00:00
So now let's take a step back, use Ohm's Law and try to use this across the whole body
instead of just one blood vessel. To do this, we use the determination of mean arterial
pressure. We'll have to determine then what is systemic vascular resistance and then we can
determine what cardiac output is. So these are going to be the three variables that we're
going to deal with. Okay, so let's talk through which ones are which. So MAP is abbreviation
for mean arterial pressure. That is going to be the cardiac output times systemic vascular
resistance plus central venous pressure. This can be seen quite well over here on the diagram
on your right. If central venous pressure is low such as 0, you can utilize this equation by
dropping the central venous pressure term. You might ask, is central venous pressure usually 0?
It's in between 0 and 4. Mean arterial pressures are much higher, maybe in the order of
95 mmHg. So when you look at a central venous pressure, it is very low and oftentimes people
will drop that portion of the equation just to make things simpler. So in this case, we have
mean arterial pressure approximates cardiac output times systemic vascular resistance.
01:38
Knowing what cardiac output equals allows us to do another substitution. Cardiac output is
simply the product of heart rate times stroke volume. That's the number of beats of the heart
times the stroke volume or the output per beat. You multiply those two together times
systemic vascular resistance and again you can approximate mean arterial pressure. So now
how do you use mean arterial pressure to get to the flow that you want to look at? First thing
about mean arterial pressure to kind of grab hold and that is really what is mean arterial
pressure and how we're calculating it. Because when you get a blood pressure measurement,
let's say with a sphygmomanometer and a stethoscope on your upper arm, you usually get a
systolic blood pressure and a diastolic blood pressure. Your systolic blood pressure is your
maximal pressure that you hear, the diastolic blood pressure is when you no longer hear any
Korotkoff sounds through the stethoscope. You would think that you could use these simple
two pressures to be able to determine what mean arterial pressure is but it's a little bit more
complex than that and that is because there is a time component to blood pressure. So if we
look at this diagram over here where we have time on the X axis and we have pressure on the
Y. Time spent in diastole is longer than a time spent in systole. So I will approximate that
here with my voice. You go through systole and then diastole, systole, diastole. The diastolic
portion lasts longer and that longer-lasting portion means that you have to wait it to a greater
degree. To do that, you utilize this particular formula where you take a third of pulse pressure
which is systolic minus diastolic pressure and you add it back in the diastolic pressure. So
diastolic pressure is waited to a greater degree than systolic pressure. Why? Because you
spend a longer duration in diastole than you do in systole. So mean arterial pressure is not
simply the mean of this systolic and diastolic pressure, you have to wait it to a greater
degree for diastolic pressure.