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Now we're going to discuss the regulation of arterial blood pressure. The best way to talk
about the regulation of blood pressure is through very acute or fast changes in arterial
pressure. These are best sensed by a baroreflex-mediated response. The baroreflex is going to
involve a sensor, the baroreceptor, some integration in the brainstem through cardiovascular
control centers and then an effect. So let's talk through some of these right now. The
cardiovascular control centers involve both an input and an output region. So let's go through
an example. If you have an increase in arterial blood pressure, that's going to be sensed by the
baroreceptors sending a positive signal of high blood pressure to the input centers of the
cardiovascular control center. There are three primary inputs that you're going to sense.
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They are going to affect the cardioinhibitory center, the cardioacceleratory center and the
vasomotor center. In this case when we have an increase in blood pressure, you're going to
stimulate the cardioinhibitory, you're going to inhibit the cardioacceleratory and you're going
to inhibit the vasomotor centers. What physiological effect happens by stimulating these
control centers? You have a decrease in heart rate, a decrease in inotropy which is
contractility of the heart, a decrease in preload. All of those combine together to decrease
cardiac output. Then along with the vasomotor center inputs the decrease in cardiac output you get
an overall decrease in blood pressure. This is a classic baroreflex response. If you have an
increase in blood pressure, how you get a lowering of that blood pressure? The opposite also
occurs. If we went through the same example when we start it off with a decrease in blood
pressure, this is how it would look. You would have a decrease in arterial blood pressure which
would inhibit the three neurons going to the cardioacceleratory region, the cardioinhibitory
region and the vasomotor center. What responses would happen? The decrease in
cardioinhibitory center, the increase in the cardioacceleratory center would increase heart
rate and inotropy. The increase in the vasomotor center would increase both preload and SVR.
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Together the increase in heart rate, inotropy and preload would increase cardiac output.
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Cardiac output increases along with SVR would increase arterial blood pressure. So really you
are correcting for whichever response you don't have, a low blood pressure's response to
raise blood pressure. If you have a high blood pressure to start, the body wants to lower it
back down again in order to maintain blood pressure homeostasis. So how does the baroreflex
work and where are these baroreceptors located? The most important arterial baroreceptors
will be in the carotid arteries. These carotid artery baroreceptors are located both on the right
and left side in the junction between where the internal and external carotid arteries branch.
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This is innervated by pressure receptors which will go through in just a second and they send
their information via cranial nerve IX which is also known as the glossopharyngeal. The other
type of arterial baroreceptor we have are located in the aortic arch. This particular
baroreceptor populations will send their signal up to the brainstem via cranial nerve X also
known as the vagus. How a baroreceptor works and how it fires is all in relation to how much
it is stretched. So here, we have a blood vessel that is going to be stretched from an increase
in pressure. So as pressure increases, it stretches the blood vessel and the nerves that are
located just outside that blood vessel sense that distention and that stretch. They will do that
by increasing their firing frequency. If there's a lowering of blood pressure, they are not
stretched as much and therefore they'll decrease their firing frequency. That can be seen in a
diagram where you have increases in blood pressure having the high firing frequencies, a
decrease in blood pressure showing low firing frequencies. The baroreflex though is so
sensitive that it can even respond within a beat of the heart. So during a systolic portion
of the cardiac cycle, the baroreflex is firing more than during diastole. So it is such a quick
acute response. The barorereflex is so important governing pressure on a millisecond to
millisecond level.