C h a p t e r 1 7
Control of Cardiovascular Function
381
Blood Flow
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
Cross-
sectional
area
1
2
3
Velocity
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
Laminar flow
Turbulent flow
