The table below shows how the usable air available for gas exchange during resting periods varies with the rate of breathing while the total amount of air breathed per minute stays the same.

Notice how while the total amount of air breathed in per minute stays at 6 litres, the amount of usable air available for gas exchange decreases with increasing breath rate from 4.8 litres per minute at 8 breaths per minute to zero at 40 breaths per minute. Explanation for this phenomenon can be found in understanding that the dead air space from the nostrils to the top of lungs stays the same at 0.15 litres while the amount of air breathed in through the nostrils per breath decreases from 0.75 litres per breath at 8 breaths per minute to 0.15 litres per breath at 40 breaths per minute.

In comparison, during moderate exercise, we may be breathing in 1 litre of air per breath while breathing at 20 breaths per minute. This works out to 17 litres of usable air per minute.

No data is available for breathing rates as low as 4 to 5 breaths per minute typical of meditation. Assuming one breathes 2 litres of air per breath at 5 breaths per minute, the usable air available per minute during meditation then would be 9.25 litres. That is almost half of the usable air available per minute during moderate exercise. During exercise oxygen consumption goes way up, while it reduces somewhat during meditation.

Usable air available for gas exchange during rest

Reference:

Sherwood, Kell and Ward. Human Physiology. Nelson, 2013. (p. 470)