Upright Lung – Hypoxemia and Hypercapnia

by Thad Wilson, PhD

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    00:01 So now to apply the ventilation to perfusion inequality from that theory example that we just went through to something that is practical, this is the upright lung.

    00:11 To do that, you’ll have to remember about blood flow in the different zones of the upright lung.

    00:16 Remember in the apex of the lung, there’s going to be a low blood flow.

    00:22 In the base of the lungs, there’s going to be high blood flow.

    00:25 This will be important as we start trying to match ventilation to perfusion.

    00:31 Now, pulmonary blood flow can also be looked at on its effects of gravity.

    00:35 The lung in the upright condition can be broken into three different zones.

    00:41 Zone 1, zone 2 and zone 3.

    00:44 Zone 1 is denoted by this type of a situation in which there’s very little with any blood flow travelling through it.

    00:52 And that is because the arterial side of the capillary to venous side of the capillary is being impinged on by the alveolar pressure.

    01:02 So in this case, P, capital A, for alveolar is larger than P small A for the arterial side of the capillary to P, small V which is the venous side of the capillary.

    01:18 A condition where there is higher blood flow occurs in this condition in which P, small A, is greater than P, capital A, which is greater than P, small V.

    01:29 How this condition works is on the arterial side of the capillary, there is enough pressure in it that it will be able to push pass the alveolar pressure.

    01:41 And therefore, there will be some flow.

    01:46 Where there is a lot of flow or the highest flow in the lungs is in zone 3 where P, small A, is larger than P small V, which is larger than P, capital A.

    01:57 And this condition, the arterial side of the capillary has enough pressure in it that there is no effect of the alveoli.

    02:07 And therefore, flow goes through continuously and that has a highest amount of blood flow.

    02:12 When dealing with ventilation to perfusion inequalities, we also need to now take into account the ventilation component.

    02:19 Now ventilation will be very dependent upon what part of the lung you’re in and this is based upon the gravity effect.

    02:26 Just like this particular coil, it’s stretched out at the top of the lung or the apex and it’s not stretched out as much on the bottom part of the lung or the base.

    02:39 The lung works in the same parameter in which the top parts of the lung are pulled and the bottom part is compressed.

    02:47 And what this allows for is there to be different interpleural pressures between each of these particular components of the lung.

    02:57 At the base of the lung, there is compression which increases pleural pressure and that is different from the apex of the lung in which it is stretched and therefore pleural pressure is larger.

    03:11 This allows for a decrease in compliance in the apex of the lung and an increase in the compliance at the base of the lung.

    03:21 What this is going to mean is as flow goes into the lung, it will preferentially go to the area that has the higher compliance or the base rather than going to the top portions of the lung because of a decrease in compliance.

    03:38 So now let’s take with some practical examples.

    03:42 So here, we have the apex of the lung so this will be a PAO2 of 125 up here.

    03:55 Now at the apex of lung, there is a very large decrease in blood flow that we just went through because of this effect of gravity.

    04:05 There’s also a decrease in ventilation because of the lower compliance.

    04:11 When we look at the ventilation to perfusion ratio, this increases the ventilation to perfusion ratio.

    04:18 Why? Because the denominator in this particular component went down to a greater degree than the numerator.

    04:24 The result or the rationale that happens because of this increase in ventilation to perfusion ratio, there’s an increase in PO2, a decrease in PCO2, and pH rises slightly.

    04:38 In the base of the lung, so this goes down here to the P, capital A, of 90.

    04:43 There’s an increase in blood flow, an increase in ventilation, but the overall effect is a decrease in the ventilation to perfusion ratio.

    04:52 Again, because blood flow increases to a greater degree than the amount of ventilation increase.

    04:59 What happens to the portions of blood that is leaving the bottom portion of the lung or the base, PO2 is lower, PCO2 is higher and pH is also lower.

    05:13 Therefore, a person that is in the upright position always has a ventilation to perfusion inequality.

    05:22 Because there is a lower ventilation to perfusion ratio in the base of your lungs compared to the apex.

    05:28 All due to gravity, its effect on blood flow and compliance.

    05:35 We can graph this in terms of a curve that we look at the PO2 on the X-axis and on the Y-axis, PCO2.

    05:47 So if we have a place in which there’s a high ventilation to perfusion ratio, it’s at the edge of the ventilation to perfusion ratio graph and it can be seen in zone 1.

    06:00 Both of those have a high ventilation to perfusion ratio.

    06:04 A normal ventilation to perfusion ratio will then continually move towards the left.

    06:13 And if you have a low ventilation to perfusion ratio such as in zone 3, it continues to move towards the left on this graph and finally here.

    06:24 So we can quantify what part of a ventilation to perfusion ratio we have based upon the PO2s and PCO2s.

    06:36 Okay. We’ve covered the four different areas of the hypoxemias.

    06:40 Now, let’s cover hypercapneas.

    06:43 Hypercapneas are a lot simpler but what it has to do with in the red portion of this particular graph is a high CO2 component.

    06:53 So that PaCO2 is higher than normal.

    06:58 This could occur because of a decrease in alveolar ventilation, a very severe ventilation to perfusion inequality and finally, I could occur if there’s an increase in CO2 production without an appropriate ventilation compensation.

    07:17 So let’s go through these couple hypercapneas.

    07:22 Hypercapnea is all related to one particular formula which is known as the alveolar ventilation equation.

    07:29 Luckily, unlike the alveolar gas equation, it is not something that we calculate all that often but I’ll show it to you here because of the relationship that you can see as this occurs.

    07:42 And that is there’s an inverse relationship between alveolar ventilation and carbon dioxide.

    07:49 If alveolar ventilation goes up, carbon dioxide goes down.

    07:53 And the opposite occurs as carbon dioxide levels go up, alveolar ventilation goes down.

    08:00 These always are inverse of each other.

    08:03 So if you measure a CO2 level that is too high, you know that alveolar ventilation rate is low.

    08:10 If you measure a CO2 rate that is too low, you’ll know that alveolar ventilation rate is high.

    08:17 These will always be opposite of each other.

    08:20 So we utilize this equation simply because of its relationship between these two variables.

    About the Lecture

    The lecture Upright Lung – Hypoxemia and Hypercapnia by Thad Wilson, PhD is from the course Respiratory Physiology.

    Included Quiz Questions

    1. Zone 2
    2. Zone 1
    3. Zone 3
    4. Zone 4
    1. Zone 3
    2. Zone 1
    3. Zone 2
    4. Zone 4
    5. Zone 5
    1. pH
    2. Compression of alveoli
    3. Blood flow
    4. Ventilation
    5. Partial pressure of carbon dioxide
    1. Zone 2
    2. Zone 1
    3. Zone 3
    4. Zone 4
    5. Zone 5
    1. 50 mmHg.
    2. 10 mm Hg.
    3. 20 mmHg.
    4. 30 mmHg.
    5. 40 mmHg.

    Author of lecture Upright Lung – Hypoxemia and Hypercapnia

     Thad Wilson, PhD

    Thad Wilson, PhD

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