Baroreceptor Reflex – Regulation of Ambulatory Blood Pressure (ABP)

00:00 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.

01:04 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.

02:55 Together the increase in heart rate, inotropy and preload would increase cardiac output.

03:02 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.

03:55 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.