>>Dr. Ketchum: So to begin, we want to think about what can change your mean arterial pressure.
MAP is the average pressure in the aorta during one cardiac cycle. Remember what MAP is equal
to; it’s equal to cardiac output times total peripheral resistance, TPR. And cardiac output
is equal to heart rate times stroke volume. So if I substitute, then, your equation for
cardiac output (which is heart rate times stroke volume and that’s your cardiac output)
and then I multiply by total peripheral resistance, I have yet another way to determine MAP. So
how can we change MAP? Because if I change heart rate, if I change stroke volume or I
change TPR, that will alter MAP. So how do we do that? We’re going to talk about two
different ways to do this. One of those is by increasing your cardiac output.
So here this is an increase in cardiac output. If I increase CO, that means I have more blood
leaving the heart. And when I have more blood leaving the heart –if this is the aorta,
then we know when blood leaves the heart, when the ventricles contract, then that’s going
to cause this aorta to expand outward. So here we’re increasing cardiac output, so
that’s going increase the volume of blood in the aorta. Remember the walls are expanding
in response to an increased cardiac output. And if we increase the volume of the blood
in the aorta, that’s going to increase MAP. So just by increasing cardiac output. Now
please notice here that we did not change the total peripheral resistance. There’s
no vasoconstriction or vasodilation in this instance; here we are only increasing cardiac
output. Again, look at your formula. Cardiac output—if I increase this, that’s going
to increase MAP.
Now let’s try another option here, and that is let’s change your total peripheral resistance.
Let’s increase it, but keep cardiac output constant. So here we have a constant cardiac
output, but we are going to vasoconstrict these arterials. So if we vasoconstrict the
arterials, this vasoconstriction, as we know, is going to increase total peripheral resistance.
So remember total peripheral resistance means the resistance across all of your arterials
at one time. So if we increase total peripheral resistance, that’s going to increase MAP.
So that gives us two ways to increase MAP. You increase cardiac output or you increased
total peripheral resistance.
How do we regulate MAP? So we’re going to be using neural and hormonal controls to do that.
Here are the two ways to regulate MAP. So you have to keep MAP at a constant, right?
You’ve got to maintain homeostasis; you don’t want it too high, you don’t want
it too low. All right, so the two ways to do that are through short-term regulation,
which occurs within seconds to minutes, and then there’s long-term regulation, and that
occurs within minutes to days.
All right, in short-term regulation, this involves baroreceptors. It also involves chemoreceptors,
but we won’t worry about those with respect to MAP. We talked about chemoreceptors with
respect to respiration already. Baroreceptors are located in the carotid sinuses as well
as the aortic arch. These baroreceptors are stretch receptors. So they are able to detect
stretch, and then they will send action potentials to the cardiovascular control center in the
medulla. Hopefully you’re asking yourself, “Well what will cause them to stretch?”
Well, pressure. If we change the pressure in the vessels, then that can either increase
stretch or decrease the amount of stretch. In long-term regulation it involves the kidneys.
The kidneys, then, will regulate your blood volume, because if you regulate your blood
volume, then we can regulate your mean arterial pressure.