Liver...The Largest Gland in the Body!

Medical Author: Leslie J. Schoenfield, M.D., Ph.D.
Medical Editor: Jay W. Marks, M.D.

The liver is the largest solid organ in the body. I think a lot of people probably know that. But they may not know that it is also the largest gland in the body. You see, the liver is also considered a gland because, among its various functions, it makes and secretes bile. (Just for your reference, the stomach and intestine are large hollow organs. Glands are organs or parts of organs that make and secrete substances. And bile is a fluid that both aids in digestion and transports fats as well as waste products into the intestine.)

Actually, there are all sorts of glands in the body that make and secrete substances, including the pancreas (digestive enzymes), thyroid and other endocrine glands (hormones), gastric glands in the stomach (acid), and lymph glands or nodes (lymph). The liver is even larger, I think, than most mammary glands (milk).

• So, how large is the liver?
• Where is the liver located?
• Just what does the liver do?
• What special features enable the liver to do so much?
• What diseases affect the liver?
• How do liver diseases cause symptoms?
• What about blood tests for the diagnosis of liver disease?
• Why does the doctor examine the liver?
• What about liver biopsy?
• What else can I say about the liver?

So, how large is the liver?

The liver weighs about three and a half pounds (1.6 kilograms). It measures about 8 inches (20 cm) horizontally (across) and 6.5 inches (17 cm) vertically (down) and is 4.5 inches (12 cm) thick.

You know, sometimes I'm surprised by how little information some people have about the liver. Therefore, in this article, I want to relate a few other special facts about the liver, which, I must admit, is my favorite organ.

Where is the liver located?

The liver is located just below the diaphragm (the muscular membrane separating the chest from the abdomen), primarily in the upper right part of the abdomen, mostly under the ribs. However, it also extends across the middle of the upper abdomen and part way into the left upper abdomen. An irregularly shaped, dome-like solid structure, the liver consists of two main parts (a larger right lobe and a smaller left lobe) and two minor lobes. As you can see in the diagram below, the upper border of the right lobe is at the level of the top of the 5th rib (a little less than 1/2 inch below the nipple) and the upper border of the left lobe is just below the 5th rib (about 3/4 inch below the nipple). During inspiration (breathing in), the liver is pushed down by the diaphragm and the lower edge of the liver descends below the margin of the lowest rib (costal margin).

Just what does the liver do?

The liver has a multitude of important and complex functions. Some of these functions are to:

• Manufacture (synthesize) proteins, including albumin (to help maintain the volume of blood) and blood clotting factors
  
  
• Synthesize, store, and process (metabolize) fats, including fatty acids (used for energy) and cholesterol
  
  
• Metabolize and store carbohydrates, which are used as the source for the sugar (glucose) in blood that red blood cells and the brain use
  
  
• Form and secrete bile that contains bile acids to aid in the intestinal absorption (taking in) of fats and the fat-soluble vitamins A, D, E, and K.
  
  
• Eliminate, by metabolizing and/or secreting, the potentially harmful biochemical products produced by the body, such as bilirubin from the breakdown of old red blood cells and ammonia from the breakdown of proteins
  
  
• Detoxify, by metabolizing and/or secreting, drugs, alcohol, and environmental toxins

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). (There are numerous acini in the liver.) In each acinus, the liver cells (hepatocytes) are grouped into 3 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 (e.g., 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.