A cell can not survive without oxygen (O2) or nutrients. A single-celled organism
can get both O2 and nutrients by simple diffusion. Since our body is built up of
~100 trillion (1014) cells, most of which never come into contact with outside air, we
need to transport both O2 and nutrients actively in order to get them to all of our
cells. The substance that mammals, like ourselves, use to accomplish this is blood.
Blood is transported through the body using blood vessels, the complex of which is
called the circulation. The movement of blood through the circulation is not a
passive process, but is driven by one of the most amazing achievements of
evolution: the heart (Fig. 1). The heart is a pump that, on average, contracts 60
times each minute at rest, a number which can increase to 200 contractions during
maximum exercise. By contracting, the heart builds up the pressure, in the
arterial side of the circulation, which is required for blood to flow.http://repub.eur.nl/res/pub/23180/110428_Beer%2C%20Vincent%20Jacob%20de.pdf
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