Download 3 - The Arterial System

The Arterial System
The Windkessel Effect
Arterial Pressure Determinants
– CO
– TPR
Changes in the Pulse Waveform
The hydraulic filter converts
pulsatile flow to steady flow
Hydraulic filtering converts the intermittent
output of the heart to a steady flow through the
capillaries.
Part of the energy of cardiac contraction is
dissipated as forward capillary flow.
The remaining energy is stored as potential
energy in the distensible arteries.
During diastole, the elastic recoil of the arterial
walls converts this potential energy into blood
flow.
Mean Arterial Pressure
The mean arterial pressure is the
pressure in the large arteries,
averaged over time.
MAP can be measured from the
arterial pressure tracing by measuring
the area under the pressure curve.
It can also be estimated by the
formula: MAP = Pd + (Ps-Pd)/3
The arterial blood pressure is
determined by physical and
physiological factors
The physiological (alterable) factors
are the cardiac output and the
peripheral resistance.
The two physical (dependent) factors
are the blood volume within the
arterial system and the compliance of
the system.
Cardiac Output
The change in pressure in response
to a change in cardiac output can be
appreciated by considering:
Let the cardiac output = 5 L/min and
MAP = 100 mmHg.
The total peripheral resistance is; R
= P/Q = 100/5 = 20 mmHg/L/min
(Omh’s Law: R=V/I)
Cardiac Output
If cardiac output (Qh) suddenly increased to
10 L/min, P will initially remain unchanged.
Outflow (Qr) remains unchanged at 5 L/min
because it depends on depends on P and R.
When Qh > Qr, mean arterial blood volume
increases.
Because P depends on the arterial blood
volume (V) and on the arterial compliance
(C), an increase in V will increase P.
Peripheral resistance
Similar reasoning can explain the changes in
P that accompany alterations in peripheral
resistance.
Increase R suddenly to 40 mmHg/L/min.
Qr = P/R = 2.5 L/min.
Thus, the peripheral runoff would only be 2.5
L/min, even though cardiac output was 5
L/min.
If Qh remained constant at 5 L/min, Qh would
exceed Qr and V would increase; therefore, P
would rise.
The pressure curves change in
arteries at different distances from
the heart
The radial stretch of descending aorta
from ejection initiates a pressure wave that
is propagated down the aorta and its
branches.
The pressure wave travels much faster
than does the blood itself.
The velocity of the pressure wave varies
inversely with the vascular compliance.
The pulse velocity increases progressively
as the wave travels from aorta towards the
periphery.
The arterial pressure contour becomes
distorted as the wave is transmitted down
the arterial system.
– Three major changes occur in the arterial pulse
contour.
First, the incisura (a notch that appears at the end of
ventricular ejection), is damped and disappears.
Second, the systolic portions of the pressure wave become
narrow and elevated.
Third, a hump may appear on the diastolic portion of the
pressure wave.
– The dampening of the high-frequency components of
the arterial pulse is caused largely by the viscoelastic
properties of the arterial walls.
– The precise mechanism for the peaking of the
pressure wave is controversial.
– Several factors contribute to these changes, including
reflection, tapering of the arteries, resonance, and
velocity of transmission.
The Windkessel Effect
Arterial Pressure Determinants
– CO
– TPR
Changes in the Pulse Waveform
Questions?