Introduction – Pulmonary Blood Flow

by Thad Wilson, PhD

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    00:02 In this lecture, we’re going to deal with pulmonary blood flow.

    00:05 We have a number of learning goals that we want to obtain.

    00:08 The first of these is is after this lecture, you’ll be able you understand the mechanism of pulmonary blood flow, both increases and decreases.

    00:18 After this lecture, you will also be able to identify the factors that affect pulmonary blood flow especially pulmonary artery pressure, lung volumes, hypoxia and gravity.

    00:30 Pulmonary blood flow can probably be best understood if we use a little bit of a compare and contrast approach.

    00:37 You are very familiar with the arterial circulation.

    00:40 And so in this, this is what everyone’s circulation is on the arterial side of the circuit, that’s from the left ventricle, all the way around to the capillaries.

    00:48 This is characterized by very high blood pressure, usually 120/80.

    00:53 There’s high resistance in this particular circuit.

    00:56 And the nice thing is, it’s carrying a lot of oxygen, having a high PaO2 and a very low PaCO2.

    01:04 After you go through a capillary bed, you’re going to extract O2 and give up CO2.

    01:11 So on the venous side of the circulation, the pressures are lower, the resistances are lower.

    01:17 You also have lower O2 and higher CO2.

    01:21 So that’s the systemic side of the circulation.

    01:24 If we compare now this to the pulmonary circulation, the pulmonary artery still has very low blood pressure.

    01:33 It has very low resistance, low O2.

    01:36 Why still low O2? It’s because it’s going from the right ventricle to the pulmonary vasculature to undergo gas exchange.

    01:44 It hasn’t gone through gas exchange yet, so O2 is still low.

    01:49 CO2 is high.

    01:51 But once you’ve gone through the lungs, on the pulmonary vein side, you have still low pressure, low resistance, but now you have oxygenation.

    02:01 So O2 is high and CO2 is low.

    02:05 So this is a nice way to think about pulmonary blood flow.

    02:10 Hopefully, you’ll recognize by looking at the two different blood flows that pressures are very low on both sides of the lung.

    02:17 Resistances are low on both sides of the lung.

    02:20 So there are certain factors that affect blood on the pulmonary side that don’t affect blood flow on the systemic side.

    02:28 So let’s go through how blood flow changes through the pulmonary circuit.

    02:35 This is a graph that looks at pulmonary vascular resistance, which is the resistance of blood flow through the tubes of the lung.

    02:42 And then we’re comparing that to mean pulmonary artery pressure.

    02:47 So this is the pressure pushing the blood through those tubes.

    02:51 And this forms a very unique relationship in which at low pulmonary artery pressures, resistance is high.

    02:59 But as pulmonary artery pressure increases, you actually get a decrease in pulmonary vascular resistance.

    03:06 And this is fairly unique.

    03:08 So as pressure goes up, you have lowering of the resistance.

    03:13 What does this do for blood flow is as pulmonary blood flow increases as pulmonary artery pressure increases.

    03:22 So then they are more linearly related in a positive manner.

    03:26 This is a very interesting process.

    03:29 It means that it’s not governed in the same way as your systemic vasculature, that will be covered in the cardiovascular section.

    03:37 So this brings up a very unique point of view and that is we have from the previous slide, an increase in pulmonary artery pressure.

    03:46 And as pressure increases, the pulmonary vascular resistance decreases.

    03:52 So what that meant was as pulmonary artery increased, blood flow also increased.

    03:58 How does this happen? Well, it seems to occur via two mechanisms.

    04:04 And let me go through what those are.

    04:06 The first is going to be a recruitment and the second is a distension issue.

    04:10 But let’s kind of walk through these step by step.

    04:13 In normal pulmonary vasculature, some of the blood vessels are collapsed even during the rested state.

    04:21 There are some of the blood vessels that are not collapsed, but yet the blood is not moving very rapidly through those particular capillaries.

    04:32 So both of those items have low blood flow through them.

    04:39 There are some of the capillaries though, just as you would expect, are open and do conduct blood.

    04:45 So there’s a little bit of a notion here that you only get blood flow through some of the capillaries.

    04:51 Other parts of the capillaries are collapsed and some have just low blood flow.

    04:57 As pressure though increases, what you get is an opening of these blood vessels.

    05:04 First the ones that were previously collapsed start to open and the ones that were not conducting flow through very rapidly as the pressure increases, those are pushed through to a much greater extent.

    05:19 The ones that had had normal blood flow, widen.

    05:24 And so that is the distension component versus the recruitment component.

    05:30 So we have recruitment that occurs through the previously collapsed blood vessels.

    05:36 We had distension that occurred through the blood vessels that were already open that became wider.

    05:43 Therefore, we have an end result of an increase in perfusion or blood flow through the entire lung.

    05:50 So this is how the process of increasing pulmonary artery pressure increases pulmonary blood flow through both a recruitment and a distension mechanism.

    06:02 Now the other factor that dramatically affects blood flow through the lungs is the lung volume itself.

    06:11 So lung volume can be thought of as how inflated the lung is versus how much you’ve been able to breathe out.

    06:19 So if we plot pulmonary vascular resistance over lung volume.

    06:24 With low lung volume being on the left hand side of the graph and high lung volume being on the ride hand side of graph and those are denoted as RV or residual volume at low lung volume.

    06:36 And TLC or total lung capacity at high lung volume.

    06:41 There is an effect of compressing the capillaries.

    06:47 So as you compress capillaries, you get an increase in resistance at high lung volumes.

    06:54 Oppositely, in terms of the blood vessels themselves, as you go to a low lung volume, there is an increase in pulmonary vascular resistance.

    07:08 So we have two different factors that are affecting here.

    07:13 One having high resistances at low lung volume and then coming down and the other is having high resistance at high lung volumes.

    07:25 This yields a total pulmonary vascular resistance in this kind of a U shaped curve.

    07:33 That means that at a moderate lung volume, the resistance through the lungs, in terms of the blood flow, is lowest.

    07:43 While you’re at a low lung volume, you have high resistance.

    07:46 And at high lung volume, you have high resistance to pulmonary blood flow.

    07:51 Okay. So how in the world do these happen? Let’s first take the compression effect and discuss that.

    07:57 And then we'll discuss the mechanism behind the supply vessel dilation effect.

    08:03 So if we look at the compression effect and that’s denoted here as low lung volumes, low resistance, high lung volumes, high resistance.

    08:13 You can see on this graph in the slide here where we have capillaries denoted as red.

    08:20 And the alveoli as a kind of hexagon white shape.

    08:25 As you inflate those alveoli, you can notice that they will squeeze out some of the red and this is compressing these pulmonary capillaries.

    08:38 You might think that seems very odd that the capillaries can be compressed such as like this.

    08:44 However, you should think of there is low blood pressure in the pulmonary vasculature.

    08:51 So factors such as inflating these air sacs can actually increase the resistance of flow through that as there’s no longer as large of a luminal diameter.

    09:03 So that is the compression effect.

    09:05 Inflate the air sacs, you have more resistance than you did at low lung volumes.

    09:12 Now, if you look at the vessel dilation effect, for this, we need to remember that the blood flow supply is surrounded by not only alveoli, but also other connective tissue.

    09:24 And what you can do is pull on this connective tissue by creating a negative pressure.

    09:30 And we call that a negative pleural pressure or P sub PL, At this point, you have all the air sacs pulling on that particular blood vessel.

    09:42 And what happens is it makes that blood vessel wider or have a larger luminal diameter.

    09:49 And in that case, it would decrease its pressure.

    09:53 And as you decrease the pressure then, you’d have more flow through that area because of lower resistance.

    09:59 And so that denotes the low lung volume, having higher resistance, and high lung volumes, having lower resistance to pulmonary blood flow.

    10:09 Putting all these together, it gives us that U shape of pulmonary blood flow to pulmonary vascular resistance in terms of the lung volume effect.

    10:22 Now, the last factor that really affects pulmonary blood flow is the amount of oxygen that is in each air sac.

    10:30 And this is a difference that occurs in the pulmonary vasculature versus systemic vasculature.

    10:37 So low PO2s cause pulmonary blood vessels to vasoconstrict only within a very, very small area.

    10:45 And this is different from systemic vasculature and it’s usually a low PO2 vasodilates the blood vessel.

    10:54 So this is an opposite kind of effect.

    10:57 Now why this is important.

    10:59 This can actually be a beneficial effect as you can decrease the blood flow to alveoli that have a low oxygen concentration and preferentially shunt that blood flow to alveoli that have a high oxygen concentration.

    11:16 So in fact, it allows you to oxygenate blood even to a greater extent than would occur if you perfuse all the different alveoli.

    11:26 Okay.

    11:27 That seems a little bit complex, I know, but I have to add one more factor to this.

    11:32 And that factor is that if you have condition in which PO2 decreases across all the alveoli, you get a vasoconstriction throughout the lung.

    11:44 And so in this particular case, it overrides particularly that local effect and you get vasoconstriction across the whole lung.

    11:52 So you have both a local effect and a kind of systemic effect of low PO2, both of which cause vasoconstriction in the lungs.

    12:01 If you vasoconstrict these blood vessels, you increase the amount of resistance which decreases pulmonary blood flow.

    12:08 Okay.

    12:09 The last kind of thing that affects blood flow is gravity.

    12:14 So this is really only going to affect you when you’re in the upright position.

    12:18 So for example, even if you’re sitting now in front of your monitor at home, what you have is an effect of being upright on the lung.

    12:27 Gravity is naturally pulling blood flow to the lower aspects of the lung.

    12:34 It seems a little bit complex.

    12:36 So let’s go through these step by step.

    12:38 We have broken the lung up into three zones.

    12:41 So I’m going to first talk you through zone number 1.

    12:44 It has the lowest amount of blood flow.

    12:47 In fact, it may have very little or at all much blood flow in it.

    12:51 And the reason is is because P, small A, which is the pressure within the pulmonary arterial, is lower than what is in P, capital A, and that P, capital A is what’s the pressure within the alveoli itself.

    13:10 And if the pressure in the alveoli is greater than the pressure in the arterial that’s moving through, you have very low flow because the arterial is compressing the capillary.

    13:23 Okay? In zone 2, this is a little bit easier process to think of.

    13:29 There’s higher amounts of blood flow and the reason why there’s a higher amount of blood flow is because the pressure within the arterial side of the capillary is large enough to get some blood flow past the alveolar pressure.

    13:47 And so you have a moderate amount of blood flow.

    13:50 You have the highest amount of blood flow, when P, small A, which is the partial pressure – Sorry. The pressure within the arterial side of the capillary is greater than the pressure, which is in the alveoli.

    14:04 If that’s larger, there’s no impedance of blood flow through the circuit.

    14:10 So zone 1, zone 2, and zone 3 are all based upon what is the pressure within the arterial side of the capillary versus the pressure that’s inside the alveoli or air sac.

    14:23 If you have higher pressures on the arterial side of the capillaries, you will get more blood flow past the air sac or alveoli.

    14:33 The more that that alveoli has a higher pressure, than what’s on the arterial side that will impede blood flow.

    14:40 And that happens as a lowering of zone effect.

    14:47 And the reason for that is because there’s a hydrostatic effect on the blood that affects its pressure.

    14:55 So you can think of this as a process in which at heart level, you have a certain pressure.

    15:01 Anytime you go below the level of the heart, the pressure will increase.

    15:05 And you’ve probably all experienced this if you have, let’s say, take your socks off and look down at the tops of your feet.

    15:14 If you’re standing upright, you’re blood vessels in the tops of your feet may be distended because there’s a higher amount of pressure as you are below the level of the heart.

    15:25 If you raise your feet up to heart level, the pressure within those vessels goes down and they will no longer be distended or harder to see.

    15:33 So this is simply an effect of gravity on blood pressure.

    15:39 Why does it affect pulmonary blood flow so much but doesn’t affect systemic blood flow? It’s because pulmonary blood flow is so low to start with.

    15:48 Therefore, only small effects such as things like gravity, lung inflation, the amount of hypoxia that you might have affect the pulmonary vasculature so much more because pressure started off so low.

    16:06 We’ve covered a lot of the important topics in this particular lecture.

    16:09 Of these, you’ll need to make sure you understand the mechanisms of how pulmonary blood flow both increases and decreases.

    16:18 Also, you’re able to understand and be able to identify the factors of pulmonary blood flow such as pulmonary artery pressure, lung volumes, hypoxia and gravity.

    About the Lecture

    The lecture Introduction – Pulmonary Blood Flow by Thad Wilson, PhD is from the course Respiratory Physiology.

    Included Quiz Questions

    1. Blood in the pulmonary artery has a low concentration of oxygen.
    2. It has high resistance.
    3. Blood in the pulmonary artery has a high concentration of oxygen.
    4. Blood in the pulmonary vein has a high concentration of carbon dioxide.
    5. Blood in the pulmonary artery comes from the left ventricle.

    Author of lecture Introduction – Pulmonary Blood Flow

     Thad Wilson, PhD

    Thad Wilson, PhD

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