A vein tranfers clean blood from the heart through the whole body and since clean blood carries oxygen it helps transfer oxygen through the whole body too! ~Allyssa!
Well they do alot of things:1) they help transport blood to your heart and brain2) they help keep your blood in one place so that if you get a bruise or cut all of your blood wont "gush" out. I hope...
The hepatic portal vein is created by the anastamoses between the superior mesenteric vein and spleinc vein. It is arguable that the inferior mesenteric vein joins at this same anastomoses but...