Ventilation–Perfusion Coupling – External Respiration (Nursing)

00:01 So the final influence is going to be what we know as ventilation perfusion coupling.

00:08 So just a couple of vocabulary words when we say perfusion.

00:12 We're referring to the blood flow that is reaching the alveoli.

00:17 When we refer to ventilation, we're talking about the gas flow or the amount of gas that is reaching the alveoli.

00:25 So for optimal efficient gas exchange, we want our ventilation and our perfusion rates to be equal to each other.

00:36 These are going to be regulated by auto-regulatory mechanisms in the body.

00:42 When we look at our partial pressure of oxygen.

00:45 This is going to actually control perfusion by changing the diameter of our arterioles.

00:54 We look at the partial pressure of carbon dioxide.

00:57 This is actually going to control ventilation and it's going to do this by changing the diameter of our bronchioles.

01:07 So changes in the partial pressure of oxygen as I said before, is going to lead to changes in the diameter of arterioles.

01:16 When our alveolar oxygen is high.

01:19 So we have a high partial pressure of oxygen.

01:22 This is going to cause the arterioles to dilate.

01:26 But when the alveolar oxygen levels are lower the arterioles are going to constrict.

01:34 This constriction of the arterioles is then going to direct blood away from the alveoli that have low oxygen toward alveoli that have a higher oxygen load.

01:45 So that the blood can pick up more oxygen.

01:49 This is an interesting phenomenon because it is opposite of what we see and our systemic arterioles.

01:56 So in our systemic arterioles, we dialate our arterioles when our oxygen levels are low in order to get more oxygen and we constrict them when our oxygen levels are high or adequate.

02:12 So changes and our partial pressure of carbon dioxide in our alveoli, are going to affect the diameter of our bronchioles.

02:20 So it is going to have more of an effect on ventilation.

02:25 When our alveolar carbon dioxide levels are high we dilate are bronchioles in order to allow for more exhalation of the carbon dioxide.

02:35 But when our alveolar concentrations of carbon dioxide are low we're going to constrict.

02:41 The bronchioles.

02:45 The dilation again of these bronchioles.

02:48 It's what's going to allow for more elimination of our carbon dioxide.

02:55 So changes in the diameters of our local arterioles and bronchioles are what we refer to as ventilation perfusion.

03:04 So this ventilation perfusion is never going to be balanced across all of our alveoli.

03:10 And the reason why is because our alveoli themselves are never balanced.

03:15 There are regional variations between the alveoli due to things such as the effect of gravity on blood and air flow.

03:25 Also sometimes are alveolar ducts can become plugged and therefore those alveoli are not ventilated or have very low oxygen.

03:34 And because of that we do not want our blood going to these and adequately oxygenated alveoli and so we divert the blood to other alveoli that are more well-ventilated.

03:49 So, let's see what happens when we have a mismatching of ventilation and perfusion.

03:55 So and areas were ventilation is lower than perfusion or when there is a local hypoxia, and there's less oxygen in an alveoli.

04:06 We are going to constrict the arterioles so that we divert perfusion away from this local hypoxia to areas of the lungs that have adequate oxygen amounts.

04:20 We are going to do this until we match the ventilation and perfusion.

04:27 The opposite occurs when our ventilation is greater than perfusion and we have a higher than normal level of oxygen.

04:36 In this case instead of constricting arterioles and diverting blood away, we're going to dilate the arterioles so that we can adequately you get more blood to get some of this oxygen out of the alveoli into the blood.

04:53 And we do this until again we match ventilation and perfusion.