380
U N I T 5
Circulatory Function
U N D E R S T A N D I N G
Hemodynamics of
The term hemodynamics is used to describe factors such as (1) pressure and
resistance, (2) vessel radius, (3) cross-sectional area and velocity of flow, and (4)
laminar versus turbulent flow that affect blood flow through the blood vessels in the
body.
Pressure, Resistance, and
Flow.
The flow of fluid through a
tube, such as blood through a blood
vessel, is directly related to a pres-
sure difference (P
1
− P
2
) between the
two ends of the tube and inversely
proportional to the resistance (R)
that the fluid encounters as it moves
through the tube.
The resistance to flow, in peripheral
resistance units (PRUs), is determined
by the blood viscosity, vessel radius,
and whether the vessels are aligned in
series or in parallel. In vessels aligned
in series, blood travels sequentially
from one vessel to another such
that the resistance becomes additive
(e.g., 2 + 2 + 2 = 6 PRU). In vessels
aligned in parallel, such as capillaries,
the blood is not confined to a single
channel but can travel through each
of several parallel channels such that
the resistance becomes the reciprocal
of the total resistance (i.e., 1/R). As a
result, there is no loss of pressure, and
the total resistance (e.g., 1/2 + 1/2 +
1/2 = 3/2 PRU) is less than the resis-
tance of any of the channels.
1
Pressure difference
Resistance
P
1
P
1
R
1
R
1
R
2
R
2
R
3
R
3
P
2
P
2
P
i
P
i
P
o
P
o
Flow
Flow
Flow
Series
Parallel
Vessel Radius.
In addition to
pressure and resistance, the rate
of blood flow through a vessel is
affected by the fourth power of its
radius (the radius multiplied by
itself four times). Thus, blood flow
in vessel B with a radius of 2 mm
will be 16 times greater than in ves-
sel A with a radius of 1 mm.
2
Vessel A
Vessel B
1 mm
2 mm
1 mL/min
16 mL/min
Pi, pressure in; Po, pressure out.
