In most vertebrates oxygen enters the bloodstream  via diffusion at the alveoli of the lungs. This diffusion (a  passive thing) is accomplished because oxygen concentration … in the  lungs is much higher than that of the blood, even than that of   oxygenated blood.es, in a sense the respiratory system removes oxygen from the air and replaces it with carbon dioxide. When the respiratory system takes in air, it reduces the concentration … of oxygen from 21% to 16%. The air is inhaled by the diaphragm into the trachea, which leads into the lungs via the bronchi.