<P> The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans . In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity . </P> <P> A typical human red blood cell has a disk diameter of approximately 6.2--8.2 μm and a thickness at the thickest point of 2--2.5 μm and a minimum thickness in the centre of 0.8--1 μm, being much smaller than most other human cells . These cells have an average volume of about 90 fL with a surface of about 136 μm, and can swell up to a sphere shape containing 150 fL, without membrane distension . </P> <P> Adult humans have roughly 20--30 trillion red blood cells at any given time, constituting approximately 70% of all cells by number . Women have about 4--5 million red blood cells per microliter (cubic millimeter) of blood and men about 5--6 million; people living at high altitudes with low oxygen tension will have more . Red blood cells are thus much more common than the other blood particles: there are about 4,000--11,000 white blood cells and about 150,000--400,000 platelets per microliter . </P> <P> Human red blood cells take on average 60 seconds to complete one cycle of circulation . </P>

How many red blood cells are in the body