Liver performs many vital functions in the body. From making carbohydrates, proteins and fats to synthesizing bile to help digest food. As such, the liver requires significant blood supply to carry out its functions properly.
Ingested food that enters intestinal tract contains nutrients and toxins as part of ingested food. Before these nutrients are released into systemic blood stream, the blood goes through a detoxification process which is mainly carried out by liver. About 75% of blood entering the liver consists of venous (oxygen-deplated) blood. Blood to the liver is supplied through portal venous system, a system of veins coming from stomach, intestine, spleen, and pancreas. These veins merge into heptatic portal vein which then branches into smaller vessels and travel througout the liver. All of the blood returning from small intestine, stomach, pancreas and spleen converges into the portal vein. One consequence of this is that the liver gets "first pickings" of everything absorbed in the small intestine, which is where virtually all nutrients are absorbed. The liver detoxifies the blood through a network of microscopic vessels known as sinusoids and sends it out into the main circulatory system. Portal vein is the main vein that carries deoxegenated and prefiltered blood from digestive tract and its related organs to the liver. A blood pressure greater than 13 H2O, or 10 mm Hg in portal vein is referred to as portal hypertension.
There are two main causes of portal hypertension.
An increased volume of blood flowing through the vessels
Increased resistance to the blood flow through the liver
Increased volume of blood flowing through the vessels (increased portal flow) occurs when the portal veins attach to arteries, as they do in an arteriovenous fistula (where a new passage is formed between a vein and an artery), or it can occur as a result of blood being diverted (shunted) from the arteries to the liver. Increased resistance to blood can occur in the portal vein prior to its entry into the liver (prehepatic); in the portal vein inside the liver (hepatic); or, it can occur in the hepatic veins in the inferior vena cava (the largest vein in the body, which feeds blood from the lower body to the heart), after blood has exited the liver (posthepatic).
In either of these conditions, portal hypertension causes the formation of multiple portosystemic shunts (PSS), a condition in which the circulatory system bypasses the liver. Increased pressure in the portal blood vessels may cause protein-containing (ascitic) fluid from the surface of the liver and intestine to leak and to accumulate within the abdomen. This condition is called ascites. A more serious complication is the development of hepatic encephalopathy in which the toxins travel to brain via blood stream and cause seizures and problems with mobility.
Common symptoms related to portal hypertension include
Yellow skin and eyes (jaundice)
Secondary hepatic encephalopathy
Portal vein blocked by a blood clot
Lack of energy
Lack of appetite