The editors would like to acknowledge that this chapter is abridged from a chapter originally written by Dr. Michael Ramsay.
HEPATIC PHYSIOLOGY & ANESTHESIA
Normal hepatic blood flow is 25–30% of the cardiac output and is provided by the hepatic artery and portal vein. The hepatic artery supplies approximately 30% of the blood supply and 50–70% of the liver’s oxygen requirements, and the portal vein supplies 70% of the blood supply and the remaining 30–50% of the liver’s oxygen requirements. Hepatic arterial flow is dependent on metabolic demand (autoregulation), whereas flow through the portal vein is dependent on blood flow to the gastrointestinal tract and the spleen. A reciprocal, though somewhat limited, mechanism exists, such that a decrease in either hepatic arterial or portal venous flow results in a compensatory increase in the other.
The hepatic artery has α1-adrenergic vasoconstriction receptors as well as β2-adrenergic, dopaminergic (D1), and cholinergic vasodilator receptors. The portal vein has only α1-adrenergic and dopaminergic (D1) receptors. Sympathetic activation results in vasoconstriction of the hepatic artery and mesenteric vessels, decreasing hepatic blood flow. β-Adrenergic stimulation vasodilates the hepatic artery; β-blockers reduce blood flow and therefore decrease portal pressure. The drug vasopressin causes a reduction in splanchnic blood flow.
The abundance of enzymatic pathways in the liver allows it to play a key role in the metabolism of carbohydrates, fats, proteins, and other substances. The final products of carbohydrate digestion are glucose, fructose, and galactose. With the exception of the large amount of fructose that is converted by the liver to lactate, the hepatic conversion of fructose and galactose into glucose makes glucose metabolism the final common pathway for most carbohydrates.
The liver and kidney are unique in their capacity to form glucose from lactate, pyruvate, amino acids (mainly alanine), and glycerol (derived from fat metabolism). Hepatic gluconeogenesis is vital in the maintenance of a normal blood glucose concentration. Glucocorticoids, catecholamines, glucagon, and thyroid hormone greatly enhance gluconeogenesis, whereas insulin inhibits it.
The liver performs a critical role in protein metabolism. The steps involved in protein metabolism include (1) deamination of amino acids, (2) formation of urea (to eliminate the ammonia produced from deamination), (3) interconversions between nonessential amino acids, and (4) formation of plasma proteins. Deamination is necessary for the conversion of excess amino acids into carbohydrates and fats. The enzymatic processes, most commonly transamination, convert amino acids into their respective keto acids and produce ammonia as a byproduct.
Ammonia formed from deamination (as well as that produced by colonic bacteria and absorbed through the gut) is highly toxic to tissues. Through a series of enzymatic steps, the liver combines two molecules of ammonia with CO2 to form ...