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I am reading about Inspiratory Reserve Volume and Expiratory Reserve Volume in my textbook. So my general question is aren't the two terms show the same amount of air ?

I would like to tell you why I think so. Like IRV tells us about the maximum amount of air we can intake other than normal intake. So this extra amount is what going inside our lungs and I think this is the part which comes out in the form of ERV .

So shouldn't the total capacity of lungs be equal to (Tidal volume + IRV + Reserved Volume) only ? Why do we need to include the term ERV in the above equation ?

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You're misunderstanding a bit. Here are some definitions from Wikipedia, where I added some bold for emphasis:

Expiratory reserve volume: the maximal volume of air that can be exhaled from the end-expiratory position

Inspiratory reserve volume: the maximal volume that can be inhaled from the end-inspiratory level

In plain terms, ERV is how much you can still breathe out after you've breathed out all you normally would. You can try it out yourself: when you're finished breathing out and ready to start breathing in, try to blow out a bit, you'll find there is still air there. You don't need to breathe in extra to be able to breathe out this extra.

Similarly, IRV is how much you can still breathe in after you've breathed in all you normally would.

If you were to do a "fullest possible breath" that would include inspiring as much extra as possible, plus the normal breath volume, plus exhaling the last little bit you can:

IRV + Tidal Volume + ERV

There's still a little bit of residual air left over (RV), though, so:

IRV + Tidal Volume + ERV + RV

gives you the total capacity.

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