Effects of Hypoxia – Pulmonary Blood Flow

00:01 Now, the last factor that really affects pulmonary blood flow is the amount of oxygen that is in each air sac.

00:09 And this is a difference that occurs in the pulmonary vasculature versus systemic vasculature.

00:16 So low PO2s cause pulmonary blood vessels to vasoconstrict only within a very, very small area.

00:24 And this is different from systemic vasculature and it’s usually a low PO2 vasodilates the blood vessel.

00:33 So this is an opposite kind of effect.

00:36 Now why this is important.

00:38 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.

00:55 So in fact, it allows you to oxygenate blood even to a greater extent than would occur if you perfuse all the different alveoli.

01:04 Okay.

01:05 That seems a little bit complex, I know, but I have to add one more factor to this.

01:10 And that factor is that if you have condition in which PO2 decreases across all the alveoli, you get a vasoconstriction throughout the lung.

01:22 And so in this particular case, it overrides particularly that local effect and you get vasoconstriction across the whole lung.

01:31 So you have both a local effect and a kind of systemic effect of low PO2, both of which cause vasoconstriction in the lungs.

01:40 If you vasoconstrict these blood vessels, you increase the amount of resistance which decreases pulmonary blood flow.

01:46 Okay.

01:47 The last kind of thing that affects blood flow is gravity.

01:53 So this is really only going to affect you when you’re in the upright position.

01:57 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.

02:06 Gravity is naturally pulling blood flow to the lower aspects of the lung.

02:13 It seems a little bit complex.

02:15 So let’s go through these step by step.

02:16 We have broken the lung up into three zones.

02:20 So I’m going to first talk you through zone number 1.

02:23 It has the lowest amount of blood flow.

02:26 In fact, it may have very little or at all much blood flow in it.

02:29 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.

02:49 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 alveolar is compressing the capillary.

03:02 Okay? In zone 2, this is a little bit easier process to think of.

03:08 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.

03:26 And so you have a moderate amount of blood flow.

03:28 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.

03:43 If that’s larger, there’s no impedance of blood flow through the circuit.

03:49 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.

04:02 If you have higher pressures on the arterial side of the capillaries, you will get more blood flow past the air sac or alveoli.

04:12 The more that that alveoli has a higher pressure, than what’s on the arterial side that will impede blood flow.

04:19 And that happens as a lowering of zone effect.

04:26 And the reason for that is because there’s a hydrostatic effect on the blood that affects its pressure.

04:34 So you can think of this as a process in which at heart level, you have a certain pressure.

04:40 Anytime you go below the level of the heart, the pressure will increase.

04:43 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.

04:52 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.

05:03 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.

05:12 So this is simply an effect of gravity on blood pressure.

05:17 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.

05:26 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.