Please read it till the end patiently and you will get the answer. Fresh air ventilating the alveoli of human lungs is a mixture of gases. It contains oxygen, carbon dioxide, carbon monoxide, nitrogen etc. Among these only oxygen and carbon dioxide easily diffuse. Oxygen diffuses from alveoli into the pulmonary blood and carbon dioxide in the opposite direction. The diffusion of individual gas in the alveolar gas mixture depends on its partial pressure. The higher the partial pressure the greater will be the rate of diffusion. The partial pressure (P) is the pressure caused by the individual gas on the total pressure. Therefore each as in the mixture contributes to the total pressure. This partial pressure of individual gas is directly proportional to its concentration. Since the concentration of nitrogen gas is comparatively high in the inhaled atmospheric air, its partial pressure is also high (PN {-2} : 569 mm Hg) when compared to other gases in the mixture (PO {-2} : 104 mm Hg & PCO {-2} : 40 mm Hg). So, one can expect that nitrogen because of its high partial pressure can easily diffuse in the alveoli. But it is not true. The partial pressure of each gas in the respiratory gas mixture tends to force molecules of that gas into solution first in the alveolar membrane and then in the blood of the alveolar capillaries. The pressure of gas in the solution is determined not only by its concentration but also by the solubility coefficient of the gas. Therefore the solubility is the major factor that determines the rate of gas diffusion in a fluid. The other important factor is the velocity of kinetic movement, which is inversely proportional to the square root of the molecular weight. Greater the velocity greater is the rate of diffusion of the gas. It is obvious that the two factors namely solubility and molecular weight are the characteristics of the gases in the alveolar mixture, which quantify the net rate of diffusion in fluid medium. These together constitute another property called diffusion coefficient of gas. The relative diffusion coefficients for different gases of respiratory importance in the body fluid are as follows. For oxygen it is one unit. It is 20.3, 0.81, 0.53 for CO {-2}, CO and N {-2} respectively. The relative rates at which different gases will diffuse are proportional to their diffusion coefficient. That is why CO {-2} and O {-2} diffuse easily during respiratory process. In these two gases, the direction of net diffusion is determined by the difference in the partial pressures between two phases (gaseous phase and liquid phase). If the partial pressure is greater in the gaseous phase, as is normally true for oxygen, then more molecules will go into the blood than in the other direction. Alternately, if the pressure of the gas is greater in the blood, which is normally true for carbon dioxide, then the net diffusion will occur towards the gaseous phase in the alveoli. That is why we take only oxygen from the gas mixture we inhale.