Gas Exchange

Okay guys we’re going to talk about the basics of gas exchange. We won’t go into any advanced concepts, but, it’s really important that you understand the basic physiology of gas exchange, because it affects every disease process we’re going to talk about.

The first thing is you really need to know is the difference between these four words. Sometimes people use them interchangeably, but they are not the same. Respiration is the actual act of breathing in and breathing out, moving air in and out of your lungs. Oxygenation is the process of putting oxygen into the bloodstream to be circulated. So that happens within the lungs as well. Ventilation is actually defined as the process by which oxygen and CO2 are exchanged. Think about a ventilation system in a building – the purpose is to remove the unclean air. So ventilation is the process of getting rid of CO2 in exchange for oxygen. If you don’t have adequate respirations, it is impossible to get adequate oxygenation and ventilation, because there’s no air moving. But, you could have perfect respirations, but struggle to oxygenate and ventilate because of problems within the lungs themselves. Finally, perfusion is the ability to get oxygen to the tissues. The lung tissue needs to be perfused with blood, but, the act of perfusion takes place within the blood vessels and tissues as opposed to within the lungs.

So remember that we said the alveoli are the functional unit of the lung, this is where all gas exchange happens. Deoxygenated blood comes into the capillaries that surround the alveoli, they release their CO2 into the alveoli, and oxygen diffuses from the alveoli into the blood. Then, oxygenated blood leaves the capillary bed to return to the heart. What we’ll see is that there needs to be contact, so-to-speak, between the air in the alveoli and the blood in the capillaries. All of this happens by diffusion, but if anything causes a disconnect here, it’s going to impair gas exchange.

The two big things that can cause problems are collapsed alveoli and decrease blood flow. With collapsed alveoli you have your normal blood vessel here with blood flowing in and flowing out normally, but you have a deflated alveoli here and therefore the contact points between the alveoli and the capillaries are decreased. The two most common causes of this are atelectasis like we talked about in the previous lesson, or pulmonary edema because fluid builds up around the alveoli causing them to collapse. The second problem we see causing gas exchange problems is decreased blood flow to the alveoli. So you have a normally inflated alveoli here except that for whatever reason the blood flow is not getting to this capillary bed adequately. This could be because of a clot that blocks blood flow all together, it could be because of significantly low blood volume so there’s just less perfusion, or because of severe vasoconstriction that reduces blood flow to the lungs. Both of these situations are going to limit the body’s ability to perform gas exchange. As we look at different disease processes in this course you’ll see these situations more clearly.

Now, we know that the lungs play a role in acid-base balance, but we can also monitor gas exchange using arterial blood gases. You’ll see that your SpO2 and PaO2 will tell you about your oxygenation ability, while your pCO2 tells you about ventilation. Make sure you check out the ABG lesson in the labs course to learn more.

So remember the key terms are respiration, ventilation, oxygenation, and perfusion. Make sure that you review those terms so that you can follow discussions of pathophysiology in the coming lessons. Remember that the alveoli and capillaries need to have good contact in order for gas exchange to occur. And remember that we use arterial blood gases to monitor the effectiveness of our gas exchange.

We hope this basic discussion of gas exchange helps in the coming lesson so that you can better understand the pathophysiology and disease processes we’re going to talk about. Now go out and be your best selves today, and, as always, happy nursing!