Let's say that the cross-sectional area of the aorta is 1 cm2 = 100 mm2. The blood pumps through the aorta at 100 mm/s but only through the capillaries at 1 mm/s. If each capillary has a cross-sectional area of 20 mm2, how many capillaries must there be in the body?
Blood is pushed out of the heart through a large vessel, called the aorta. The aorta then divides into smaller and smaller blood vessels, eventually reaching vessels called capillaries. Capillaries are so small that oxygen and nutrients can diffuse across the vessel walls and into the tissues of the body, with CO2 and waste diffusing back into the blood.
Let's say that the cross-sectional area of the aorta is 1 cm2 = 100 mm2. The blood pumps through the aorta at 100 mm/s but only through the capillaries at 1 mm/s. If each capillary has a cross-sectional area of 20 mm2, how many capillaries must there be in the body?
Note: these numbers are not physiologically correct, but were chosen to make the math easier. The blood velocity through the aorta is closer to 300 mm/s. The capillaries have a total cross-sectional area of ~600,000 mm2 (6000 cm2), with a blood flow rate closer to 0.2 mm/s.
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