We've said that in the systemic circulation, blood from the tissues drains straight back into the right side of the heart via the veins.

In fact, there is one notable exception to this general arrangement. This is the portal circulation.

The portal circulation is an offshoot of the systemic circulation in which all the blood from the gut first of all flows to the liver, via the portal vein. Only after passing through the liver does the blood enter the systemic veins, which return it to the heart in the normal way.

One of the liver’s most important functions is to store and modify nutrients from food. It also plays an important role in removing toxins from the blood. The portal circulation ensures that blood from the gut goes directly to the liver before going to the rest of the body, so that the liver can carry out these key tasks.