The Liver
Introduction
The liver is the largest gland in the
body and normally weighs between 1 - 2.3 Kg. It is situated in upper part of
abdominal cavity on right side, under the costal margin ( right hypochondriac
region). The upper and anterior surfaces are smooth and curved to fit the under
surface of the diaphragm. The posterior surface is irregular in outline. The
stomach, bile ducts and duodenum are inferior. The organ is enclosed in thin
hepatic capsule and incompletely covered by a layer of peritoneum, folds of
which form supporting ligaments to the diaphragm. The liver is in two main
lobes, the left and right as well as two smaller lobes.
Is the normal liver palpable?
No if the liver is palpable there is
an anatomical abnormality
Can you think of any conditions
which might cause an enlarged liver, ie hepatomegaly?
Alcoholic cirrhosis
Viral hepatitis, eg A or B
Secondary tumour deposits
Hepatocellurar carcinoma, (primary
liver cancer)
Congestive cardiac failure
Blood supply
As with all other organs the liver has
an arterial blood supply to bring in oxygenated blood to fuel tissue
metabolism, this blood is supplied via the hepatic artery. However in addition
the liver also receives blood from the hepatic portal vein. This vein begins in
capillaries which are found in the wall of the gastrointestinal tract. This
means that the products of digestion are brought directly to the liver. In the
liver there is a mixing of the blood supplied from the hepatic artery and the
hepatic portal vein and all the blood is drained via the hepatic vein.
Why does the liver have a
venous blood supply?
This is to carry the products of
digestion from the gastrointestinal tract to the liver to be biochemicaly
processed or stored.
Histology
The functional unit of the liver is
the individual liver cell, these are termed hepatocytes. The liver structure is
made up of tiny lobules. In the centre of each lobule is a branch of the
hepatic vein. The hepatocytes are arranged in columns, which form walls of
liver cells. Between these there is an area referred to as a sinusoid. These
are in effect capillaries, which allow the blood to come into close contact
with the individual liver cells. Small branches of the hepatic artery and
hepatic portal vein both circulate blood into the sinusoids, so in the
sinusoids there is mixing of this arterial and venous blood. Because these
sinusoid blood vessels have incomplete walls, larger molecules can interchange
the blood and the liver cells. Once the blood has passed through the sinusoid it
is collected in the central vein, which is a branch of the hepatic vein.
Why do you think larger
molecules may need to pass between the liver cells and the blood?
As the liver is involved in plasma
protein production, the proteins produced must be able to be exported from the
hepatocytes into the blood.
Why is it important that the
sinusoids receive blood from the hepatic artery as well as the hepatic vein?
The blood from the hepatic artery is
essential to supply oxygen to the metabolicaly active hepatocytes.
The blood flow through the liver may
be summarised as follows,
Aorta --- hepatic artery --- small
branch of hepatic artery ---- sinusoid --- central lobular vein ---- hepatic
vein ---- inferior vena cava
Capillaries of hepatic portal vein (in
wall of GI tract)----- hepatic portal vein ----- interlobular branches of hepatic portal vein ---- sinusoid
---- hepatic vein ---- inferior vena cava
Supporting the lobules of the liver
and maintaining the overall structure there is connective tissue largely made
up of collagen.
One of the functions of the liver
cells is to form bile, by the concentration of bilirubin.
Where does bilirubin come from?
(Clue..think blood cells)
Bilirubin is a pigment which comes
from the breakdown of haemoglobin when red cells are broken down.
Bile is collected from the rows of
hepatocytes. However the bile leaves the from the other side of the cells from
the sinusiods and collects in bile canaliculi. These merge to form larger bile
ducts and eventually leave the liver in the right and left hepatic ducts.
Where is bile stored and
concentrated before it enters the duodenum?
The gall bladder
The liver sinusoids are also patrolled
by its own phagocytic cells termed Kupffer cells.
Function of the liver
As mentioned the functional unit of
the liver is the individual liver cell. This is the site of much of the
biochemistry being carried out by the body. Because there is a lot of chemistry
being carried out in liver cells a lot of energy is required to facilitate
these reactions. This means the liver produces heat.
Individual functions of the liver
1.
Converts glucose to glycogen in presence of insulin, for storage.
2.
Breaks down protein (deaminates protein), forms urea, and uric acid.
3.
Desaturates fats - converts stored fat to a form in which it can be used
by the tissue to provide energy.
4.
Heat production
5.
Produces and secretes bile
6.
Stores the anti-anaemic factor, ie, vit B12 (extrinsic factor)
7.
Stores iron
8.
Stores Vit A.D.E.K.
9.
Synthesis of Vit A.
10.
Forms Plasma Proteins - serum
albumin and globulin
11.
Forms prothrombin and fibrinogen
Hepatic Position Palpation Diseases
Hepatic artery Mesenteric
artery Hepatic portal vein Hepatic veins
Inferior vena cava
Hepatocytes, numerous mitochondria
Bile – red blood cells – haemoglobin –
bilirubin – liver – bile ducts - gall bladder - duodenum
Jaundice
Lobules 1 – 2 mm in diameter Sinusoids
(blood channels)
Canaliculi (bile channels) Kupffer
cells
Triad – branch of HPV, hepatic artery,
bile ducts Central vein
Intracellular enzymes present in
hepatocytes
Glucose – glycogen
Glycogen – glucose
Galactose and fructose – glucose
Amino acids – glucose
Protein – fat Carbohydrate – fat
Fat – fat Alcohol and fat
metabolism – fatty liver, cirrhosis
Deamination – waste nitrogen, ammonia
Urea production
Transamination – 20 amino acids in
human proteins, 10 essential.
Synthesis of albumin
Synthesis of prothrombin and
fibrinogen
Synthesis of angiotensinogen
Stores fat soluble vitamins Stores vitamin B12 Storage of iron as ferritin
Breaks down toxins from gut bacteria
Drugs – metabolites using cytochrome
p450
Alcohol – metabolites using
dehydrogenase systems
Breaks down hormones
Produces heat