Oxygen comes from alveoli to blood by ?

Ans. is 'a' i.e., Diffusion Oxygen is transferred from the alveoli to the pulmonary capillaries by the process of diffusion. The diffusion occurs across the respiratory membrane. As oxygen diffuses into the blood, it combines with hemoglobin. However, the oxygen, in combination with hemoglobin, does not contribute to the PO2 of blood. Therefore the alveolar - capillary gradient of PO2 is maintained much longer than it would be if hemoglobin were not present. The sequence mey be visualized as follows:- i) Oxygen diffuses from the alveoli into the plasma due to the PO2 gradient. ii) To sta with, the PO2 in the RBC fluids is the same as in the plasma. As soon as diffusion from the alveoli to plasma raises the PO2 of plasma above that of the RBC fluids, oxygen diffuses from the plasma into the RBC. iii) As soon as oxygen enters the RBC, it combines with hemoglobin, which mops up oxygen but does not let PO2 rise. The process continues till hemoglobin cannot pick up any more oxygen. iv) Then the PO2 in the RBC fluids rises to equal that in the plasma and diffusion from the plasma to RBC stops. v) After that the PO2 in the plasma also rises to equal that in the alveoli, and consequently diffusion from the alveoli to the plasma stops. Thus, the presence of hemoglobin makes an enormous difference to the amount of oxygen transferred. By mopping up oxygen without letting the PO2 rise, it lets the diffusion of oxygen continue much longer. While the PO2 gradient determines the direction of diffusion of oxygen, it is actually hemoglobin that lays the trap for carrying large amounts of oxygen in the blood.