Pressure Flow and Resistance

The last task in this survey of the design of the cardiovascular system is to introduce the concepts of pressure, flow, and resistance. In all parts of the system, blood flow (F) is always from a region of higher pressure to one of lower pressure. The pressure exerted by any fluid is termed a hydrostatic pressure, but this is usually shortened simply to "pressure" in descriptions of the cardiovascular system and denotes the force exerted by the blood. This force is generated in the blood by the contraction of the heart, and its magnitude varies throughout the system for reasons to be described in subsequent sections. The units for the rate of flow are volume per unit time, usually liters per minute (L/min). The units for the pressure difference (AP) driving the flow are millimeters of mercury (mmHg) because historically blood pressure was measured by determining how high a column of mercury could be driven by the blood pressure.

It must be emphasized that it is not the absolute pressure at any point in the cardiovascular system that determines flow rate but the difference in pressure between the relevant points (Figure 14-10).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

14. Circulation

© The McGraw-Hill Companies, 2001

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

Flow rate = 10 ml/min

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