Function of liver

Function of liver

The liver is a large, solid, gland situated in the right upper quadrant of the abdominal cavity. In the living subject, the liver is reddish brown in colur, soft in consistency,and very friable. It weighs about 1600 g in males and about 1300 g in females. The liver occupies the whole of the right hypochondriac, the greater part of the triumphalist,and extends into the left hypochondriac reaching up to the left lateral line. From the above it will be obvious that most of the liver is covered by ribs and coastal cartilages, except in the upper part of the triumphalist where it is in contact with the anterior abdominal wall. The liver is the largest gland in the body. It secretes bile and performs various other metabolic functions. The liver is also called the 'heparin' from which we have the adjective 'hepatic' applied to many structures connected with the organ.

One prominent border : The inferior borber is sharp anteriorly where it separates the anterior surface from the inferior surface. It is somewhat rounded laterally where it separates the right surface from the inferior surface.

Two lobes

The liver is divided into right and left lobes by the attachment of the falciform ligament anteriorly and superiorly; by the fissure for the ligamentum teres inferiorly; and by the fissure for the ligamentum venosum posteriorly. The right lobe is much larger than the left lobe and forms five sixth of the liver. It contributes to all the five surfaces of the liver and presents the caudate and quadrate lobes. The caudata lobe is situated on the posterior surface. It is bounded on the right by the groove for the inferior vena cava, on the left by the fissure for the ligamentum venosum, and inferiorly by the porta hepatis. Above it is continuous with the superior surface. Below and to the right, just behind the porta hepatis, it is connected to the right lobe of the liver by the caudate process. Below and to the left it presents a small rounded elevation called the papillary process. The quadrate lobe is situated on the inferior surface, and is rectangular in shape. It is bounded anteriorly by the inferior border, posteriorly by the porta hepatis, on the right by the fossa for the gall bladder, and on the left by the fissure for the ligamentum teres. The porta hepatis is a deep, transverse fissure about 5 cm long, situated on the inferior surface of the right lobe of the liver. It lies between the caudate lobe above and the quadrate lobe below and in front. The portal vein, the hepatic artery and the hepatic plexus of nerves enter
the liver through the porta hepatis, while the right and left hepatic ducts and a few lymphatics leave it. The relations within  the porta hepatis are from behind forwards the portal vein the hepatic artery and the hepatic ducts. The lips of the porta hepatis provide attachment to the lesser omentum. The left lobe of the liver is much smaller than the right lobe and forms only one - sixth of the liver. It is flattened from above downwards. Near the fissure for the ligamentum venosum, its inferior surface presents a rounded elevation, called the omental tuberosity or tuber omentale.                   

Visceral relations

Anterior surface : The anterior is triangular and slightly convex. It is related to the xiphoid process and to the anterior abdominal wall in the median plane; and to diaphragmon each side. The diaphragm separates this surface from the pleure above the level of a line drawn from the xiphisternal joint to the 10th rib in the midaxillary line; and from the lung above the level of a line from the same joint to the 8th rib.The falciform ligament is attached to this surface a little to the right of the median plane.

Posterior surface: The posterior surface is triangular. Its middle part shows a deep concavity for the vertebral column.
1. The bare area is related to the diaphragm; and to the right suprarenal gland near the lower end of the groove for the inferior vena cava.
2. The groove for the inferior vena cava lodges the upper part of the vessel, and its floor is pierced by the hepatic veins.
3. The caudate lobe lies in the superior recess of the lesser sac. It is related to the crura of the diaphragm above the aortic opening, to the right inferior phrenic artery, and to the coeliac trunk.
4.The fissure for the ligamentum venosum is very deep and extends to the front of the caudate lobe. It contains two layers of the lesser omentum. The ligamentum venosum lies on its floor. The ligamentum venosum is a remnant of the ductus venosus of foetal life; it is connected  below to the left branch of the portal vein, and above to the left hepatic vein near its entry into the inferior vena cava.
5 .The posterior surface  of the left lobe is marked by the oesophageal impression.

Superior surface: The superior surface is quadrilateral and shows a concavity in the middle. This is the cardiac impression. On each side of the impression the surface is convex to fit the dome of the diaphragm. The diaphragm separates this surface from the pericardium and the heart in the middle; and from lieura and lung on each side. Right surface: The right surface is quadrilateral and convex. It is related to the diaphragm opposite the 7th to 11th ribs in the midaxillary line. It is separated by the diaphragm from the pleura up to the 10tyh up rib, and from the lung up the 8th rib. Thus the upper one-third of the surface is related to the diaphragm, the pleura and the lung; the middle one - third, to the diaphragm and the costodiaphragmatic recess of the pleura; and the lower one - third to the diaphragm alone.

Blood supply

The liver receivees 20% of its blood supply through the hepatic artery, and 80% through the portal vein. Before entering the liver, both the hepatic artery and the portal vein divide into right and left branches. Within the liver, they redivide to from segmental vessels which further divide to from interlovular vessels which run in the portal canals.Further ramifications of the interlobular branches open into the hepatic sinusoids. Thus the hepatic arterial blood mixes with the portal venous blood in the sinusoids. There are no anastomoses between adjoining hepatic arterial territories and hence each branch is an end artery.

Hepatic segments : On the basis of the intrahepatic distribution of the hepatic artery, the portal vein and the biliary ducts, the liver can be divided into the right and left functional lobes. These do not correspond to the anatomical lobes of the liver. The physiological lobes are separated by a plane passing on the anterosuperior surface along a line joining the cystic notch to the groove for the inferior vena cava. On the inferior surface the plane passes through the fossa for the gall bladder; and on the posterior surface it passes through the middle of the caudata lobe.The right lobe is subdivided into anterior and posterior segments, and the left lobe into medial and lateral segments.

Histology : Liver is covered by Glisson's  capsule. In the pig there are hexagonal lobules with portal redicles at 3-5 corners. Each radicle contains bile ductule, branch each of portal vein and hepatic artery. Central vein lies in the center and all around the central vein are the hepatocytes in from of laminae. On one side of the lamina is the sinusoid and on the other side is a bile canaliculus. Portal lobule seen in human is traingular in shape with three central veins at the sides and portal tract in the centre. The liver acinus is defined as the liver parenchyma around a preterminal branch of hepatic arteriole between two adjacent central veins. The liver acinus is the functional unit of liver. Blood reaches the acinus via branches of portal vein and hepatic  artery to open into the sinusoids to reach the central vein. On the other hand, the flow of bile is along bile canaliculi,
bile ductules and the interlobular bile ducts. Hepatocytes in zone i close ti preterminal branch are better supplied by oxygen, nutrients and toxins. The liver cells in zone III close to central vein  are relatively hypoxic while cells in zone II are intermediate in oxygen supply. Histology of the liver can be studied by liver biopsy.  

Arterial supply : The spleen is supplied by the splenic artery which is the largest branchof the coeliac trunk. The artery is tortuous in its course to allow for movements of the spleen. It passes through the lienorenal ligament to reach the hilum of the spleen where it divides into five or more branches. These branches enter the spleen to supply it. Within the spleen it divides repeatedly to from successfully the straight vessels called penicilli,ellipsoids and arterial capillaries. Further course of the blood is controversial. According to closed theory of splenic circulation,the capillaries are continuous with the venous sinusoids that lie in the red pulp; the sinusoids join together to from veins. However, according to open theory of splenic circulation, the capillaries end by opening into the red pulp from where the blood enters the sinusoids through their walls. Still others believe in a compromise theory, where the circulation is open distended spleen and closed in contracted spleen. The splenic circulation is adapted for the mechanism of separation and storage of the red blood cells.