>>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