In Section 4

In Section 4.2.3 we concluded that the pumping power of the human heart is approximately 2 W (assuming a normal cardiac output of 5 liters/min). Like any pump, however, the heart is not 100% efficient, and, therefore, the power that is supplied to the heart muscle will actually exceed 2 W. In this question we will estimate the heart’s pumping efficiency η by calculating how much energy is supplied to the heart muscle from the blood. (Note that the heart muscle has its own vasculature, called the coronary circulation.)

(a) When we calculated the pumping power of the heart, we only considered the head gain across the heart from the pressure increase. In general, there could also be changes in elevation (very small) and in kinetic energy. Estimate the ratio of kinetic energy head to pressure head at peak systole, when pressure is 120 mmHg and blood velocity is 100 cm/s. Can we safely neglect kinetic energy gains in calculating pumping power?

(b) At rest, the coronary blood flow is 225 ml/min, and 65% of the O₂ is removed from the blood as it passes through the coronary vasculature. The oxygen capacity of blood is 19.4 ml O2/100 ml blood, and in a normal diet 4.83 kcal of food energy is released for every liter of O₂ consumed. From this data, estimate η for the heart. State assumptions.

(c) The basal metabolic rate of a normal individual is 72 kcal/h. What fraction of this is consumed by the heart?