Metabolism

After digestion by the intestines, the nutrients travel through the portal circulation to the liver where other mechanisms do their stuff:

Carbohydrates

Glucose is essential for neuronal function and, if levels fall too low, can cause coma/death. Overly high levels can also cause widespread damage, especially to blood vessels.

The liver stores glucose through glycogenesis in the form of glycogen, which can be broken back down to glucose by glycolysis – with enough spare for around 18hours normal activity. It also forms new glucose from amino acids in a process called gluconeogenesis.

This processes are all regulated by insulin and glucagon, secreted by the pancreas.

Amino acids and proteins

Essential amino acids cannot be synthesised in high enough quantities within the body, so must be derived from the diet. The liver produces a full complement of amino acids by transamination and other modifications to dietary aminos, and exports them for protein synthesis throughout the body.

Excess aminos are converted to ammonia and then to urea and excreted in the urine. Lack of aminos can be dealt with through the liver metabolising muscle proteins, etc, in order to provide essential energy.

Excess acids are converted to ketones, which can be used for energy through the citric acid cycle, for example in neurones, in fasting, but is not generally a useful source of energy.

The liver produces many proteins, including albumin which makes up 50% of plasma proteins keeping osmotic pressure balanced. It leaks at about 5% an hour and collected by lymphatics and returned to the circulation at the thoracic duct – known as the transcapillary escape rate. Important, used for binding with stuff like warfarin, free radicals. Also has some kind of anticoagulant effect. Decrease is caused by decrease in synthesis, increased catabolism, increased loss.

The liver also produces coagulation factors such as Prothrombin – this only last a few hours in the circulation, so the Prothrombin time is one of the most sensitive measures of rapidly deteriorating liver function.

Lipids

Dietary lipids (eg. Chylomicrons) are taken up from the circulation by the liver and broken down to component parts (ie. fatty acids, phospholipids and cholesterol). The liver then repackages them as lipoproteins. The main types exported from the hepatocyte are very-low-density-lipoproteins (VLDLs) and high-density-lipoproteins (HDLs).

The majority of body cholesterol is also produced in the liver, and most comes from hepatic synthesis rather than diet. Statin drugs, which work at reducing hypercholesterolaemia, work by inhibiting HMG-CoA reductase, which reduces the rate of production of cholesterol. Cholesterol in used to synthesise bile salts.