Disorders of the liver
Introduction to the liver and liver disease
Introduction to the liver
The liver is divided into two lobes, left and right (which includes the caudate). It has two blood supplies: 25% of its blood originates from the hepatic artery (oxygenated) and 75% originates from the portal vein that drains the gastrointestinal tract and spleen. This blood is therefore relatively low in oxygen, but rich in glucose, lipids and amino acids.
The functions of the liver are carried out by the hepatocytes, which have a special architectural arrangement. Blood enters the liver through the portal tracts, which contain the triad of hepatic artery, portal vein and bile duct. It then filters from the edges of the lobule to the central (efferent) vein. The lobule is classically used to describe the histology of the liver (see Fig. 5.4a) but the acinus forms the functional unit (see Fig. 5.4b).
The hepatocytes in zone 1 of the acinus receive well-oxygenated blood from the portal triads, whereas the hepatocytes in zone 3 receive poorly-oxygenated blood and are therefore more vulnerable to damage when the blood supply is compromised.
The liver has multiple functions, which may be impaired or disrupted by liver disease:
· Carbohydrate metabolism: The liver is one of the major organs in glucose homeostasis under the control of pancreatic insulin. Excess glucose following a meal is converted to glycogen and stored within the liver. In early starvation states this glycogen is mobilised, and once this store is exhausted glucose is synthesised via the gluconeogenesis pathway within the liver.
· Protein: The liver is involved in the synthesis of all circulating proteins apart from immunoglobulin. This includes carrier proteins, albumin and coagulation factors. The liver is also involved in the breakdown of amino acids producing ammonia, which is converted to urea and excreted by the kidneys.
· Fat: The liver is involved in synthesis of lipoproteins (lipid protein complexes), triglycerides and cholesterol.
· Bile synthesis and metabolism.
· Drug and hormone inactivation and excretion.
Liver disease can be considered according to the aetiology, the pathology (such as acute hepatitis) or the clinical picture of presentation.