Animation of the heart pumping arterial blood to the legs when lying down. The heart pumps blood that is full of oxygen and food to the tissues thorough blood vessels called Arteries.

Blood then returns to the heart through the Veins.

When you are lying down, your heart pumps blood to your legs (see animation above).

The blood pumps the blood through the arteries at high pressure and so it moves quickly down the leg and into the tissues.

However getting blood back from the legs isn't so easy. There is no "heart" in the leg that can pump the heart back and so the body needs a different way to get blood back to the heart.

After the blood has been through the tissues, delivering oxygen and food to keep the cells in the legs alive, it collects together in the veins.

When you are lying down, there is just enough pressure left in the blood to get flow back to the heart.

Therefore when lying down, blood flows steadily back to the heart on its own accord.

However, as soon as you sit up or stand, things change.

This is explained on the next page.