Transcript:

Let's look at blood flow. So, the heart pumps oxygenated blood into the arteries. This is at high pressure. So it's simply pipe flow. The high-pressure blood is flowing in these pipes called the arteries, and eventually, these arteries keep splitting, and the point at which the oxygen from the blood is transferred to the cell, it happens via capillaries. These are tiny, tiny channels.

But the capillaries have no pressure. At the capillary, the blood goes into the cell, the oxygen goes into the cell, the waste from the cell comes back into the blood, and the capillary collects that, and now the capillary is joined to form the veins that bring the oxygen-depleted blood back to the heart.

But how does it come back to the heart? There is no pressure there. The way it comes back is ingenious.

In the veins, there are these flaps. These flaps allow the blood only to go in one direction. So, let's say the blood is going towards the heart this way; the flaps are like this. The muscles, when they move, they compress the veins and it forces the blood up, and if the blood was to go down, the flap just closes, which is why muscle activity is of primary importance for the blood to have good circulation.

It's ingenious design.