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 sup-ply 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 de-graded waste products that are delivered back to the intestine for disposal. It also contains substances that bind to metabolites in the intes-tine to aid absorption. A series of ducts of increasing diameter and complexity, beginning with bile 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 and  -globulins, prothrombin, and glycoproteins, including fibronectin. Glucose, released from stored glycogen, and triiodothy-ronine (T3), 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.
Liver, human, H&E ×65; inset ×65.
At the low magnification shown here, large numbers of hepatic cells appear to be uniformly disposed throughout the specimen. The hepatocytes 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 below (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 tis-
Liver lobule, liver, human, H&E ×160.
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 66.
It is best to examine low-magnification views of the liver to define the boundaries of a lobule. A lobule is best identifed when it is cut in cross section. The central vein then appears as a circular profle, and the hepatic sue 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 cuboidal 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.