Introduction to Hemodynamics - Online Tutor, Practice Problems & Exam Prep

Hemodynamics involves understanding blood flow, blood pressure, and resistance within the cardiovascular system. Blood flow (f) is the volume of blood passing a point per minute, driven by the blood pressure gradient (ΔP) and inversely affected by resistance (R). The equation f = ΔP / R illustrates these relationships. Factors influencing resistance include blood viscosity, vessel length, and diameter, with viscosity and length directly proportional to resistance, while diameter is inversely proportional. Understanding these concepts is crucial for grasping cardiovascular dynamics and their physiological implications.

Hemodynamics

In this video, we're going to begin our introduction to hemodynamics. So the term hemodynamics refers to the physical principles of blood circulation throughout the cardiovascular system. And when it comes to the study of hemodynamics, there are 3 physiologically important terms for you to understand, and those are blood flow, blood pressure, and resistance. And notice that down below in the image, we have a section for each of these three important terms. Now it's also very important to know that these three terms are very highly interconnected with each other, and so changing any one of these three terms will have an impact on the other two terms. And so later in our course, we will talk about the equation that links these 3 physiologically important terms. But for now, let's take a closer look at what blood flow is. And so blood flow is really just the total volume of blood that flows through any particular point in the cardiovascular system within a given time period. And, usually, blood flow is expressed in units of milliliters per minute, so how many milliliters of blood pass through any particular point within the cardiovascular system within one minute. And blood flow can vary drastically throughout the cardiovascular system, and so different tissues, different blood vessels, and different organs can have different blood flows. And blood flow is also dynamic and can change, and again, changing any of these other two variables can also impact the blood flow. And so let's take a look at our image down below on the bottom left, which is showing you a silly cartoon for blood flow. And notice that in this silly cartoon, we've got these red blood cells that are all dressed up in their workout gear, and they are out on a jog throughout the cardiovascular system. And notice that the very first red blood cell here is saying, let's speed up. Blood flows only 5 milliliters per minute. And then in the next scene, you can see the red blood cells have sped up, and the first one is saying, 20 milliliters per minute, that's more like it.

And so, let's move on to the next physiologically important term here, which is blood pressure. Now, blood pressure refers to the force that blood exerts on the walls of the blood vessel, and usually blood pressure is measured in units of millimeters of mercury. And again, blood pressure can vary drastically throughout the cardiovascular system. And so blood pressure tends to be highest in the arteries that are closest to, the heart, where the blood pressure can be about 120 millimeters of mercury, and blood pressure is at its lowest in the large systemic veins, where the blood pressure can be as low as about 2 millimeters of mercury. And, again, blood pressure can be impacted by many variables, and that is going to include the blood flow and resistance. And so let's take a look at this image down below in the middle here, which, again, is a silly cartoon to help you understand blood pressure. And so notice that in this cartoon, we've got this red blood cell that is exerting force on the blood vessel walls, and that force that it exerts on the blood vessel walls is the blood pressure.

Now last but not least, we have the term resistance, which refers to any opposition to blood flow, or in other words, how difficult it is for blood to flow through the cardiovascular system. And so the greater the resistance is, the harder it is for blood to flow through the cardiovascular system. And ultimately, resistance becomes a significant measure of the amount of friction that blood encounters as it travels through the cardiovascular system. Now it turns out that the greatest amount of resistance is encountered away from the heart in the periphery of the body, and so sometimes resistance is referred to as peripheral resistance. And recall from previous lesson videos that arterioles, the smallest of the arteries, are also sometimes called resistance vessels since they play a huge and critical role in the regulation of resistance in the cardiovascular system, and this is due to how numerous the arterioles are and their ability to vasoconstrict and vasodilate plays a major role in resistance. And so let's take a look at our image down below on the far right hand side, which is focusing in on resistance. And again, we have a silly cartoon that shows you some red blood cells strolling through one of the blood vessels, and notice the first one saying low resistance. Flowing through here will be a breeze. And so, when there is low resistance, blood flow will be relatively easy, if you will. And so there will not be a lot of opposition to the blood flow with low resistance. And then notice that the bottom part of the cartoon here is showing you some red blood cells flowing through a much tighter and narrower blood vessel where the resistance is greater. The friction of the blood with the walls is greater. And so notice this red blood cell is saying higher resistance will make it through, but it'll be tougher to flow through here.

And so really that is what resistance is about, how difficult it is for blood to flow through the cardiovascular system. And so this here concludes our brief lesson on the introduction to hemodynamics, and as we move forward in our course, we'll be able to apply these concepts and continue to learn more about hemodynamics. So I'll see you all in our next video.

Introduction to Hemodynamics Example 1

So here we have an example problem that asks which of the following is an appropriate analogy for increasing resistance to blood flow in blood vessels. And we've got these four potential answer options down below. Notice that in each of these four potential answer options, the analogy used compares water flowing through a garden hose to blood flowing through blood vessels. Now, notice that the problem wants us to select the option that is an appropriate analogy for increasing resistance, and recall that resistance is any opposition to blood flow, or in other words, how difficult it is for blood to flow through the blood vessels. And so the greater the resistance is, the more difficult it is for blood to flow through. And so, really, we want to choose an answer option that is consistent with that.

Notice answer option A says, lubricating a garden hose so that water can flow through it faster. But if water is flowing through the garden hose faster, that's not making it more difficult for the water to flow through, it's making it less difficult since it's flowing faster. And so, for that reason, we can eliminate answer option A because that's consistent with decreasing resistance, not increasing resistance. Now answer option B says, cutting the end of a garden hose so that water has a shorter distance to travel. Now if water has a shorter distance to travel, that means less friction between the water and the garden hose, and less friction means that it's going to be less resistance. And so, that's not consistent with increasing resistance, so we can eliminate answer option B. Now answer option C suggests cutting a hole in the side of a garden hose so that some water leaks out as it flows through.

But again, this doesn't suggest that it will make it more difficult for the water to flow through the garden hose, and so for that reason, we can eliminate answer option C. And, of course, this leaves answer option D as the best answer, which says squeezing a garden hose, thereby slowing down the water flowing through it. Squeezing the garden hose is going to narrow down the diameter of the garden hose, and that's going to create more friction between the water and the garden hose. And so that is going to be consistent with, you know, slowing down the speed of the water going through it is consistent with making it more difficult for the water to flow through. So answer option D is an analogy consistent with increasing resistance to blood flow in blood vessels. So that here concludes this example, and I'll see you all in our next video.

Arteries tend to have thicker walls than veins. Which of the following is a reason for this?

• A. Arteries need to provide less resistance to blood flow than veins, and a thinner wall provides resistance.
• B. Veins need to provide more resistance to blood flow than arteries, and a thinner wall provides resistance.
• C. Arteries need to withstand higher blood pressure than veins.
• D. Veins need to withstand higher blood pressure than arteries.

Relationship Between Blood Flow, Pressure, & Resistance

In this video, we're going to continue our lesson on hemodynamics as we focus in on the relationship between blood flow, blood pressure, and resistance. Recall from our last lesson video that blood flow refers to the total volume of blood flowing through any particular point in the cardiovascular system in a given time period, and it's usually expressed in units of milliliters per minute. Blood flow can be symbolized with the capital letter f, which is why we have the f bolded here. Blood flow or f is actually directly driven by a blood pressure gradient or a difference in the blood pressure between two points in the cardiovascular system. Recall from our last lesson video that blood pressure refers to the force that the blood exerts on the walls of the blood vessels, and it's usually expressed in units of millimeters of mercury. The blood pressure gradient can be symbolized as Δp, where the Greek letter delta represents the difference and the letter p represents the blood pressure, which is why we have the p bolded here. The blood pressure gradient is also referred to as the hydrostatic blood pressure.

Again, blood flow or f is directly driven by a blood pressure gradient or Δp, because blood will always flow from areas of high blood pressure toward areas of low blood pressure. However, it's important to note that blood flow, or f, is impacted not only by the blood pressure gradient or Δp but also by the resistance, which refers to any opposition to blood flow, or in other words, how difficult it is for blood to flow through the cardiovascular system. The greater the resistance, the harder it is for blood to flow through. Resistance is symbolized with the capital letter r.

It's essential to understand that these three variables, blood flow or f, blood pressure gradient or Δp, and resistance or r, are interconnected, and changing any one of these variables can impact the others. Blood flow or f is directly proportional to the blood pressure gradient or Δp, which means that the greater the Δp, the greater the blood flow will be. Conversely, blood flow or f is inversely proportional to resistance or r, which means that the greater the value of r, the lower the blood flow will be. Referring to our equation: f = Δp r

If you increase the value of Δp, it will lead to an increase in the blood flow. Conversely, increasing the resistance will decrease the value of the blood flow. This equation shows that blood flow and resistance are inversely proportional with each other.

We can algebraically rearrange this equation to isolate any one of these variables. To isolate the resistance, multiply both sides of the equation by r and divide both sides of the equation by f, resulting in: r = Δp f

To isolate Δp, multiply both sides of the equation by resistance, and you get: Δp = f ⋅ r

This concludes our lesson on the relationship between blood flow, blood pressure gradient, and resistance. We'll be able to get some practice applying these concepts and continue to learn more as we move forward in our course. I'll see you all in our next video.

Introduction to Hemodynamics Example 2

So here we have an example problem that says, blood is flowing from point A to point B at a constant rate. Then a physiological change causes the resistance to blood flow between the two points to decrease. And then the problem asks, what will happen to the rate of blood flow? We've got these three potential answer options down below that say that it will increase, it will decrease, or it will remain the same. Now, in order to solve this problem, I've gone ahead and drawn a little sketch just to help us better visualize the scenario. Notice that I've drawn a blood vessel right here and indicated where point A is and where point B is. Again, the problem tells us that blood is flowing from point A to point B at a constant rate, but then there's some kind of physiological change that occurs that causes the resistance to blood flow between the two points to decrease. Now recall from our previous lesson videos that the resistance refers to any opposition to the blood flow or how difficult it is for blood to flow through the cardiovascular system. It's a measure of the amount of friction that the blood encounters as it travels through the cardiovascular system. Recall that the greater the resistance is, the lower the blood flow will be. So there is an inverse relationship between the two. Notice that this problem mentions that there is a decrease in the resistance, which means there is less opposition to the blood flow, and that will make it easier for the blood to flow through the cardiovascular system between the two points. And so what that means is that the blood flow is going to increase if there's a decrease in the resistance. So that means that we can indicate that answer option A, it will increase, is the correct answer to what will happen to the rate of blood flow. So we can indicate that A here is correct, and these other options are not correct.

Now another approach that we could have taken to solving this problem is remembering the equation that we introduced in our last lesson video that relates the important hemodynamics variables, which recall are blood flow, blood pressure gradient, and resistance. Recall that the equation is that blood flow, or F, is equal to the blood pressure gradient, or ∆P, divided by the resistance, or R. The problem tells us that the resistance between the two points is decreasing, so the value of R is going down. And, of course, because the resistance is in the denominator, that causes this entire value here, which is the blood flow, to increase. Now, we could also use some random numbers here to help us better understand how this equation could have worked. Let's say that for the blood pressure, we used a value of 2, and for the resistance, we also used a value of 2. And, of course, 2 divided by 2 is 1, so that would make the blood flow 1. Now let's imagine that this was the case, initially before the physiological change. Now after the physiological change, there was a decrease in the resistance, which is the R value in the bottom. So, let's make it 1 this time. The blood pressure difference we'll say is the same, just 2. And so, 21 is going to be 2. That shows that when the resistance decreases, as you see here, that causes the value of the blood flow to increase, which, again, validates that answer option A is correct. So, hopefully, this was helpful for you all, and that concludes this example. I'll see you all in our next video.

The pressure at point A in the circulatory system is 15 mm Hg, & the pressure at point B is 8 mm Hg. Blood is flowing from point A to point B, then a physiological change causes the pressure at point B to increase to 10 mm Hg. What will happen to the rate of blood flow?

• A. It will increase.
• B. It will decrease.
• C. It will remain the same.

Which of the following physiological changes would likely occur in someone’s blood vessels when they begin an intense exercise session?

• A. Rate of blood flow needs to increase, so the ΔP will increase & the resistance will increase.
• B. Rate of blood flow needs to decrease, so the ΔP will decrease & the resistance will decrease.
• C. Rate of blood flow needs to increase, so the ΔP will increase & the resistance will decrease.
• D. Rate of blood flow needs to decrease, so the ΔP will increase & the resistance will decrease.

Altering Resistance in Blood Vessels

In this video, we're going to continue our lesson on hemodynamics as we discuss altering resistance in blood vessels. And so, really, there are 3 important factors that affect resistance of blood flow through the cardiovascular system. And recall from our previous lesson videos that resistance refers to any opposition to blood flow, or in other words, how difficult it is for blood to flow through the blood vessels of the cardiovascular system. And, ultimately, resistance is a measure of the amount of friction that the blood encounters as it flows through the cardiovascular system. And so recall that the greater the resistance is, the more difficult it is for the blood to flow through the cardiovascular system. And so, again, there are 3 important factors that affect resistance that we have numbered down below in our text, 1, 2, and 3. And these are, blood viscosity, or how thick the blood is, blood vessel length, or how short or how long the blood vessel is, and blood vessel diameter or how wide or how narrow the lumen is of the blood vessel. And so notice down below in the table, we have a column for each of these three important factors and how they affect resistance. Now before we continue, I want to address the elephant in the room, which is this delicious looking milkshake or smoothie in the top right of the screen. And so you may be thinking, why is this in the lesson? And the reason is that if you've ever had a really thick milkshake or a really thick smoothie with a straw, then even though you may not realize it yet, you already have a pretty good understanding of how these three factors impact resistance. And so notice that in this milkshake or smoothie, that there are 2 different types of straws sticking out of it, which you can think of as blood vessel straws. And so notice that one of these straws is really tall and narrow, kind of like a coffee stirrer straw. And the other straw is short, but broad and wide. Now when you try drinking a really thick milkshake or a really thick smoothie with a tall and narrow straw like this one, you're going to encounter a lot of resistance. And this is because the really thick milkshake or the really thick smoothie is going to flow really, really slowly through this tall and narrow straw. Now on the other hand, when you try drinking a really thick milkshake or a thick smoothie with a short but broad and wide straw, you're not going to encounter much resistance at all. And this is because the really thick milkshake or smoothie is going to flow through this straw relatively quickly. And so your milkshake and smoothie drinking experiences can actually be helpful when it comes to understanding how these three important factors affect resistance. So let's take a look at the very first factor once again, which is blood viscosity. And again, viscosity is really just the thickness of the blood, and so the greater the viscosity, the more thick the blood is. And so if we were to say that water had a relatively low viscosity, because water is pretty fluid, then a substance such as a thick milkshake, or a thick smoothie, or even honey would be substances that are very viscous because they are very thick. And so the more viscous the fluid is, the harder it is to get that fluid flowing through the vessels. And so just like a milkshake, the greater the viscosity or the more thick the fluid is, the more resistance will be encountered, and so the harder it will be for the fluid to flow through the vessel or to flow through the straw. Now the next factor that we have here is blood vessel length, which, of course, refers to how short or how long the blood vessel is. Now, the longer the blood vessel is, the more opportunities there are for blood to encounter resistance with the blood vessel walls, and so the more resistance there will be. And so, again, just like drinking a milkshake, the longer the blood vessel is or the longer the straw is, the more resistance will be encountered. Now, last but not least, we have blood vessel diameter, which, again, refers to how wide or how narrow the blood vessel is. And it turns out that this factor is the most easily altered physiologically, and this is because we know that many blood vessels have smooth muscles in their walls that allow them to contract to narrow down their diameters, and then the smooth muscle can relax to widen their diameters. Now, again, just like drinking a smoothie, the larger the diameter of the blood vessel or the larger the diameter of the straw, the less resistance is encountered. And so it turns out that blood viscosity and blood vessel length, both are directly proportional to resistance. So the greater the viscosity and the greater the length, the more resistance. However, blood vessel diameter is inversely proportional to resistance, and so the greater the diameter, the less resistance there is. So let's take a look at this image down below, which is really just a table, and notice that in this table, again, we have columns for viscosity, vessel length, and vessel diameter. And what you'll notice is that, this row that you see here are all going to be, factors that would lead to lower resistance. Whereas the second row that you see here are going to be factors that lead to higher resistance. And so, again, when it comes to blood viscosity, it is directly proportional with resistance. And so the lower the viscosity, the lower the resistance. And notice that the blood flowing through this blood vessel is very, very fluid, and so it's kind of flowing similar to how we would expect water to flow through. Since water is not very viscous, it's very, very fluid. Now, down below, what we have is a blood vessel with higher viscosity, and so the blood flowing through here is very, very thick and has the consistency of honey, if you will. Now the next one here is blood vessel length. And, again, the shorter the blood vessel is, the less resistance there will be because there will be fewer opportunities for the blood to encounter friction with the walls. And the longer the blood vessel is, the more opportunities for the blood to encounter friction with the walls, and so that will create higher resistance. And then last, but not least, vessel diameter is inversely proportional with resistance. So the larger the diameter is, the fewer opportunities there are for resistance and friction, and so, that will lead to lower resistance. And the smaller the diameter is, the more opportunities for friction there will be between the blood and the blood vessel walls. And so it's going to be a lot harder for the blood to flow through the smaller diameter than it is the larger diameter. And so this here concludes our lesson on altering resistance in blood vessels, and moving forward, we'll be able to apply these concepts and problems. So I'll see you all in our next video.

Introduction to Hemodynamics Example 3

So here we have an example problem that says, imagine two individuals sharing a thick, viscous milkshake. One individual drinks from a broader, shorter straw and the other individual drinks from a thinner, taller straw. And then it asks, which individual encounters more resistance drinking the milkshake? And so notice down below we've got this cartoon showing two individual red blood cells sharing this thick, viscous milkshake. Notice that the individual on the left-hand side is drinking from the broader, shorter straw, which you can think of these straws as blood vessel straws, if you will, and notice the individual on the right-hand side is drinking from the thinner, taller straw. Now, notice the individual on the left is saying this milkshake is awesome, and they've actually got a little bit of a brain freeze, if you will. That's because the fluid flowing through this broader, shorter straw is going to encounter less resistance. And so what this means is that there will be less friction encountered by the fluid as it flows through the broader shorter straw, and that will allow the fluid to flow relatively quickly. And so this individual was able to get a lot of milkshake. Now, the individual on the right-hand side, on the other hand, notices is saying, "I've barely had any," and that's because the fluid flowing through this thinner, taller straw is going to encounter more resistance and more friction, and so it is going to flow through the thinner taller straw relatively slowly. And so this individual on the right encounters more resistance. So, in terms of the question, which individual encounters more resistance drinking the milkshake, it's going to be the individual on the right-hand side drinking through the thinner, taller straw. So we can go ahead and indicate that the individual drinking from the thinner, taller straw encounters more resistance drinking the milkshake. So that concludes this example problem, and I'll see you all in our next video.

Assuming each of these blood vessels have the same difference in pressure along their length, which would have the lowest resistance and therefore, the greatest rate of blood flow?

• A.
• B.
• C.
• D.

Blood vessels primarily impact resistance to blood flow by:

• A. Altering their length.
• B. Altering their diameter.
• C. Altering their wall thickness.
• D. Altering the viscosity of blood.

Here’s what students ask on this topic:

What is hemodynamics and why is it important in the cardiovascular system?

Hemodynamics refers to the physical principles governing blood circulation within the cardiovascular system. It is crucial because it helps us understand how blood flow, blood pressure, and resistance interact to ensure efficient blood distribution throughout the body. Blood flow (f) is the volume of blood passing a point per minute, driven by the blood pressure gradient (ΔP) and inversely affected by resistance (R). The equation $f=\frac{\mathrm{\Delta P}}{R}$ illustrates these relationships. Understanding hemodynamics is essential for diagnosing and treating cardiovascular diseases, as it provides insights into how changes in blood pressure, vessel diameter, and blood viscosity affect overall cardiovascular health.

How do blood flow, blood pressure, and resistance relate to each other in hemodynamics?

Blood flow (f), blood pressure (ΔP), and resistance (R) are interrelated in hemodynamics. Blood flow is directly proportional to the blood pressure gradient, meaning that an increase in ΔP leads to an increase in f. Conversely, blood flow is inversely proportional to resistance; as R increases, f decreases. The relationship is mathematically expressed as $f=\frac{\mathrm{\Delta P}}{R}$. This equation highlights that blood will flow more easily when the pressure gradient is high and resistance is low, ensuring efficient circulation throughout the cardiovascular system.

What factors affect resistance in blood vessels?

Three main factors affect resistance in blood vessels: blood viscosity, vessel length, and vessel diameter. Blood viscosity refers to the thickness of the blood; higher viscosity increases resistance. Vessel length impacts resistance because longer vessels provide more opportunities for friction between blood and vessel walls. Vessel diameter is the most easily altered factor; narrower vessels increase resistance, while wider vessels decrease it. These relationships are crucial for understanding how changes in these factors can impact overall blood flow and cardiovascular health.

How does blood viscosity influence resistance in the cardiovascular system?

Blood viscosity, or the thickness of the blood, directly influences resistance in the cardiovascular system. Higher viscosity means thicker blood, which encounters more friction as it flows through blood vessels, thereby increasing resistance. For example, substances like honey or a thick milkshake have high viscosity and flow slowly through a narrow straw, similar to how thick blood flows through vessels. Conversely, lower viscosity, like water, results in less resistance and easier blood flow. Understanding this relationship is essential for managing conditions that affect blood viscosity, such as dehydration or polycythemia.

Why is blood vessel diameter considered the most easily altered factor affecting resistance?

Blood vessel diameter is considered the most easily altered factor affecting resistance because many blood vessels contain smooth muscle in their walls. This smooth muscle can contract to narrow the vessel (vasoconstriction) or relax to widen it (vasodilation). These changes in diameter significantly impact resistance; narrower vessels increase resistance, while wider vessels decrease it. This ability to alter vessel diameter allows the body to regulate blood flow and pressure dynamically, adapting to various physiological needs and maintaining homeostasis.