The percentage of oxygen in inhaled air is 21%. Oxygen is transported within the body in two ways- a small amount is carried in the plasma as a dissolved gas while majority of it is carried in the blood in a bound state with hemoglobin. You breathe in and out in each second which will regulate (balance) the amount of oxygen required by our body. The gas exchange process takes place at a rate more than you expect.
Respiratory rate:The number of breaths per minute or, more formally, the number of movements indicative of inspiration and expiration per unit time.