The portal vein drains most of the GI tract, from the lower parts of the oesophagus to the middle part of the rectum. The portal vein drains into the liver, where nutrient-rich blood is filtered through the hepatic sinusoids before draining into the inferior vena cava.
There are multiple points in the body where veins that drain into the portal system and veins that drain into the vena caval system anastomose. They are called porto-caval anastomoses, and can be found in:
- The oesophagus
- The rectum
- The paraumbilical region
When the pressure increases inside the portal circulation for any reason is there portal hypertension. This occurs most frequently due to an increase in the resistance of the portal circulation. This increased resistance can come from prehepatic, intrahepatic or posthepatic causes.
- Prehepatic – reduced flow in portal circulation
- Portal vein thrombosis
- Splenic vein thrombosis
- Compression due to tumor
- Intrahepatic – reduced portal flow through the liver
- Presinusoidal type
- Polycystic liver
- Hepatic metastasis
- Sinusoidal type
- Postsinusoidal type
- Veno-occlusive disease
- Presinusoidal type
- Posthepatic – reduced flow after the liver
- Inferior vena cava thrombosis
- Right-sided heart failure
The most common cause is cirrhosis, and it’s probably the only intrahepatic cause you need to know. Intrahepatic causes can be divided into presinusoidal, sinusoidal and postsinusoidal causes depending on where the obstruction is, however that’s not an important detail right now. You’ll see in topic 18 that ascites develops only in sinusoidal and postsinusoidal types.
No matter the cause are the consequences the same:
- Splanchnic congestion
- Mesenterial thrombosis
- Decreased effective circulating arterial blood volume -> hepatorenal syndrome
- See topic 18
- Portal blood is shunted through porto-caval anastomoses
- Oesophageal varices
- Rectal varices
- Caput medusae
- Toxins and stuff from the gut directly enter the systemic circulation
As the pressure increases inside the portal circulation will there be congestion of the splanchnic circulation. This causes splenomegaly with haemolysis, malabsorption and increased tendency for mesenterial thrombosis due to the slower circulation. Increased haemolysis may cause thrombocytopaenia.
In portal hypertension will there be more vasodilation in the splanchnic circulation due to increased production of vasodilators like NO. This “steals” perfusion from other organs to the abdominal organs, which causes a relative increase of blood in the splanchnic circulation and a relative decrease of blood in the non-splanchnic circulation. This causes a reduction of effective circulating arterial blood volume, which mimics hypovolaemia. Especially the renal perfusion is decreased, which is the beginning of hepatorenal syndrome.
Portal hypertension forces the portal blood through the porto-caval anastomoses. They are then called varices and are fragile and have a large probability of breaking and bleeding. As cirrhosis often involves a coagulopathy and thrombocytopaenia is often also present can ruptures oesophageal varices cause life-threatening bleeding. As portal blood is shunted through the paraumbilical veins will caput medusae be visible.
The liver usually sifts through the nutrient-rich portal blood and filters or metabolizes everything that would be harmful to the body. As more blood bypasses the liver by going through anastomoses will more harmful substances absorbed from the GI tract enter the systemic circulation directly. These substances include ammonia, short-chain fatty acids, biogenic amines and GABA. This contributes to portal encephalopathy, a component of hepatic encephalopathy. Symptoms like confusion are especially prominent after protein-rich meal, as digested proteins are partly converted to ammonia.
Treatment: Transjugular intrahepatic portosystemic shunt (TIPS) is a procedure where an artificial channel is created inside the liver that communicates between the portal vein and the hepatic vein. This relieves the portal hypertension. It’s performed in patients who have uncontrolled variceal bleeding.
16. Cirrhosis. Causes, mechanisms and consequences. Hepatic cachexia.
18. Ascites and hepatorenal syndrome