Effects of Lung Volume – Pulmonary Blood Flow

00:00 Now the other factor that dramatically affects blood flow increases pulmonary blood flow through both a recruitment and a distension mechanism.

00:04 through the lungs is the lung volume itself.

00:10 So lung volume can be thought of as how inflated the lung is versus how much you’ve been able to breathe out.

00:17 So if we plot pulmonary vascular resistance over lung volume.

00:22 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.

00:35 And TLC or total lung capacity at high lung volume.

00:39 There is an effect of compressing the capillaries.

00:45 So as you compress capillaries, you get an increase in resistance at high lung volumes.

00:52 Oppositely, in terms of the blood vessels themselves, as you go to a low lung volume, there is an increase in pulmonary vascular resistance.

01:07 So we have two different factors that are affecting here.

01:11 One having high resistances at low lung volume and then coming down and the other is having high resistance at high lung volumes.

01:23 This yields a total pulmonary vascular resistance in this kind of a U shaped curve.

01:32 That means that at a moderate lung volume, the resistance through the lungs, in terms of the blood flow, is lowest.

01:41 While you’re at a low lung volume, you have high resistance.

01:45 And at high lung volume, you have high resistance to pulmonary blood flow.

01:49 Okay. So how in the world do these happen? Let’s first take the compression effect and discuss that.

01:55 And then we'll discuss the mechanism behind the supply vessel dilation effect.

02:01 So if we look at the compression effect and that’s denoted here as low lung volumes, low resistance, high lung volumes, high resistance.

02:12 You can see on this graph in the slide here where we have capillaries denoted as red.

02:19 And the alveoli as a kind of hexagon white shape.

02:23 As you inflate those alveoli, you can notice that they will squeeze out some of the red and this is compressing these pulmonary capillaries.

02:36 You might think that seems very odd that the capillaries can be compressed such as like this.

02:43 However, you should think of there is low blood pressure in the pulmonary vasculature.

02:49 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.

03:01 So that is the compression effect.

03:03 Inflate the air sacs, you have more resistance than you did at low lung volumes.

03:10 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.

03:22 And what you can do is pull on this connective tissue by creating a negative pressure.

03:28 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.

03:41 And what happens is it makes that blood vessel wider or have a larger luminal diameter.

03:47 And in that case, it would decrease its pressure.

03:51 And as you decrease the pressure then, you’d have more flow through that area because of lower resistance.

03:57 And so that denotes the low lung volume, having higher resistance, and high lung volumes, having lower resistance to pulmonary blood flow.

04:07 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.