CHAPTER OUTLINE

The macroscopic and microscopic anatomy of the liver is difficult to understand, partly because of its inherently complicated three-dimensional structure and partly because of the recent trend to replace simple (but misleading) morphological descriptions with more accurate, but less obvious, functional descriptions.

The liver has a dual blood supply. Arterial blood, direct from the aorta, is supplied via the hepatic artery from the coeliac axis. The second source is the portal vein, which is formed by the joining of the superior mesenteric and splenic veins and which collects blood from the gut. After passage through the sinusoids (see below), blood drains from the liver via the hepatic veins at the posterior aspect of the liver into the inferior vena cava and thence to the right side of the heart. The portal venous system delivers about 80% of the blood and 20% of the oxygen supplied to the liver. A major cause of abnormal function in chronic liver disease is disturbance of blood flow through the liver, consequent on the fibrosis that follows chronic liver cell damage, but if the portal vein is blocked, relatively normal liver function can be maintained for many years. Arterial occlusion occurring suddenly, for example as a result of trauma, leads to acute liver failure, but more gradual occlusion by a tumour mass is compatible with normal liver function for quite long periods.

Hepatocytes, the hepatic parenchymal cells, comprise about 80% of the total cell mass of the liver. As viewed under the microscope, the functional unit of the liver appears to be the acinus, often termed the lobule, and it is in terms of this structure that pathological changes are described. Since liver biopsy has, under many circumstances, become the de facto ‘gold standard’ of liver disease diagnosis, it is important to understand this terminology. It is also important to understand that this unit does not comprise a homogeneous collection of hepatocytes and that there is marked functional heterogeneity across the acinus, with different zones having different physiological and biochemical functions (see below).

The fact that the liver has a great capacity for regeneration is the rationale for much of the practice of clinical hepatology, particularly for undertaking prolonged periods of liver- intensive care during acute liver failure and for the feasibility of extensive resection, particularly for the management of primary and, less commonly, secondary malignant liver tumours.

Hepatic regeneration has been studied mainly in animals, most extensively the rodent model, after partial hepatic resection. Twenty-four hours after liver resection, there is extensive division of the remaining cells accompanied by a surge in DNA synthesis; this is followed 24 h later by increased replication of the non-parenchymal cells. The initiation of these events is by growth factors that are now being identified. Epidermal growth factor (EGF), transforming growth factor α (TGFα) and hepatocyte growth factor (HGF) are all involved in switching on regeneration, and transforming growth factor β (TGFβ) is involved in switching it off. Normally quiescent differentiated hepatocytes replicate rapidly after resection, while intrahepatic precursor cells, termed oval cells, proliferate and generate lineage only in situations where hepatocyte proliferation is blocked or delayed. Bone marrow cells can generate oval cells, but such transdifferentiation is now known to be inefficient and occurs only rarely.

Because the various functions of the liver may fail at different times, creating differing clinical pictures, it is necessary to consider each function separately. The physiology and biochemistry of the liver subsume most of intermediary metabolism and, as such, are clearly beyond the scope of this chapter. Other than giving a brief outline, emphasis is placed on those functions whose measurement or disturbance is important in liver tests or pathology, respectively.

As has been known for many years, removal of the liver in an animal leads to death from hypoglycaemia. A major metabolic function of the liver is to store sugar and reduce the variations in blood glucose consequent upon the human habit of eating intermittently. Thus, during a meal, the liver stores glucose as glycogen and then releases it (glycogenolysis) slowly when food is not being eaten. This is particularly important for those organs such as the brain and red blood cells that have an obligatory requirement for glucose. Specific glucose transporter molecules located in the sinusoidal membrane that mediate facilitated diffusion are involved in both processes. Between meals, as the supply of glycogen decreases (only about 75 g can be stored), the liver starts to make glucose (gluconeogenesis) from other sources, particularly lactate, but also pyruvate, glycerol and alanine, although only from alanine is there a significant net production of glucose. During more prolonged periods of starvation, the total body requirement for glucose falls and energy demand is increasingly met by production of ketone bodies, derived mainly from fatty acids via acetyl-CoA. Apart from being stored, glucose is used by the liver as an energy substrate via glycolysis and the citric acid cycle, or for the synthesis of fatty acids and triglycerides.

After a meal, dietary triglycerides are hydrolysed to free fatty acids and monoglycerides by pancreatic lipases and dissolved in an aqueous medium, facilitated by the detergent action of bile salts excreted by the liver into the gut. The liver meets its own metabolic energy requirements, and those of the body as a whole, by mitochondrial β-oxidation of short chain fatty acids. The resultant acetyl-CoA either enters the citric acid cycle or reacts with another molecule of acetyl-CoA to form ketone bodies. Although the role of the liver is central to the oxidation of fatty acids, most tissues contain the enzymes required to undertake complete oxidation. The liver also synthesizes fatty acids, triglycerides, cholesterol, phospholipids and lipoproteins. Disturbances of fatty acid metabolism, including decreased oxidation (as in excessive alcohol consumption), increased hepatic fatty acid synthesis and decreased breakdown of triglycerides to fatty acids and glycerol, may all be involved in the development of fatty liver (‘steatosis’), an increasingly prevalent problem found in obesity and as an additional or incidental histological finding in many liver disorders.

Hepatic protein metabolism is central to the assessment of liver function, and its disturbance underlies many of the clinical complications that occur in severe liver cell dysfunction.

A 70 kg man on a normal diet needs to excrete between 10 and 20 g of nitrogen per 24 h. This derives, in the form of ammonia, from amino acids that are surplus to requirements (and cannot be stored) and those that are not reutilized after normal turnover. The ammonia is converted into urea in the liver and excreted by the kidneys. The liver processes dietary amino acids arriving via the portal vein and from breakdown of muscle proteins, both for its own requirements and for export to peripheral tissues. Aromatic amino acids (AAA: phenylalanine, tyrosine and tryptophan) are metabolized by the liver, but hepatic extraction of branched chain amino acids (BCAA: leucine, isoleucine and valine) is small and these are taken up largely by muscle. The ratio of BCAA/AAA is decreased in acute liver failure and this alteration forms the basis of one theory of the pathogenesis of hepatic encephalopathy, namely that it is due to the toxic effects of increased concentrations of ammonia on the brain.

The lungs and kidneys are effective in excreting volatile and water-soluble substances, respectively, but many compounds, both exogenous (such as drugs) and endogenous, including end products of metabolism, are lipid soluble and non-volatile. Many of these compounds are toxic, and a vital function of the liver is to render such substances more water soluble so that they can be excreted in urine or bile.

Two phases of biotransformation of metabolites by the liver are recognized. In phase I, a suitable polar group is made available, which is conjugated in phase II. Phase I reactions occur in the smooth endoplasmic reticulum and are mediated mainly by the mixed function oxidase system (cytochrome P450 isoenzymes) that utilize atmospheric oxygen, typically generating hydroxylated or carboxylated compounds. Phase II reactions involve their subsequent conjugation by the action of glucuronyl transferases with glucuronic acid, acetyl or methyl radicals or, in the case of bile acids, with glycine, taurine or sulphate (see below). There is considerable functional heterogeneity of the glucuronyl transferases, of which there are several isoenzymes that have varied substrate specificities, particularly for exogenous compounds. An alternative, non-oxidative pathway for biotransformation of both endogenous and exogenous substances is by conjugation with reduced glutathione by glutathione S-transferases.

Liver function tests have four potential applications.

2. As an aid to making a specific diagnosis. While functional tests are clearly distinct from diagnostic tests, it is still reasonable to assume that certain patterns of dysfunction may be characteristic of particular diseases.

3. To establish the severity of liver dysfunction or damage once a specific diagnosis has been established. This is important from a prognostic point of view, although the standard biochemical ‘liver function tests’ do not always reflect accurately the severity of tissue damage.

4. To monitor the progression of the disease and any response to therapeutic intervention.

Within this framework, two classes of tests will be considered. The first are the standard liver function tests – a group of tests often applied irrespectively of the suspected diagnosis and to which all the caveats about lack of true functional assessment apply. The second group includes those biochemical assays used for assessment of liver disease in specific situations, for example α₁-antitrypsin in suspected deficiency of this protein and α-fetoprotein in suspected primary hepatic cancer (hepatocellular cancer). The latter are described briefly here, for the sake of completeness, and in more detail in the next chapter in the context of their diagnostic application. The standard liver function tests are usually considered to include the plasma total bilirubin concentration, the activities in plasma of certain enzymes (particularly alanine and/or aspartate aminotransferases, alkaline phosphatase and γ-glutamyltransferase), and the total plasma protein, albumin and globulin concentrations. The prothrombin time (discussed later) also provides useful information about the synthetic capacity of the liver in the short term, as it is a functional measure of factor VII (among others) concentration and this clotting factor has a short half-life in plasma (see below). It has been estimated that this group of tests will correctly allocate patients to a liver disease/non-liver disease category in about 75% of cases.