Portal hypertension

Portal hypertension means increased blood pressure in the hepatic portal system - or portal venous system.

Most commonly, this happens because of hepatic cirrhosis, which is when the liver tissue is replaced by fibrotic, functionless tissue.

Now, the portal venous system comprises the portal vein and its tributaries - namely, the splenic, and mesenteric veins.

This blood contains all the nutrients absorbed in the GI tract, but it also carries toxins that the liver metabolizes so that they can be safely excreted by the kidneys.

Once the liver processes all these substances, it sends the blood to the heart, through the inferior vena cava, to enter the systemic venous system.

Now, there’s a few points in the boundaries of the hepatic portal system, where it could be connected with the systemic venous system that collects blood from the rest of the body: the inferior portion of the esophagus, the superior portion of the anal canal, and the round ligament of the liver - which used to be the umbilical vein during fetal life.

At birth, the umbilical cord is cut, and the umbilical vein collapses to form the round ligament.

Normally, the round ligament stays shut because pressures in the portal venous system and the systemic venous system are the same, between 5 and 10 Millimeters of Mercury

But in some situations, an obstruction may prevent blood flow from the portal vein towards the inferior vena cava.

When this happens, venous blood accumulates in the hepatic portal system, causing pressure to rise above 5 to 10 12 mmHg - which defines portal hypertension of mercury.

Portal hypertension leads to the formation of portosystemic shunts - which is when blood is diverted away from the portal venous system and backs up into systemic veins.

So first, less blood gets to the liver, causing diminished liver function and decreased blood detoxification, which leads to a buildup of toxic metabolites, like ammonia, in the blood.

Ammonia and other toxins can pass through the blood brain barrier, and cause hepatic encephalopathy.

Second, blood backing up in the systemic veins leads to portosystemic shunts, which happens in the three points where the systemic venous system and the hepatic portal system are connected.

In the esophagus, this causes esophageal varices, or enlarged esophageal veins.

In fact, portal hypertension is the most common cause of esophageal varices.

These varices are very fragile, and could easily rupture, causing massive upper GI bleeding.

In the rectum and anal canal, there may be hemorrhoids, which are also enlarged veins that can bleed as well.

Finally, portal hypertension causes the round ligament to re-channel, allowing blood from the portal system to pass into the systemic veins of the abdomen, which dilate, making the abdomen look like the head of the greek mythological creature “Medusa” - the one with snakes for hair.

So this consequence is frequently termed caput medusae.

Portal hypertension can also cause blood to back up into the spleen, causing congestive splenomegaly, meaning an enlarged spleen.

This causes hypersplenism, meaning the spleen traps blood elements like red blood cells, causing anemia, white blood cells, causing leukopenia, and platelets, causing thrombocytopenia.

Another consequence of portal hypertension is that the endothelial cells lining the blood vessels release more nitric oxide.

The reason behind this is unclear, but nitric oxide makes peripheral arteries dilate, so blood pressure drops.

This stimulates the release of aldosterone, which tries to bring blood pressure back up by telling the kidneys to retain more sodium and water.

In time, plasma volume expands so much, that fluid in blood vessels is more likely to get pushed into tissues and across tissues into large open spaces like the peritoneal cavity.

The accumulation of fluid in the peritoneal cavity is called ascites.

As if that wasn’t enough, bacteria can also invade the peritoneal cavity, causing spontaneous bacterial peritonitis.

So, the features of portal hypertension can be remembered as ABCDE: where “A” stands for Ascites, “B” for Bleeding, “C” for Caput medusae, “D” for Diminished liver function, and “E” for Enlarged spleen.

Now, causes of portal hypertension can be classified as prehepatic, intrahepatic or posthepatic, depending on where the obstruction is.