And so what happens is this increases and expands the cross sectional area of our vein and therefore increases the amount of volume that can pass along that given vein. It also ensures that there isn't a build up of pressure within our veins. And that's exactly why in arteries we have a high pressure because the walls can easily push back. But in veins, the walls cannot push back. So we have expansion taking place and the pressure remains at a relatively low value. So in A, we have a relaxed vein, but in this diagram, let's call it B when the blood flows and easily pushes against, it expands, but the walls cannot push back and so it remains expanded and the volume increases.