Veins and valve anatomy. Animation showing the anatomy of veins, the blood vessels that bring blood from the body back towards the heart. The animation starts with a view of the internal tissues of human legs (including bones and muscles), zooming in on the blood vessels. These are the arteries (red) and veins (blue). A vein is shown in longitudinal sectioned view, with blood (red) passing through a valve. Blood is pushed through the valve by blood pressure from the beating heart, and from surrounding pressure from tissues. As the upwards pressure falls and the blood begins to flow back downwards, the flaps of the valve close to prevent the continued backwards flow. This animation uses a transparent alpha channel.