Suppose that in response to some stimulus a small blood vessel narrows to \(90 \%\) of its original diameter. If there is no change in the pressure across the vessel, what is the ratio of the new volume flow rate to the original flow rate?

A. 0.66

B. 0.73

C. 0.81

D. 0.90

The blood pressure at your heart is approximately \(100 \mathrm{~mm} \mathrm{Hg}\). As blood is pumped from the left ventricle of your heart, it flows through the aorta, a single large blood vessel with a diameter of about \(2.5 \mathrm{~cm}\). The speed of blood flow in the aorta is about \(60 \mathrm{~cm} / \mathrm{s}\). Any change in pressure as blood flows in the aorta is due to the change in height: the vessel is large enough that viscous drag is not a major factor. As the blood moves through the circulatory system, it flows into successively smaller and smaller blood vessels until it reaches the capillaries. Blood flows in the capillaries at the much lower speed of approximately \(0.7 \mathrm{~mm} / \mathrm{s}\). The diameter of capillaries and other small blood vessels is so small that viscous drag is a major factor.