The biggest vein in the body is the ‘inferior vena cava’ – this is the vein that returns the blood from the bottom of your body to your heart 🙂
The blood is de-oxygenated, and enters the right atrium of your heart – this is at the lower right, back side of your heart. The vein is behind your abdomen and runs next to your spine… 🙂
First, lets think about the fattest (widest) vein:
Veins tend to get bigger as they get closer to the heart (because they’re draining blood from more places, so need to be able to cope with bigger flows. So, the biggest vein from the body going back into the heart is probably the last one … except that there are two “last ones”, the superior and inferior vena cavae. However, because there is more blood flow from the lower parts of the body, I’d suppose that its going to be the inferior vena cava.
However, there are a few potential tricks here …
(1) The entire blood flow has to pass back from the lungs to the heart through the pulmonary veins, so these are big, but there are several of them, so I’d guess that these are smaller
(2) Lots of blood flows to the gut (which extends from the oesophagus [the food pipe in your neck] all the way down to your rectum … the far end …) and it all has to flow through the liver, ultimately through a very big vein called the hepatic portal vein. However, this blood still has to flow into the inferior vena cava, so ..
I think its the inferior vena cava, or “IVC” for short!
Secondly, what’s the longest? Well, the IVC is fairly long (stretching almost the length of the abdomen and about half the length of the chest), but legs are quite long … so, I’ve checked it out on me (with a tape measure, using where I estimate the veins to start and end) and I think that longest vein in the body will be the longest one in the legs: the great saphenous vein.