Lung volumes and capacities

Definitions & Key takeaways

Lung volumes refer to the volume of air in the lungs, measured at various phases of the respiratory cycle. Major lung volumes include the tidal volume, inspiratory reserve volume, expiratory reserve volume, and the residual volume.

The tidal volume is the amount of air inhaled or exhaled during a single breath. The inspiratory reserve volume is the additional air that can be inhaled after a tidal volume has been breathed in. The expiratory reserve volume is the additional air that can be exhaled after a tidal volume has been exhaled. And the residual volume is the amount of air left in the lungs after you've finished exhaling.

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Introduction0:00–0:31

The main job of the lungs is gas exchange, pulling oxygen into the body and getting rid of carbon dioxide. Normally, during an inhale - the diaphragm contracts to pull downward, and chest muscles contract to pull open the chest to suck in air like a vacuum cleaner, and then during an exhale - the muscles relax, allowing the lungs to spring back to their normal size pushing that air out.
Now, we can use a spirometer to measure the volume of air that moves in and out of the lungs with each breath using an instrument called a spirometer; the test is called spirometry.

Spirometry0:31–3:40

At this point there are more sophisticated electronic spirometers, but a classic example is having an air chamber submerged in water that the person can breathe into.
As they take air in, the chamber moves down into the water, which moves a pencil that traces as it moves, then when they breathe out, the chamber moves up and the pen moves down.
So, if this is a healthy adult woman, as she breathes the spirometer makes a wave-like tracing on the paper. The plot you end up with therefore has volume of air on the vertical axis, and the horizontal axis shows time.
During normal, quiet breathing the volume of air moving in and out with each breath is represented by the height of the wave and it’s called the tidal volume; it’s typically around 0.5 L or 500 ml.
After a few cycles, we might ask the woman to inhale the maximum volume of air that she can, and then exhale the maximum volume of air that she can.
The volume of air that she maximally inhales above the tidal volume is known as the inspiratory reserve volume, and it’s typically around 3 liters.
This is sort of a like a massive backup capacity that you don’t typically use, but might need to in a specific situation like if you’re going for a dive in the ocean.
Similarly, the expiratory reserve volume is the volume of air that she maximally exhales below the tidal volume, and it’s typically around 1.2 liters.
Now, even after she attempts to exhale all the air from the lungs, it turns out that some air still remains in the lungs and this is known as the residual volume, and it’s typically around 1.2 liters as well.
Combining the expiratory reserve volume and the residual volume together gives you the functional residual capacity, which would be about 2.4 liters.
Similarly, combining the tidal volume and the inspiratory reserve volume results gives us the inspiratory capacity which is 0.5 liters plus 3 liters or about 3.5 liters.
Going one step further and including the expiratory reserve volume as well, you get the vital capacity, at about 4.7 L, which is the volume of air that can be exhaled after a maximal inspiration - so inspiring all the way in and then exhaling all the way out.
Finally, adding the vital capacity and residual volume, you get the total lung capacity which is the total volume of air that the lungs can hold, and it adds up to 4.7 liters plus 1.2 liters - which based one what we have, is 5.9, nearly 6 L.
Finally, keep in mind that all of the lung volumes that we just calculated are considered static lung volumes rather than dynamic lung volumes, because they don’t involve the rate of airflow in and out of the lungs.
Now, even though we had the average residual volume of 1.2 L here using this spirometer the way it’s set up, wouldn’t actually be possible to figure out, since it’s the amount of air that a person cannot exhale even when they try.

Helium dilution method3:40–7:10

But it can be measured using something called the Helium dilution method. To do it, a known concentration of helium is placed into the spirometer to breath in - this can be written out as “CBEFORE”, and we also know the volume of the spirometer, which is “VSPIROMETER “.
So to find the total mass of helium, M, you just multiply the concentration times the volume or (CBEFORE X VSPIROMETER), since units are mg/mL times mL, which equals mg.
Next, the person is asked to breathe in the helium mixed air. The helium is insoluble in blood and lung tissue, so the helium stays in the lungs and within a few cycles of breathing, is redistributed equally between the spirometer and the lungs.
At that point the person is asked to exhale normally, which means that the volume of air in the lungs is the functional residual capacity.
Let’s call this FRC, and then you’ve also again got the volume in the spirometer. Neglecting the small volume in the trachea, the total volume after must therefore the volume of the spirometer plus the FRC.
At this point, the concentration of helium in the air which has now equilibrated between the lungs and the spirometer can be called CAFTER.
So, just like in the first system, you can take concentration C-after times the new volume, Vafter, and get the total amount of helium.
Since we didn’t actually change the amount of helium in the system, Mhelium is the same as it was at the start, and so these two equations can be combined to be Cbefore times Vspirometer equals Cafter times the sum of Vspirometer and FRC.
CBEFORE X VSPIROMETER = CAFTER X (VSPIROMETER + FRC) The concentration of helium before and after can be measured and the volume of the spirometer is known, so that leaves only the FRC as the unknown value.
So at this point, it’s a matter of plug and chug! So, let’s say that the spirometer has a volume of 300 milliliters, and we put some helium in the spirometer and measure the concentration, and it turns out to be 0.54 mg per ml.
Now when the person is connected to the spirometer, let’s say the concentration of helium is measured to be 0.06 mg/ml after the person exhales normally.
Now, solving for FRC—.54 times 300 equals 162, which equals .06 times 300 plus FRC, and 162 divided by 162 minus 300 equals the FRC, which ends up being 2400 mL.
Now, remembering back to our crazy plot, FRC is equal to the expiratory reserve volume plus the residual volume, simple spirometry gives us an ERV or 1.2 liters, SO, therefore, our residual volume is 2400 minus 1200 ml, which is 1200 mL, or 1.2 Liters!

Plethysmography7:10–9:48

Another way of measuring the FRC involves body plethysmography. In this technique, a person sits inside a plethysmograph, which is an airtight box.
The air pressure and volume of the box, as well as any changes in them, can be measured - so let’s call the initial values PBOX1 for the pressure and VBOX1 for the volume of that box.
Remember that Boyle’s law states that pressure times volume of any gas is constant at a given temperature, so k for some constant in the box.
So the person is asked to breathe in and out through the mouth through a mouthpiece which measures air pressure in the mouth.
The mouthpiece is remote-controlled and is closed to the outside world right after a normal exhalation. At this point the pressure is measured as Pmouth1, and since they just exhaled, the air in the lungs is the FRC, so at this point the Pressure in the mouth times the volume in the lungs or FRC is equal to another constant.
Next, the person is asked to try inhaling even though the mouthpiece is closed. As they try to inhale, the lungs expand a tiny bit, but no air can actually come in since it’s a closed system, and therefore the pressure in the lungs decreases.
Here’s the neat part - since the person and the box are in a closed system, these changes cause an increase in pressure in the box as well as a decrease in volume of the box!
Both of which can be measured! So the new pressure in the box is Pbox2, and the new volume of the box is the original volume Vbox1 minus some small change in volume delta V.
Applying Boyle’s law to the box, we get: PBOX1 X VBOX1 = PBOX2 X VBOX1 - ΔV) And from this, since we know everything else, we can solve for that small change delta V.
Sweet, and again applying Boyle’s law to the mouthpiece, the new pressure is Pmouth2, and the new volume is the original volume FRC plus that small change delta V.
PMOUTH1 X FRC = PMOUTH2 X (FRC + ΔV) Since again the pressures are known, and now delta V is known, the only thing left to solve for is the FRC!
Wow. Normally, the value of FRC obtained by plethysmography is equal to the FRC obtained by the helium dilution method.
But if a person has an obstructive lung disease, then the FRC from plethysmography ends up being slightly more than the FRC from helium dilution, since that helium doesn’t diffuse into every part of the lungs.
All right, as a quick recap, some common lung volumes include the tidal volume, inspiratory reserve volume, expiratory reserve volume, and the residual volume - every other measure is combination of these four.

Review9:48–11:11

Calculating the residual volume is tricky and can be done by first calculating the FRC which is done using either the helium dilution method or body plethysmography.