The liver is the largest mass of glandular tissue in the body and the largest internal organ. It is unique because it receives its major blood supply from the hepatic portal vein, which carries venous blood from the small intestine, pancreas, and spleen. Thus the liver is directly in the pathway that conveys materials absorbed in the intestine. This gives the liver the first exposure to metabolic substrates and nutrients; it also makes the liver the first organ exposed to noxious and toxic substances absorbed from the intestine. One of the major roles of the liver is to degrade or conjugate toxic substances to render them harmless. It can, however, be seriously damaged by an excess of such substances.
Each liver cell has both exocrine and endocrine functions. The exocrine secretion of the liver, called bile, contains conjugated and degraded waste products that are delivered back to the intestine for disposal. It also contains substances that bind to metabolites in the intestine to aid absorption. A series of ducts of increasing diameter and complexity, beginning with canaliculi between individual hepatocytes and ending with the common bile duct, deliver bile from the liver and gallbladder to the duodenum.
The endocrine secretions of the liver are released directly into the blood that supplies the liver cells; these secretions include albumin, nonimmune a- and ^-globulins, prothrombin, and glycoproteins, including fibronectin. Glucose, released from stored glycogen, and triiodothyronine (TJ, the more active deiodination product of thyroxine, are also released directly into the blood.
Functional units of the liver, described as lobules or acini, are made up of irregular interconnecting sheets of hepatocytes separated from one another by the blood sinusoids.
Figure 1, liver, human, H&E x65; inset x65.
At the low magnification shown here, large numbers of hepatic cells appear to be uniformly disposed throughout the specimen. The hepatic cells are arranged in one-cell-thick plates, but when sectioned, they appear as interconnecting cords one or more cells thick, depending on the plane of section. The sinusoids appear as light areas between the cords of cells; they are more clearly shown in Figure 2 (asterisks).
Also present in this figure is a portal canal. It is a connective tissue septum that carries the branches of the hepatic artery (HA) and portal vein (PV), bile ducts (BD), and lymphatic vessels and nerves. The artery and vein, along with the bile duct, are collectively referred to as a portal triad.
The hepatic artery and the portal vein are easy to identify because they are found in relation to one another within the surrounding connective tissue of the portal canal. The vein is typically thin walled; the artery is smaller in diameter and has a thicker wall. The bile ducts are composed of a simple cuboiclal or columnar epithelium, depending on the size of the duct. Multiple profiles of the blood vessels and bile ducts may be evident in the canal because of either branching or their passage out of the plane of section and then back in again.
The vessel through which blood leaves the liver is the hepatic vein. It is readily identified because it travels alone (inset) and is surrounded by an appreciable amount of connective tissue (CT). If more than one profile of a vein is present within this connective tissue, but no arteries or bile ducts are present, the second vessel will also be a hepatic vein. Such is the case in the inset, where a profile of a small hepatic vein is seen just above the larger hepatic vein (HV).
Figure 2, liver, human, H&E x160.
The terminal hepatic venules or central veins (CV) are the most distal radicals of the hepatic vein, and like the hepatic vein, they also travel alone. Their distinguishing features are the sinusoids that penetrate the wall of the vein and the paucity of surrounding connective tissue. These characteristics are shown to advantage in Plate 62.
It is best to examine low-magnification views of the liver to define the boundaries of a lobule. A lobule is best identified when it is cut in cross section. The central vein then appears as a circular profile, and the hepatic cells appear as cords radiating from the central vein. Such a lobule is outlined by the dashed line in Figure l.
The limits of the lobule are defined, in part, by the portal canal. In other directions, the plates of the lobule do not appear to have a limit; i.e., they have become contiguous with plates of an adjacent lobule. One can estimate the dimensions of the lobule, however, by approximating a circle with the central vein as its center and incorporating those plates that exhibit a radial arrangement up to the point where a portal canal is present. If the lobule has been cross-sectioned, the radial limit is set by the location of one or more of the portal canals as indicated by the bile ducts (BD) in this figure.
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