existing amino
acids into other types of amino acids (interconversion).
Plasma proteins, which are an integral part of blood, are also produced by
the liver.
Proteins produced and secreted
by the liver:-
§ A large part of amino acid synthesis
§ The liver is responsible for the
mainstay of protein metabolism, synthesis as well as degradation
Metabolism of Fats
The
liver breaks down fats into
acetyl-CoA (acetyl coenzyme A) to for energy production. Conversely, fat may be
synthesized by carbohydrates and proteins as it is the more efficient form of
nutrient storage. Certain types of lipids like cholesterol,
phospholipids and lipoproteins are also produced within the liver.
Filtration and Storage of Blood
The
liver plays an important role in filtering
the blood and storing
blood so that it is not in circulation. Approximately 1.5 liters of
blood pass through the liver every minute. This blood enters through the portal
vein and hepatic artery. Excess nutrients are removed and processed as
described above or stored, toxins are neutralized and secreted and specialized
liver cells also play a part in removing pathogens. Calcium and the body’s own
hormones are also neutralized and/or removed from the body by the liver.
Up
to 500 milliliters of blood is stored within the liver in the hepatic sinuses
and hepatic vein. This excess blood can be released into the circulation should
the body require it. In the event of any congestion that results in backflow of
blood, the liver may hold up to 1 liter of blood or more before it becomes
damaged. This can significantly increase the weight of the liver (normal weight
in adults is approximately 1.5 kg).
Production of Bile
Bile is
an important compound for the emulsification of fats in the gastrointestinal
tract. It is also a means for the body to get rid of bilirubin from
the bloodstream as well as cholesterol produced
by the liver.
Formation of Blood Clotting Factors
The
liver produces essential clotting factors which are crucial for the coagulation
of blood. This includes :
- Factor I (fibrinogen)
- Factor II (prothrombin)
- Factor V (accelerator globulin)
- Factor VII (antihemophilic factor A)
- Factor IX (plasma thromboplastin component / antihemophilic factor B)
- Factor X (Stuart-Prower factor)
- Factor XI (plasma thromboplastin antecedent)
- Factor XII (Hageman factor)
- Factor XIII (fibrin-stabilizing factor)
* The liver requires vitamin K to produces
these factors.
Storage of Vitamins and Minerals
The
liver stores many micronutrients including vitamins A, D and B12 and
minerals like iron (stored
as ferritin). The storage capacity of the liver is so large that it has enough
of some of these vitamins to sustain the body from a few months to several
years should there be no further nutrient intake.
Breakdown
§ The breakdown of insulin and other hormones
§ The liver breaks down hemoglobin, creating metabolites that are added to bile as pigment (bilirubin and biliverdin).
§ The liver breaks down or
modifies toxic substances (e.g., methylation) and most medicinal
products in a process called drug
metabolism. This
sometimes results in toxication, when the metabolite is more toxic
than its precursor. Preferably, the toxins are conjugated to avail excretion in bile or
urine.
§ The liver converts ammonia to urea.
Other functions
§ The liver is responsible for
immunological effects- the reticuloendothelial
system of
the liver contains many immunologically active cells, acting as a 'sieve' for
antigens carried to it via the portal system.
§ The liver produces albumin, the major osmolar component of blood serum.
§ The liver synthesizes angiotensinogen, a hormone that is responsible for raising the blood
pressure when
activated by renin, an enzyme that is released when the kidney senses low blood pressure.
SYMPTOMS OF LIVER DISEASE
Patients with mild liver disease may
have few or no symptoms or signs. Patients with more serious disease develop
symptoms and signs that may be nonspecific or specific.
Nonspecific symptoms, that is,
symptoms that don't suggest that the liver is their cause, include:
·
Fatigue,
·
Weakness,
·
Vague abdominal pain, and
·
Loss of
appetite.
Symptoms and signs that are specific
for liver disease include:
·
Yellowing
of the skin (jaundice) due to the accumulation of bilirubin in the blood,
·
Itching,
and
·
Easy
bruising due to decreased production of blood clotting factors by the diseased
liver.
Severe, advanced liver disease with
cirrhosis can produce symptoms and signs related to cirrhosis; these symptoms
include:
·
Fluid
accumulation in the legs (edema) and abdomen (ascites),
·
Mental
confusion or coma,
·
Kidney failure,
·
Vulnerability
to bacterial infections, and
·
Gastrointestinal bleeding.
The first sign of damage to the liver is an increase in liver enzyme levels in the blood. When the liver is damaged, its enzymes are released into the bloodstream, where the levels can be measured by blood tests. These are called liver function tests (LFTs). Enzyme levels that are routinely checked as part of LFTs include:
·
alanine aminotransferase (ALT)
·
aspartate aminotransferase (AST)
·
gamma-glutamyltransferase (GGT)
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