In oxygen dissociation curve , the partial pressure of O2 in tissues which is 40mmHg corresponds to the pO2 in tissue capillaries, similarly the pO2 in alveoli is 100mmHg which corresponds to the pO2 in pulmonary capillaries. Why is it so?
Oxygen and carbon dioxide are transported in the blood mostly by red blood cells. The transfer of O2 and CO2 between the alveoli and capillaries occurs by the diffusion from the area with high partial pressure to the area with low partial pressure of these gases. Because of this process, the partial pressure of O2 in pulmonary capillaries rises to 100mHg and the pressure of CO2 decreases.
When highly oxygenated blood reaches the tissues, oxygen goes towards the tissues and carbon dioxide moves to capillaries, because of the difference between partial pressure of gases in the blood and in the tissues. Therefore, the partial pressure of O2 in tissues decreases to 40mmHg and the partial pressure of CO2 increases. Then the blood goes back to the lungs and the process repeats.