Baroreflex & Baroreceptor Function Alteration, Resetting & Adaptation – Regulation of Ambulatory Blood Pressure (ABP)

00:00 Now, let's talk through this example one more time looking at a fallen blood pressure. I think it's helpful to go through these examples because it brings this process home a little bit more to understand what responses we should be getting to something like a fallen blood pressure.

00:22 So if you have a decrease in blood pressure, you'll become hypotensive. What's going to happen? You notice this fallen blood pressure, therefore there is some reason why there is a change or a change needed in systemic vascular resistance. So you need to increase SVR. This causes also a venoconstriction so you decrease blood flow to the various systemic circulatory organs, you venoconstrict to squeeze more blood back to the heart. This increases venous preload, this increases the strength of contraction through two mechanisms. One is through direct involvement of the sympathetic nervous system and the baroreflex to increase inotropy but also the increase in the preload will also cause more stretch of the heart to get it to contract harder. Finally, through the cardiac stimulatory or acceleratory region you'll get an increase in heart rate. Hopefully, all those things are enough to bring blood pressure back to normal. So you're operating on this principle of working on the factors that affect blood pressure, cardiac output and systemic vascular resistance. There are a number of factors that alter the function of the baroreflex. Exercise is one. So once you engage in exercise or aerobic exercise specifically, you need to reset the baroreflex to a different level. Why? Because pressures are going to be different. Just because your blood pressure is high during exercise doesn't mean you also have to not be able to respond to a change in blood pressure. So by resetting the baroreflex, it gives us that ability to modulate or to change blood pressure on a second by second basis even when you're in exercise. Chronic pain, high blood pressure, even heart failure and things like vascular disease are all items that might reset the baroreflex in order to be able to respond to this new environment. So now let's talk through what or how does the baroreceptor reset in response to these chronic conditions. To do this, let's look at how a normal baroreceptor curve looks. In this, we will have arterial blood pressure on the X axis and on the Y axis we're going to have a baroreceptor firing frequency. In a normal individual, we're going to have this sigmoidal curve in which there is the highest firing frequency along the steepest portion of the curve. Eventually though, that curve flattens out and flattens out at the bottom as well as the top. A reset in the curve means the whole curve shifts to a new pressure. So, if the person was hypertensive it would shift to a higher blood pressure but you still need to maintain that same sigmoidal relationship. Why? Because the hypertensive person still needs to do things like stand up, move around, they need to be able to respond to acute changes in blood pressure even if they're hypertensive. So that rightward shift in the baroreflex curve makes them less sensitive to changes than it would be in the normal condition.

04:30 The effects of catecholamines on arterial blood pressure are also present. These are part of our neurohumoral factors and this will primarily be released from the adrenal medulla and it's normally in both epinephrine and norepinephrine, although epinephrine is more important because it's released in the greater quantity about 80%. There are also sympathetic nerves that may contribute to the amount of norepinephrine circulated around in the blood. These are from norepinephrine spillover into the blood. So we have direct adrenal release of catecholamines and an indirect sympathetic nervous system spillover. This increases cardiac output, heart rate and stroke volume. This works in the heart by beta-adrenergic receptors and this also is enacted by increases in beta-adrenergic receptors at the kidney to increase blood volume and cardiac preload. Catecholamines also have vascular effects. They can cause vasodilation through beta 2 adrenergic receptors but more importantly they cause vasoconstriction through alpha 1 and alpha 2 receptors. Circulating catecholamines can also be changed by other activities. Increases in exercise or physical activity. When your body is stressed either through mental stress or thermal stress, those can release catecholamines. In certain systemic conditions such as heart failure, such as shock, even certain cancers can cause increases in catecholamine concentrations.