kidney and adrenal gland Posteriorly - oesophagus, inferior vena cava,
aorta, gall bladder, vertebral column
Laterally - lower ribs and diaphragm.
The liver is enclosed in a thin inelastic capsule and incompletely covered by a layer of peritoneum. Folds of peritoneum form supporting ligaments attaching the liver to the inferior surface of the diaphragm. It is held in position partly by these ligaments and partly by the pressure of the organs in the abdominal cavity.
The liver has four lobes. The two most obvious are the large right lobe and the smaller, wedge-shaped, left lobe. The other two, the caudate and quadrate lobes, are areas on the posterior surface (Fig. 12.35).
The portal fissure
This is the name given to the region on the posterior surface of the liver where various structures enter and leave the gland.
The portal vein enters, carrying blood from the stomach, spleen, pancreas and the small and large intestines.
The hepatic artery enters, carrying arterial blood. It is a branch from the coeliac artery, which is a branch from the abdominal aorta.
Nerve fibres, sympathetic and parasympathetic, enter here.
The right and left hepatic ducts leave, carrying bile from the liver to the gall bladder.
Lymph vessels leave the liver, draining some lymph to abdominal and some to thoracic nodes.
Structure.The lobes of the liver are made up of tiny functional units, called lobules, which are just visible to the naked eye (Fig. 12.36A). Liver lobules are hexagonal in outline and are formed by cubical-shaped cells, the hepatocytes, arranged in pairs of columns radiating from a central vein. Between two pairs of columns of cells are sinusoids (blood vessels with incomplete walls) containing a mixture of blood from the tiny branches of the portal vein and hepatic artery (Fig. 12.36B). This arrangement allows the arterial blood and portal venous blood (with a high concentration of nutrients) to mix and come into close contact with the liver cells. Amongst the cells lining the sinusoids are hepatic macrophages (Kupffer cells) whose function is to ingest and destroy worn out blood cells and any foreign particles present in the blood flowing through the liver.
Blood drains from the sinusoids into central or centrilobular veins. These then join with veins from other lobules, forming larger veins, until eventually they become the hepatic veins, which leave the liver and empty into the inferior vena cava. Figure 12.37 shows the system of blood flow through the liver. One of the functions of the liver is to secrete bile. In Figure 12.36B it is seen that bile canaliculi run between the columns of liver cells. This means that each column of hepatocytes has a blood sinusoid on one side and a bile canaliculus on the other. The canaliculi join up to form larger bile canals until eventually they form the right and left hepatic ducts, which drain bile from the liver.
Lymphoid tissue and a system of lymph vessels are also present in each lobule.