Portal hypertension

Question

"Portal hypertension is more frequent and manifests in more complex ways in chronic liver failure than in acute liver failure (Fig. 14.7). It stems from increased vascular resistance coupled with increased portal blood flow. The increased resistance to portal flow is at the level of the sinusoids and is caused by contraction of vascular smooth muscle cells and myofibroblasts, and disruption of blood flow by scarring and the formation of parenchymal nodules. Increase in portal venous blood flow is due to arterial vasodilation. The increased splanchnic arterial blood flow in turn leads to increased venous efflux into the portal venous system." Robbins & Kumar Basic Pathology.

In liver cirrhosis, the blood neither from the hepatic artery nor from the portal vein are able to enter the liver normally, hence blood backs up in both, right? With the following: "Increase in portal venous blood flow is due to arterial vasodilation", how does the arterial vasodilation cause an increase in portal venous blood flow? Which arteries is it talking about? If for example the hepatic arteries are in vasodilation, more blood fits them hence less blood would go into the portal venous system, so how would there be an increase in the portal venous blood flow? Am I simply missing the picture of the circulatory system around the liver?

Moreover, with "The increased splanchnic arterial blood flow in turn leads to increased venous efflux into the portal venous system", why did the splanchnic arterial blood flow increase? Venous efflux from where and why into the portal venous system?

Many thanks for any help!

Answer 1

Splanchnic refers to all the abdominal cavity organs, not just the liver.

The portal vein system carries blood from these organs to the liver instead of directly back to the heart. The more blood flows to these organs, the more goes through the portal system. Vasodilation increases blood flow because there is less resistance when the vessels are wider.

This is all a compensatory mechanism gone a bit wild; there's not enough flow through the portal system. Normally, arterial tone sets flow, so normally if there's not enough flow it's because the arteries need to relax. As they relax it does increase flow, but because it isn't actually the arteries that are the problem (the problem is the pathology in the liver) there is even more hypertension in the portal system.

Remember, too, that veins are low-pressure vessels. Hypertension in the veins is a problem at much lower pressures than normal pressure in the arteries.