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What special features enable the liver to do so much?

The liver has many special features. For example, in order to carry out its secretory functions, ducts (tubes) closely connect it to the gallbladder and intestines. Thus, bile made by the liver travels through these tubes to the gallbladder. The bile is stored in the gallbladder between meals, and then is discharged into the intestines at mealtime to aid in digestion.

For another example, the liver is appropriately situated in the body to directly receive the blood that comes from the intestines (portal blood). With this arrangement, the liver can readily process (metabolize) nutrients absorbed from food as well as other contents of the portal blood. Indeed, because of its numerous biochemical functions, the liver is considered the biochemical factory of the body.

What's more, the liver is organized strategically to coordinate its structure, including its blood circulation, with its functions. Four key features of this organization of the liver are as follows.

  1. The basic unit of the liver is called an acinus (pronounced as' i-nus: plural acini). (There are numerous acini in the liver.) In each acinus, the liver cells (hepatocytes) are grouped into three zones that are anatomically related to the liver's blood supply and drainage. Thus, the blood enters zone one first, and then travels through the second and third zones before leaving the liver. Each zone has its own special functions to perform. (Moreover, because of these different functions, as well as the different relationships to the flow of blood, the zones have different susceptibilities to injury.)
  2. Specialized areas of the walls of adjacent liver cells (hepatocytes) join to form bile canaliculi (pronounced kan" ah-lik' u-li). The canaliculi are microscopic tubes that transport bile that is produced by the liver cells (hepatocytes). Then, meeting with other canaliculi, they ultimately empty into tiny bile ducts. These bile ducts join with other bile ducts to form larger bile ducts that ultimately leave the liver.
  3. The liver has a unique, dual blood supply. One comes from the portal vein, as already mentioned, and the other from the hepatic artery. The hepatic artery brings to the liver oxygenated blood that comes from the lungs, heart, and branches of the aortic artery. So, finally, tiny branches of the portal vein and hepatic artery travel in the liver together with the tiny bile ducts in tracts called portal tracts (triads).
  4. The hepatic artery supplies blood to nourish the bile ducts and the liver cells (hepatocytes). This blood joins with the portal vein blood in tiny blood vessels called sinusoids. Now, these sinusoids are situated on each side of single-cell-thick plates of liver cells (hepatocytes), and they have an exceptionally porous (hole-filled) lining (epithelium). This unique arrangement enables passage of even large molecules (for example, lipoproteins) through the sinusoidal lining to and from the liver cells (hepatocytes). The blood travels in the sinusoids through the three acinar zones. Finally, the blood is drained from the liver by the hepatic veins and then heads back to the heart and lungs.

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