Of course I can help: I'M THE PERSON WHO WRITES THE EXAMS FOR THE COURSE. I'm also really qualified to be teaching the class, and that's not being a braggart, it's stating a fact.
Finally, the other night, I received this email: "I am having a hard time understanding F=P/R and P=HR x SV x R. I understand how you get to that equation, but what I don't understand is how those all regulate blood pressure. I'm not really sure how to ask a question."
Aha! So they do listen to me at times!
Well, to be quite honest, talking about blood flow (F), blood pressure (P or more accurately ΔP), and resistance (R) is a difficult thing to do via email. But I felt the need to get this student an answer. So I plunged in. And then, I thought, why not share this information to my faithful readers? Of course, you need to know a bit about the heart, blood vessels, and the body to understand the answer, but if you have any interest in your blood pressure and how it is regulated, then read on. This is what I said:
F=P/R means that flow is affected by pressure and resistance. The P should really be ΔP, which indicates a difference in pressure. If you don't have a pressure difference, then there will be no flow. That concept is proven with that formula, but it's also a concept you need to know.
Flow is also affected by resistance. If you increase the resistance, you decrease the flow. Think of it as someone blocking a doorway to let people out. The flow of people will be decreased if you increase resistance (block the doorway with your body). If you decrease resistance, flow goes up. But again, the reason we talk about flow in terms of pressure in the first place is because you can't have flow without a pressure differential. The pressure is high in the aorta and then gets lower as you go through the hierarchy of vessels. Once you get to the veins, the pressure is very low. That differential is what helps the flow of blood to occur.
Remember that if you don't have blood flow, you don't have oxygen getting to your tissues, and tissues will die.
So, you need to have a ΔP, and we need to maintain that pressure. That's where P=HR x SV x R comes in. We essentially took F=P/R, multiplied by R on both sides to get P = F x R, and then substituted CO for F [CO means cardiac output, which is equivalent to flow]. Which leads us to P = CO x R. And we know that CO = HR x SV, so if we substitute those into the P = CO x R, we now have P = HR x SV x R. [HR = heart rate and SV = stroke volume, in case you're interested.]
That's a great formula to know because it helps us understand what affects the maintenance of blood pressure. We know that an increase in HR, SV, or R will increase the pressure (increase in R means we vasoconstrict). If we decrease HR, SV, or R, then we decrease the pressure (decrease in R means we vasodilate). So, our body can adjust these variables to make changes in the BP if necessary. We can easily adjust HR and R and we routinely do when our body faces a homeostatic imbalance.
Think about what we discussed in class. What happens when our BP goes up? How does our body adjust to that? First and foremost, we want to bring the BP back down. How do we do that? The baroreceptors that sensed the BP change will send a message to the brain. The brain says, we need to do something to fix this situation, and we need to get the BP to come back down. So, let's inhibit impulses to the sympathetic nervous system, which will decrease HR, AND it will allow for a decrease in R (via vasodilation). We also see a decrease in CO. All of the decreases lead to a decrease in BP. One of the main things here is to remember that this is a short term control and it mainly works with the resistance side of things (called peripheral resistance).
The picture I've included in the notes [see below] goes through that scenario (at the top) and the opposite scenario (at the bottom). But that control is pretty much short term and we can't sustain it long term, so then we need help from the kidneys. The kidneys alter the blood volume to make changes in BP. We don't know about kidney function yet, so I'm not requiring you to know all the details. Mainly, if blood pressure does down, we need to get it back up. If we increase blood volume (an increase in CO), then we increase BP (using that above formula again). Of course, there are details about conserving water and the direct and indirect mechanisms that the kidney uses in this control, but those details--right now--aren't needed.
After that, we have to ask ourselves if we have transient (intermittent) changes in BP and our body is making adjustments, how do tissues get what they need? That's when we talked about local blood flow and local arterioles, vasoconstriction and vasodilation. I think those notes are pretty straight forward.
Hope that helps.
|For educational purposes only, Pearson Education Inc.|