Portal Hypertension Pathophysiology

Portal hypertension is a condition that occurs when there’s abnormally high pressure in the portal venous system, which is a network of vessels that drain blood from the abdomen into the liver.

As a quick review, the portal vein is formed by the splenic vein merging with the superior and inferior mesenteric veins, so it receives blood from the spleen, stomach, pancreas, and small and large intestines.

This blood contains all the nutrients absorbed in the GI tract, as well as ingested medications and potential toxins.

The slow, low-pressure flow of blood provides adequate time for the liver to extract nutrients, metabolize medications, and remove toxins before they reach systemic circulation.

Once the liver processes the blood it receives, it flows through the hepatic veins to the inferior vena cava, and to the right atrium of the heart. The blood then flows through the heart and lungs, and oxygenated blood is pumped out to the rest of the body.

Now, portal hypertension is caused by any condition that obstructs blood flow in the portal venous system, and can be prehepatic, intrahepatic, or posthepatic.

Prehepatic causes are those that result in obstruction of blood flow before reaching the liver such as splenic vein thrombosis, or a clot within the splenic vein.

Intrahepatic causes are those which result in an obstruction within the liver. One of the most common causes is cirrhosis, a condition where healthy liver tissue is replaced by scar tissue resulting in increased resistance to blood flow through the liver.

Other causes include certain infections like hepatitis or schistosomiasis, a parasitic infection that can affect multiple organs, including the liver.

Lastly, posthepatic causes result in obstruction of blood flow from the liver, like right-sided heart failure where the right ventricle is not pumping effectively, causing blood to back up into the liver.

Okay, so when an obstruction to blood flow develops, pressure builds and blood backs up in the portal system, including the splenic and mesenteric circulation, dilating the venous channels before the obstruction.

In response to the increased pressure, portosystemic shunting occurs, meaning new pathways for the blood flow are formed, called collateral vessels, that divert blood flow around the portal venous system.

These collateral vessels involve the dilation and enlargement of existing veins along with some newly formed veins and occur at three points where the systemic venous system and the hepatic portal system are connected.

First, in the esophagus, esophageal varices, or fragile enlarged esophageal veins form.

Then in, the round ligament of the liver, which used to be part of the umbilical vein during fetal life, opens, allowing blood to flow into the systemic veins of the abdomen.

The round ligament collapses after birth, but these veins can dilate, and appear to radiate out from the belly button, called caput medusae.

Finally, in the rectum and anal canal, hemorrhoids form.

Portal hypertension can also cause blood to back up into the spleen, causing splenomegaly, or an enlarged spleen. This causes hypersplenism, meaning the spleen traps blood elements like red blood cells, white blood cells, and platelets.