Pulmonary Function 101

Understanding How your Lungs "Work "

                by Mark W. Mangus, Sr. BSRC, RRT, RPFT, FAARC

O2 and CO2 - Gas Exchange

I think that an adjustment in how many think of O2 and CO2 exchange in relation to breathing might help them better understand how that exchange occurs AND will clear up some of the confusion people express in relation to how that exchange becomes disturbed. 

What follows is long and essentially a lesson in “pulmonary physiology” that, once understood, should give readers a much better understanding of many aspects of their disease and how and why many of the changes occur AND are, or are not, ‘changeable’.

There are two major principles that we all need to understand.  Gas exchange – that is, oxygen and carbon dioxide ‘gases’ – is *constantly* occurring in the lungs, regardless of what ventilation is doing.  It is a “passive process”.  That is, the oxygen and carbon dioxide pressures in the blood strive to match the oxygen and carbon dioxide pressures in the alveoli and occurs in accordance with the principles of equilibrium.  How much oxygen and carbon dioxide are exchanged between the blood and the gas in the alveoli is dependent upon ‘time’ ; how much time the blood and alveolar gases are exposed to one another AND how much time it takes for oxygen and carbon dioxide to equilibrate as much as they can in accordance with the time they are exposed to each other. 

Remember back to your school science studies wherein you learned about “diffusion”, that is, molecules (in this case gas molecules) strive to equilibrate moving from higher pressure toward the lower pressure in an effort for the two pressures to become equal.  This is the passive principle at play in the lungs at the level or the alveoli.  However, for a future point, I want to make it clear that while this definition of “diffusion” applies to gas exchange, it is not the ONLY force that determines your “Diffusing Capacity” (DLCO or DCO), when you have that measured on a pulmonary functions test.  I’ll explain more on that later.

Secondly, when at rest after having taken in a breath, the lungs hold several liters of gas.  For purpose of this illustration, let’s use the figure of 3 liters.  If you take in the deepest breath you can, that volume expands to about 4.5 liters (Total Lung Volume or Capacity = TLV or TLC), the change representing what we call Inspiratory Reserve Volume (1500 ml) (IRV).  A normal breath is about a half liter (500 ml) and is called the “Tidal Volume” (as in the ‘tide’ coming in and going out and is connoted as V sub T or VT when subtext is not available to type it correctly).  At the end of a normal breath, there is still a large volume of gas left in the lungs, about 2500 ml (2.5 liters).  This represents three volumes that we call “resting volumes”.  First, the total amount represents what we call Functional Residual Capacity (FRC).  Within the FRC are two volumes.  The Expiratory Reserve Volume (ERV) is the amount of air that you can go on to exhale forcefully after exhaling a normal VT and takes you down to all the air you can blow out of your lungs.  When you have reached that point of maximal exhalation, there is still about 25 % of your total lung volume left within your lungs that you cannot blow out.  We call that Residual Volume (RV).

When we breathe with normal lungs, the gas within the lungs becomes “diluted” with the fresh air that comes in during tidal breathing.  The oxygen in the alveoli fluctuates between a pressure of about 110 and 90 about or about 3 % as oxygen is taken up by the blood.  Carbon Dioxide fluctuates between a pressure of about 45 and 35, or about a 22 % change from breath to breath as carbon dioxide is released by the blood into the alveoli.  This results in normal measurements of O2 and CO2 of about 100 (mmHg) and 40 (mmHg), respectively, as measured on a blood gas.

In COPD, because of the damage done to the airways and the consolidation and expansion of the damaged alveoli,  the “resting volumes” in the lungs increase (FRC,ERV and RV) representing trapped air that cannot any longer be exhaled during normal breathing.  While the TLV can increase as COPD worsens, it does not increase all that much because of the restriction of the chest wall and the resistance to displacement of the abdomen.  BUT, within the lungs, the various volumes can shift drastically, hence the causes of the progressive symptoms and breathing and gas exchange difficulties.

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This page was last updated January 19th, 2011