There is a limit to how long your neck can be. If your neck were too long, no blood would reach your brain! What is the maximum height a person's brain could be above his heart, given the noted pressure and assuming that there are no valves or supplementary pumping mechanisms in the neck? The density of blood is \(1060 \mathrm{~kg} / \mathrm{m}^{3}\).

A. \(0.97 \mathrm{~m}\)

B. \(1.3 \mathrm{~m} \quad\) C. \(9.7 \mathrm{~m}\)

D. \(13 \mathrm{~m}\)

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.