Total human male lung capacity is 6 L (+/- 20%). Female lung capacity is 22.5% less i.e. 4.7 L. For now we will take male lung capacity as the basis for our discussion. Out of total 6 L capacity, lungs retain a 1.2 L (called Residual Volume) to keep the alveoli
(the small round sacks where oxygen / CO2 exchange between inhaled air and blood stream happens) inflated. Therefore, theoretically one can force up to 4.8 L of air (called Forced Vital Capacity or FVC) out of lungs. But not everyone has the capability of using full 6 L capacity. Deep divers, swimmers and other athletes train themselves to utilize more than average capacity of their lungs. During the normal breathing cycle (respiration), we inhale and exhale up to 500 ml to 700 ml of air (called Tidal Volume). Normally, we leave 2.4 L of air in the lungs after the tidal breath out (Functional Residual Capacity). The inhaled air is oxygen rich, whereas the residual air stale and stagnant with CO2. So deep breathing helps us two ways – one it helps us use the lung capacity more effectively, and second it also helps in renewing the residual air in the lungs. The accumulative effective is increase in the net supply of oxygen through lungs to the blood stream.
Brain is an amazingly hungry organ – it is only 2% of the human body weight, yet it consumes 20% of the energy mitochondria produces and requires more than 25% of the oxygen out of the blood stream. Many experiments have shown that one can increase brain alertness by either increasing glucose supply to the brain or the supply of oxygen to make the energy production more efficient. So next time when you take a deep breath followed by a long exhale, you are doing your mind and body a favor by supplying more oxygen and removing more CO2.