The liver, as shown in Fig. 32.2, has a high blood flow compared to its size, ensuring that hepatocytes receive ample amounts of oxygen and nutrients. Hepatocytes receive more than 70% of their blood supply at rest through the portal vein, which increases to over 90% after a meal. The liver parenchyma, made up of hepatocyte plates, is supplied by sinusoids, low-resistance cavities that receive blood from the portal vein and hepatic artery. These sinusoids are different from capillaries found in other organs. While some sinusoids are collapsed during fasting, more can be recruited as portal blood flow increases after a meal. The sinusoids allow for increased blood flow without a significant increase in pressure, eventually draining into the central branches of the hepatic vein.
The sinusoids have unique endothelial cells, as illustrated in Fig. 32.3. These cells have fenestrations, specialized openings that permit the passage of large molecules like albumin. Unlike other endothelial cells, sinusoidal endothelial cells lack a basement membrane, which would hinder diffusion. These features enable albumin-bound substances to reach the hepatocytes. Kupffer cells are also present in the sinusoids. Beneath the sinusoidal endothelium, there is a thin layer of loose connective tissue called the space of Disse. This space poses little resistance to the movement of molecules, including albumin. Additionally, the space of Disse houses stellate cells, which store retinoids and produce essential growth factors for hepatocytes.
Under abnormal conditions, stellate cells become activated and produce significant amounts of collagen, contributing to hepatic dysfunction.
Fig. 32.2 ). The liver receives a high blood flow that is disproportionate to its mass, which ensures that hepatocytes receive high quantities of both O2 and nutrients. Hepatocytes receive more than 70% of their •Fig. 32.2 Diagrammaticrepresentationofahepaticlobule.Platesofhepatocytesarearrayedradiallyaroundacentralvein.Branchesoftheportalveinandhepaticarteryarelocatedontheperipheryofthelobuleandformthe“portaltriad”togetherwiththebileduct.Bloodfromtheportalveinandhepaticarterypercolatesaroundthehepatocytesviathesinusoidsbeforedrainingintothecentralvein.(ModifiedfromBloomW,FawcettDW.A Textbook of Histology. 10thed.Philadelphia:Saunders;1975.) blood supply at rest via the portal vein (rising to more than 90% in the postprandial period).
The plates of hepatocytes that constitute the liver parenchyma are supplied by a series of sinusoids, which are low-resistance cavities supplied by branches of both the portal vein and hepatic artery. The sinusoids are unlike the capillaries that perfuse other organs. During fasting, many sinusoids are collapsed, but more can gradually be recruited as portal blood flow increases during the period after a meal when absorbed nutrients are transported to the liver. The low resistance of the sinusoidal cavities means that blood flow through the liver can increase considerably without •Fig. 32.3 Interrelationshipsofthemajorcelltypesintheliver.
a concomitant increase in pressure. Eventually the blood drains into central branches of the hepatic vein.
The sinusoids are also unusual in terms of the endothelial cells that line their walls (
Fig. 32.3 ). Hepatic endothelial cells contain specialized openings known as fenestrations that are large enough to permit passage of molecules as big as albumin. Sinusoidal endothelial cells also lack a basement membrane, which might otherwise pose a diffusion barrier. These features allow access of albumin-bound substances to the hepatocytes that will eventually take them up. The sinusoids also contain Kupffer cells. Beneath the sinusoidal endothelium and separating the endothelium from the hepatocytes is a thin layer of loose connective tissue called the space of Disse, which in health likewise poses little resistance to the movement of molecules even as large as albumin. The space of Disse is also the location of another important hepatic cell type, the stellate cell. Stellate cells serve as storage sites for retinoids and in addition are the source of key growth factors for hepatocytes.
Under abnormal conditions, stellate cells are activated to synthesize large quantities of collagen, which contributes to the hepatic dysfunction.