Portal vein lobule

The portal vein lobule was first recognized in the description of the portal unit given by RP. Mall (1906). It resembles a hexagon. The periportal field constitutes the axis at the centre while the central veins form the limiting points, (s. fig. 2.13) The glandular character of the liver is the main criterion of differentiation of the portal vein lobule. Thus the direction of blood flow is from the centre towards the periphery (centrifugal) and the direction of bile flow from the periphery towards the centre (centripetal). It could also be demonstrated that the lobule periphery is enclosed by basket-like ramifications of the portal vein (= corbicula portalis). (77) This further emphasizes the significance of the hepatic lobule. 

Fig. 2.13 Diagram of the classic hepatic lobule (I), the portal vein lobule (II) and the hepatic acinus (III) CV = central vein ( ), P = portal tract ( ). Flow direction venous blood (= blue arrow), arterial blood (= red arrow) and bile (= green arrow), with the microcirculatory acinus zones 1, 2, 3. (cf. W Ekataksin et af, 1992 the microvascular unit is regarded as an area in which all hver cells receive blood from a common terminal vessel)...

Classically the liver has been divided into hexagonal lobules centred around the terminal hepatic venules. Blood enters the liver through the portal tracts that are situated at the corners of the hexagon. The portal tracts are triads of a portal vein, an hepatic artery, and a common hepatic bile duct. The vast expanse of hepatic tissue, mostly consisting of parenchymal cells (PC) or hepatocytes, is serviced via terminal branches of the portal vein and hepatic artery, which enters the tissue at intervals. The hepatocytes are organized into cords of cells radially disposed about the central hepatic venule. Between these cords are vascular sinusoids that transport the blood to the central hepatic venules. The blood is collected through the hepatic venules into the hepatic vein which exits the liver into the inferior vena cava (Figure 4.1). 

The liver contains an enormous number of hepatocytes that perform the various functions noted above. The hepatocytes are contained within minute units known as hepatic lobules, in which the cell layers (which are one or two cells thick) are in contact with networks of minute blood channels - the sinusoids - which ultimately join the venous capillaries. Capillaries carrying blood from the hepatic artery and the portal vein empty separately into the sinusoids. The walls of sinusoids and liver cells are incomplete, and blood is brought into direct contact with the hepatocytes. 

Figure 28.1. Comparison of structural liver lobule with functional acinar regions. The liver lobule is centered on the terminal hepatic venule (THV), also called the central vein, and assumes a roughly hexagonal shape with its vertices at the portal triads, which contain the portal vein (PV), hepatic artery (HA), and bile duct (BD). The liver acinus is centered upon the tract of blood vessels that branch from the hepatic artery and portal vein of the portal triads. Hepatocytes within the acinus are grouped within functional zones 1,2, and 3 located at increasing distances from the vascular tracts that interconnect adjacent portal triads.

Figure 2.3. Blood circulation and tissue perfusion in the liver, a Schematic of the blood circulation. Portal vein and liver artery branch out in a parallel fashion. From the terminal branches, the blood enters the tissue and is then eollected into the tributaries of the liver vein, b The liver tissue has a honeyeomb stmcture each hexagon is a liver lobule. The liver artery and portal vein branches are located at the comers in the middle of the lobule, we find the central vein which merges with others to form the liver vein, c Higher power view, showing the sponge-like stmcture of the liver tissue. The blood gains intimate contact with virtually every liver cell - diffusional barriers are absent, and distances extremely short.

Hepatic lobule and hepatic acinus are relatively well-accepted models to describe the structure and functional aspects of the liver. Histologically, the hepatic lobule is a hexagonal region of the liver parenchyma around the central vein. Typically, six portal triads, consisting of branches from the portal vein and hepatic artery as well as bile ductules, border the edge of the lobule. Cords of hepa-tocytes are arranged radially around the central vein and blood sinusoids form between them. The hepatic parenchyma is divided into three zones based on the proximity to... 

In the porta hepatis, the proper hepatic artery divides into the right branch (from which the cystic artery emerges) and the left branch (from which a middle hepatic artery occasionally emerges). The branches of the hepatic artery run close to the portal veins and may even (rarely) coil round them in places. An arterial sphincter is located prior to the further division of the hepatic artery into smaller branches. There are anastomoses between the arterial branches and the hepatic vein. By way of an arteriolar sphincter (46), the interlobular arteries branch into intralobular arterioles, supplying the lobules of the liver with arterial blood. The arterial blood enters the sinusoids either through terminal branches or through arterioportal anastomoses and mixes with the portal blood. The pressure in the hepatic arterioles is 30-40 mm Hg. (36, 46, 61)... 

The functional and microcirculatory hepatic unit forms the basis for assessing the hepatic acinus (A. M. Rappaport, 1954). (41-43) The portal vascular bundle, with the terminal branches of the hepatic artery and portal vein diverging fan-shaped after penetrating the lobules, is at the centre of the acinar structure. These vessels represent the central axis for the circular blood supply of the related liver parenchyma. This area is roughly the shape of a rhombus, the outer angles of which are formed by the two central veins of the adjacent lobules while the diagonal... 

Figure 1 Diagram illustrating the basic anatomical unit of the liver, the liver lobule, showing (1) the radial disposition of the liver cell plates and sinusoids around the central vein, (2) the centripetal flow of blood from branches of fhe hepafic artery and portal vein, and (3) the centrifugal flow of bile (small arrows) fo fhe small bile duct in the portal space. (Reproduced from Bloom W and Fawcett DW (eds.) (1968) A Textbook of Histology, 9th edn. Philadelphia Saunders redrawn and modified from Ham, Textbook of Histology. Philadelphia Lippincott, with permission from Lippincott.)...

To understand the possibilities and limitations of liver in vitro systems it is crucial to be aware of the organization principles of this organ. The smallest functional unit of the liver is the lobule (Fig. la). The human liver is composed of approximately one million lobules. Each lobule is supplied by branches of the portal vein which carries blood from the intestine (about 80 % of the liver s blood). Moreover, arterial blood is supplied by branches of the liver artery (about 20 %). The blood enters the lobules in the periphery, passes through microvessels where it is in close contact with hepatocytes, is finally drained off into the central veins, and leaves the liver by the hepatic vein. The oxygen concentration is about 13 % v/v (60-65 mmHg) in the periportal zone and drops to about 4 % v/v (30-35 mmHg) in the central vein [3]. 

The lobes of the liver are divided into smaller lobules with a roughly hexagonal arrangement of hepatocytes around a central vein. At the vertices of the lobules are bile ducts, terminal branches of the hepatic artery, and portal veins—termed the portal triad. Connective stromal tissues extend throughout the liver, providing support for cells and routes for blood vessels, lymphatic vessels, and bile ducts. The hepatocytes form groups of cells around small branches of the portal vein, hepatic arteriole, bile duct, lymph vessel, and nerves this functional unit is called an acinus (plural acini). The acini form part of a larger structure, which can be divided into three zones ... 

The human liver consists of two lobes, each containing multiple lobules and sinusoids. The liver receives 75% of its blood supply from the portal vein, which carries blood retnrning to the heart from the small intestine, stomach, pancreas and spleen. The remaining 25% of the liver s blood snpply is arterial, carried to the liver by the hepatic artery. 

Blood from both the portal vein and hepatic artery empty into a common con-dnit, mixing their contents as they enter the liver sinnsoids (Fig. 46.1). The sinusoids are expandable vascnlar channels that rnn throngh the hepatic lobules. They are lined with endothelial cells that have been described as leaky because, as blood flows through the sinusoids, the contents of the plasma have relatively Ifee access to the hepatocytes, which are located on the other side of the endothelial cells. 

Figure 39.1 Blood enters the liver lobules via the hepatic artery and the portal vein. It leaves via the hepatic vein.

Fig. 344. Uniform distribution of glycogen in the liver lobule (CV = central vein PV = portal vein branch) an unmedicated 465 g old male Wistar rat MR 2000 (No. 2) exsanguinated under ether anaesthesia on March 14, 1977. The tissue was fixed by immersion in Carno/s fluid (ethanol-chloroform-glacial acetic acid) and embedded in Paraplast. Periodic acid-Schiff reaction. Objective Leitz PI 40/0.65. Leitz Orthomat (eyepiece 2X). Film Agfa Pan 25...

The acinus is that portion of liver supplied by a primary afferent branch of the portal vein and hepatic artery. If we look at a bell-shaped hepatic lobule hanging from and traversed by a branch of the hepatic vein, the portal space runs on its side containing the artery, the vein, and the bile duct (see Figs. 9-21 and 9-22). At some point the vein and the arteriole yield three branches, two lateral and one apical, that are the primary afferent branches. They leave the connective tissue of the portal tract to penetrate in the substance of the lobule, where they further ramify into smaller branches that enter the sinusoid of the lobule. The territory irrigated by these primary afferent arterioles, the acinus, forms a somewhat irregular ovoid mass of tissue inserted between two central veins. The tissue mass is composed mainly of hepatic cells and sinusoids lined by Kupffer s cells. The central zone of the acinus (zone 1) receives the fresh blood supply and may therefore be the first to be injured by toxins in blood. The peripheral zone (zone 3) receives a blood supply partially exhausted in its oxygen and nutrients and is therefore more susceptible to anoxemia. 

RCURE 8-1. Comparaiive structural arxl functional features of tfte Kver. In tfie hexagonal lobule model, die central vein (CVO receives blood from the portal vein via die hepatic sinusoids. 69e, on the otfier hand, flows from die c itral vein toward the portal triad (P7). The centrilobular area, whm die central vein b located, b the least ox) enated area arid contains the highest concentration of mixed fwioion oxidases (MFOs). In die functional (acinar) model, the various zones (1.2. and 3) are characteri d by dtfferent leveb of blood flow, oxidative enzymes, arxl MFOs. 

Fig. 2.15 Diagram of the liver lobule and the acinus arranged like a clover leaf around the portal field according to the acinar structure (modified from D. Sasse, t986) central hepatic vein (CV) or terminal hepatic vein, periportal field (P). Circulatory and meta-bolically different zones zone t (periportal), zone 2 (intermediate), zone 3 (perivenous)...

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