■ FIGURE 5.7 The effects of changes of vessel radius on flow through a single vessel. This quantitative
relationship is derived from Poiseuille’s equation (Equation 5-7). Decreasing vessel radius (r) dramatically
increases resistance and decreases flow (F) at constant perfusion pressure (AP) because flow is propor-
tional to radius to the fourth power.
when radius is one-half normal (0.5 relative
radius), flow is decreased 16-fold. Therefore,
the new flow is only about 6% of the original
flow. This figure dramatically illustrates how
very small changes in vessel radius can have
profound effects on flow (and on perfusion
pressure if this were the dependent variable
and flow were held constant).
An isolated, cannulated arteriole
is perfused with an oxygenated
physiologic salt solution at a constant
flow, and the pressure gradient across
the two ends of the arteriole is initially
2 mm Hg. If the application of a drug
constricts the vessel diameter by 50%,
what will be the new pressure gradient
across the arteriole?
Laminar versus Turbulent Flow
the simplified equations for the relationship
between pressure, flow, and resistance (Equa-
tions 5-1 and 5-5) assume laminar, nontur-
hulent conditions for flow. Laminar flow is
the normal condition for blood flow in most
blood vessels. It is characterized by concen-
tric layers of blood moving down the length
of a blood vessel (see Fig. 5.8, top panel). The
orderly movement of adjacent layers of blood
moving through a vessel helps to minimize
energy losses in the flowing blood caused
by viscous interactions between the adjacent
layers and the wall of the blood vessel. Tur-
bulence occurs when laminar flow becomes
disrupted (see Fig. 5.8, bottom panel). Tur-
bulence is found distal to stenotic (narrowed)
heart valves or arterial vessels, at large artery
branch points, and in the ascending aorta at
high cardiac ejection velocities (e.g., dur-
ing exercise). Turbulence in large arteries
results in characteristic sounds (e.g., carotid
bruits) that can he heard with a stethoscope.
Because higher velocities enhance turbulence,
murmurs resulting from turbulence become
louder whenever blood flow increases across
narrowed valves or vessels.
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