Porth's Essentials of Pathophysiology, 4e

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Control of Cardiovascular Function

C h a p t e r 1 7

Blood Flow

Velocity

Cross-Sectional Area and Velocity of Flow. The velocity or rate of forward movement of the blood is affected by the cross- sectional area of a blood vessel. As the cross-sectional area of a vessel increases (Sections 1 and 3), blood must flow laterally as well as for- ward to fill the increased area. As a result, the mean forward veloc- ity decreases. In contrast, when the cross-sectional area is decreased (Section 2), the lateral flow decreases and the mean forward velocity is increased. 3 Laminar and Turbulent Flow. Blood flow is normally laminar, with platelets and blood cells remaining in the center or axis of the blood- stream. Laminar blood flow can be described as layered flow in which a thin layer of plasma adheres to the vessel wall, while the inner layers of blood cells and platelets shear against this motionless layer. This allows each layer to move at a slightly faster velocity, with the greatest velocity occurring in the central part of the bloodstream. Turbulent blood flow is flow in which the blood elements do not remain confined to a definite lamina or layer, but develop vortices (i.e., a whirlpool effect) that push blood cells and platelets against the wall of the vessel. More pressure is required to force a given flow of blood through the same vessel (or heart valve) when the flow is turbulent rather than laminar. Turbulence can result from an increase in velocity of flow, a decrease in vessel diameter, or low blood viscosity. Turbulence is usually accompanied by vibrations of the fluid and surrounding struc- tures. Some of these vibrations in the cardiovascular system are in the audible frequency range and may be detected as murmurs or bruits. 4

Cross- sectional area

1

2

3

Laminar flow

Turbulent flow