C h an g es in p re ssu re as b lo o d flow s th ro u g h th e cardiovascular sy stem • Figure 11.18
Systemic circulation
What the
heart
does:
1. Takes in a small volume of blood during diastole
2. Applies a force (contraction) to the blood that
raises its pressure during systole
3. Releases the blood into the systemic circulation
(via the aorta)
Like any other fluid, blood flows according to changes in pressure or pres-
sure gradients. The blood vessels offer resistance to that flow, so pressure
drops throughout the system. In addition, both pressure and resistance are
affected by many factors.
How blood flows in the systemic circulation:
• The blood flow through the system is constant (steady state).
• Change in pressure (
A
P) drives the flow. (
A
P is directly proportional to
flow.) So anything that increases blood pressure increases the flow.
• Vascular resistance (R) impedes the flow (R is inversely proportional to
flow). For example, if R is increased, then flow is decreased.
Heart activity sets up
the initial blood
pressures at a
steady flow rate
(cardiac output, CO).
/
. . . . . . . . . . . . . . .
I .
Elastic arteries
have low R, so
the systolic and
diastolic
pressure
variations are
maintained.
140
Arterioles,
capillaries and
venules have
high R (narrow,
rigid) so blood
pressure drops
and changes
from fluctuating
to steady.
120
-
100
80
60
40
20
\
Veins have
low R (large
diameters),
so blood
pressure
drops only a
little.
X?
/
'
*
s S
x
* 9
0
from the adrenal medullae directly increase the heart’s
contractility.
Certain
drugs,
such as
digitalis,
also
increase contractility.
Increased blood volum e.
More blood exerts more pressure
(like more water in the balloon). Also, increased blood
volume increases the stroke volume. The increased
stroke volume stretches the heart more, which generates
a higher force of contraction and higher blood pressure.
Increased cardiac output.
In the cardiovascular system,
venous return is equal to cardiac output. So, like an
increase in blood volume, increasing the cardiac output
returns more blood to the heart and increases blood
pressure.
Increa sed v a scu la r resistance.
Increased venous resistance
prevents blood from moving into the capillaries,
holding more blood in the artery and increasing
the
pressure.
Several
factors
influence
vascular
resistance, including size of the lumen, viscosity, and
total length of blood vessels. The size of the lumen
may be affected by vasoconstriction or vasodilation
resulting from autonomic nervous system activity.
Hormones and drugs may also affect the size of
the lumen. The thickness (or viscosity) of the blood
may be affected by blood cell count, dehydration, or
plasma protein levels. The total length of the blood
vessels may vary, depending on the amount of fat.
Maintaining Blood Pressure is Critical for Survival
333
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