What are the liver and bile ducts and what do they do?
The liver is a large triangular shaped organ located in the right upper abdomen beneath the diaphragm. Two blood vessels supply blood to the liver -- the hepatic artery and the portal vein. The hepatic artery carries blood from the heart and the portal vein carries blood that comes from the intestinal tract. The liver is the largest solid organ in the body and serves multiple functions including but not limited to production of bile to aid in digestion of fats from the diet, detoxifying metabolic waste products, storage of sugars and amino acids, protein production, and hormone production. Currently, there are no artificial processes or machines that can replace the functions of the liver.
The bile ducts transport bile within and outside the liver. Bile is first produced by liver cells (hepatocytes). The bile is then secreted into small bile ducts inside the liver which then flows down through a tree-like network of ducts until they meet and form the right and left hepatic duct. As the bile continues to flow downward, it is then stored in the gallbladder. When we eat, the gallbladder contracts and squeezes the stored bile out of the gallbladder causing the bile to flow down the common bile duct into the small intestine.
What are liver function tests?
Liver function tests are a group of laboratory tests that can be used to evaluate the function of the liver and the bile ducts. The labs include the bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase, albumin, prothrombin time/International normalized ratio (PT/INR). These labs are often ordered during routine visits with your primary care provider in the absence of any symptoms. However, they are also often ordered when people have symptoms of abdominal pain, nausea, vomiting, or jaundice. These tests are also frequently monitored while on certain medications that can affect the liver.
The bilirubin is a metabolic byproduct that results from the breakdown of red blood cells. Bilirubin circulates in the bloodstream after it is produced by the breakdown of red blood cells that occurs in both the liver and spleen. Both injury and/or dysfunction of liver cells as well blockage of bile ducts can cause the bilirubin levels to rise. There are also many other conditions outside of the liver/bile ducts that can cause bilirubin levels to rise
The AST and ALT represent enzymes that are present in the cells of the liver (called hepatocytes) that help drive chemical reactions in the liver. When the hepatocytes are injured or damaged, these enzymes are released and can be detected in the bloodstream. Therefore, elevation of the AST and ALT can represent inflammation, injury, or damage to the liver.
Alkaline phosphatase is an enzyme that is present in the liver, bile ducts, bones, and in pregnant women the placenta. As a liver function test, elevations in this enzyme level can be due to injury to or blockage of the bile ducts that alters flow of bile through the ducts.
Lastly, the liver is also an organ that produces many proteins. When the liver is injured, the ability of the organ to produce these proteins may be compromised. This may result in diminished production of a common protein in the blood called albumin. The production of other proteins can also be affected. For example, a number of the proteins essential to normal blood clotting are produced by the liver. Diminished production of these proteins can cause abnormal blood clotting parameters such as the PTT/INR, which are lab tests that test the ability of blood to appropriately clot.