When the fluid passes into the smaller tube the velocity increases. In fact, it is increased six times; thus, the kinetic energy increases 36-fold. There is some decrease in total fluid energy due to friction. Transmural pressure decreases from 100 to 50.
In the larger tube beyond the constriction, total fluid energy is decreased to 71 from 86. Kinetic energy is 1 once again and pressure is 70. In this case the fluid is moving from an area of lower pressure to one of higher pressure, a seeming impossibility at first glance.
Recall the example of the cardiac cycle, where blood flows from an area of lower to an area of higher pressure during the second half of ventricular ejection. The ventricle continues to empty into the aorta because the total fluid energy is still greater in the ventricle.
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