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## Module 6

HEMODYNAMICS

Dr. D. Penney

__Velocity and Pressure:__

*"Hemodynamics is the study of the movement of the blood and the forces concerned therein."*

Velocity is usually expressed in distance travelled per unit time, such as in cm/sec or m/sec, whereas flow is usually expressed in ml/sec., gal/sec, etc. The **mean velocity of blood flow** can be calculated if you know the volume flow per sec. and the cross-sectional area. For example, if the volume flow is 100 ml/sec (6 l/min) and the area is 3 square cm. - then the velocity is about 33 cm/sec. If the cross-sectional area decreased to 1 square cm., then volocity of flow in the tube would triple, to 100 cm/sec.

The cross-section of the blood vessel is assumed to be a circle, thus the area is equal to pie r^{2}. Conversely, if you know the velocity and the area, you can determine the volume flow. The velocity profile of flow is assumed to be cylindrical, whereas, in fact, it is a parabaloid, since flow is maximal on the axis and least near the wall.

**Note:** Please don't confuse the increase in velocity with decrease in cross-sectional area of a single tube with what occurs in the whole circulation when we look at velocity, for example, in the aorta vs. a capillary. While the cross-sectional area of a single capillary is a tiny fraction that of the aorta, the total cross-sectional area of all of the capillaries in the body are several hundred times that of the one aorta, thus blood velocity in the capillaries must be a small fraction of that in the aorta. See **Figure 7.03.**

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