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Anatomy and physiology of the liver related to blood flow and biochemistry

The Liver and Blood Flow 

Systemic circulation: Oxygenated blood that has returned from the lungs to the left ventricle of the heart is pumped to all of the tissues of the body. 

Pulmonary circulation: After reaching the tissues, blood is returned to the right side of the heart, from where it is pumped to the lungs and then returned to the left side of the heart after taking up oxygen and giving off carbon dioxide. 

Portal circulation: Blood from the gut and spleen flow to and through the liver before returning to the right side of the heart. The large vein through which blood is brought to the liver is called the portal vein. After passing through the liver, blood flows into the hepatic vein, which leads into the inferior vena cava to the right side of the heart. The liver also receives some blood directly from the heart via the hepatic artery. In the esophagus, stomach, small intestine and rectum, the portal circulation and veins of the systemic circulation are connected. Under normal conditions, there is little to no back flow from the portal circulation into the systemic circulation. 

The Liver and Biochemical Functions 
The liver performs many biochemical functions. Blood clotting factors are synthesized in the liver. Albumin, the major protein in the blood, is also synthesized in and secreted from the liver. The modification and/or synthesis of bile components also takes place in the liver. Many of the body's metabolic functions occur primarily in the liver including the metabolism of cholesterol and the conversion of proteins and fats into glucose. The liver is also where most drugs and toxins, including alcohol, are metabolized. 

Bilirubin Secretion 
The liver is the site of bile formation. Bile contains bile salts, fatty acids, cholesterol, bilirubin and other compounds. The components of bile are synthesized and modified in hepatocytes (the predominant cell type in the liver) and secreted into small bile ducts within the liver itself. These small bile ducts form a branching network of progressively larger ducts that ultimately become the common bile duct that takes bile to the small intestine. Bilirubin is a yellow pigment that derives primarily from old red blood cells. Bilirubin is taken up by hepatocytes from the blood, modified in the hepatocytes to a water soluble form and secreted into the bile. 

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