Observe the given and answer the following question from the picture

Dear student 
a. The biological principle involved in exchange of gases is Diffusion. It means gas molecules move from a region of higher concentration to region of their lower concentration. 

b. Gas molecules generally move down a pressure gradient. But as partial pressure of oxygen is same in alveoli and pulmonary artery, gas molecules will not diffusion. This process of diffusion doesn't take place. 

c. The partial pressure of oxygen is less in lungs compared to atmosphere because :
1. As the air enters the lungs, the partial pressure of water vapour (47 mmHg) reduces the partial pressure of oxygen as air is humidified by various pathways.
2. From lungs there is continuous uptake of oxygen by the pulmonary capillaries and also the continual diffusion of CO2 out of the capillaries into the alveoli reduces partial pressure of oxygen in lungs. 
Regards 


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