Page 10, CV Review, Essentials, Develop. Physiol., Dr. D. Penney

Misconception 11. Pressure is the only form of energy determining blood flow.

Blood flow is a function of total fluid energy (Figure 1.06). Total fluid energy, in turn, is a function of pressure (potential) energy and velocity (kinetic) energy. The relevant pressure in this case is perfusion (driving) pressure, the difference between the transmural pressures at two points in a conduit. Kinetic energy is a function of the square of the flow velocity. Thus when velocity doubles, the kinetic energy portion of total fluid energy increases 4-fold.

It is the high kinetic energy of the blood during ventricular ejection that keeps the aortic valve open during the latter part of this phase of the cardiac cycle. Thus, it is possible for flow to occur in the seemingly strange manner of taking place from an area of lower to one of higher pressure.

Ordinarily in the arterial circulation kinetic energy is only a small fraction of pressure energy, since the pressure is so high; however, in veins where pressure is low, kinetic energy may become a significant fraction of total fluid energy.

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