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How much it is reduced depends on the velocity of the fluid. In Fig. 5, we can consider the pressures that would be measured in the system described in Fig. 4. One sensor is arranged to face upstream so that any fluid particles striking it will be stopped. The deceleration results in the conversion of kinetic energy back to potential energy. This end pressure, therefore, measures total energy. The other sensor is arranged to open perpendicular to the axis of the flow. This sensor is said to sense the side pressure, which would be the same as that experienced by the walls of the vessel. Because we are ignoring frictional losses in our example, the end pressure is the same at all three points. Note that the side pressure is always lower than the end pressure at any point and that this difference increases with the velocity.

The difference between the end and side pressures in a flowing fluid is due to the kinetic component. The kinetic component can be calculated if one considers that the kinetic energy is equal to:

In Fig. 4, the flow through region 3 was 200 cm/sec. Thus:

= 20,000 dynes/cm2

To convert from dynes/cm2 to mm Hg, we divide by 1333. Dividing 20,000 by 1333 yields approximately 15 mm Hg, which is the difference between the end pressure and the side pressure in region 3. The concept that pressure decreases as the velocity of flow increases always seems illogical to the beginning student, but this is predicted by the Bernoulli principle, which, among other things, keeps airplanes aloft. It should be pointed out that for the most part the kinetic component is small in the cardiovascular system, reaching a maximum value of only 5 to 10 mm Hg at the aortic root where blood velocity reaches its highest value in the body. Thus, any measurement error due to an inappropriately positioned catheter would normally be negligible. In the case of high cardiac output syndromes, however, such as arteriovenous fistulas, regurgitant valves, or heavy exercise, the kinetic components can be much greater and then must be taken into account when directly measuring pressures in such individuals. Also, be aware that it is the side pressure that stretches the vessel wall and forces blood down an artery.

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