PSL201Y1 Lecture Notes - Mean Arterial Pressure, Common Carotid Artery, Central Venous Pressure

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20 Apr 2012
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Mean arterial pressure and its regulation
The mean arterial pressure is the average aortic pressure occurring
during a cardiac cycle. It is determined by heart rate, stroke volume
and total peripheral resistance.
Mean Arterial Pressure = Cardiac output Total peripheral resistance.
Cardiac output = Heart rate stroke volume
Therefore, MAP = Heart rate stroke volume Total peripheral
resistance
The mean arterial pressure is the pressure that is needed to drive the
blood through the system. If you have a low mean arterial pressure,
you are unable to deliver the blood to the brain and the other organs
that will be deprived of oxygen and nutriments.
In the systemic circuit normally, the MAP is 90 mm of mercury. The
central venous pressure is 0 mm of mercury. The pressure gradient
driving the blood flow through the systemic circuit is the difference
between the mean arterial pressure and the central venous pressure.
It is 90 mm of mercury.
Total peripheral resistance is whatever pressure that is resisting the
blood from flowing. In the systemic circuit, it is the combined
resistance of all the organs to and away from them that is called total
peripheral resistance.
Effects of cardiac output on mean arterial pressure
If there is constant cardiac output and constant total peripheral
resistance, the mean arterial pressure will be constant. However, an
increase in cardiac output leads to an increase in the volume of blood
contained in the aorta and an increase in mean arterial pressure when
total peripheral resistance remains the same. The blood flows into the
aorta faster than it flows out when the cardiac output is increased,
increasing the volume of blood in the aorta. The vessel expands by
stretching the walls of the aorta causing it to exert a large inward force
on the blood so that the pressure on the blood increases. Thus, an
increase in the heart rate or stroke volume will lead to an increase in
cardiac output which will eventually lead to an increase of mean
arterial pressure.
Effects of total peripheral resistance on mean arterial pressure
A constant cardiac output leads to an increase in the volume of blood
contained in the aorta and an increase in mean arterial pressure.
When there is an increase in TPR reduces the blood flow out of the
aorta at a constant rate. Therefore, volume of blood in the aorta will
increase, the walls of the aorta will stretch and the mean arterial
pressure also increases.
Neural control of Mean Arterial Pressure
Neural control of mean arterial pressure is done through negative
feedback. An example of negative feedback is body temperature
control. A change in body temperature is detected by sensors.
The sensors monitoring the mean arterial pressure are called the
arterial barrel receptors. A fall in the blood volume activates both
cardiovascular and renal reflex mechanisms. A loss of blood will lead to
an increase in sympathetic activity which in turn increases the heart
rate, stroke volume and cardiac output. This increase in sympathetic
activity will lead to an increase of vasoconstriction and total peripheral
resistance. This all results in the return of the MAP to its normal value.
This is possible due to the presence of baroreceptors which are
pressure receptors. Baroreceptors are found in the blood vessels and
the heart that respond to changes in pressure within the
cardiovascular system. Arterial baroreceptors are found in the aortic
arch and in the carotid sinuses (brain). When the mean arterial
pressure changes, activity in the neurons change.
When the pressure is increased in the baroreceptors, there is an
increase in action potential frequency. The brain detects changes in
action potential and through the sympathetic and parasympathetic
systems, causes the heart and blood vessels to alter their function in
response.