The heart is a remarkable device. It's a fully automatic pump capable of pumping up to 35 litres of blood a minute under special circumstances. It could fill an ordinary tub in less than 10 minutes.
Other parts of the circulatory system are not quite so automatic; they need a little help from the muscles, again in the shape of movement, for it is their job to pump the blood back to the heart by pressing on the veins.
The circulatory system is a closed system and must run non-stop without interruption. The vene pump, as it is called, needs help all the time when you are sitting still. It's important to use the leg muscles so that they are neither tight nor relaxed for any length of time.
We are also subject to the Law of Gravity. The whole system has to work harder when we're sitting or standing....and the direction of the heart is up.