The partial pressures (in mm Hg) of oxygen (O₂) and carbon dioxide (CO₂) at alveoli (the site of diffusion) are :

1. pO₂ = 95 and pCO₂ = 40

2. pO₂ = 159 and pCO₂ = 0.3

3. pO₂ = 104 and pCO₂ = 40

4. pO₂ = 40 and pCO₂ = 45

Match the following columns and select the correct option:

The partial pressure of oxygen in the alveoli of the lungs is:

1. equal to that in the blood

2. more than that in the blood

3. less than that in the blood

4. less than that of carbon dioxide

The figure given below shows a small part of the human lung where exchange of gases takes place. In which one of the options given below, the one part A, B, C or D is correctly identified along with its function?

   

1.	A – Alveolar cavity – main site of exchange of respiratory gases
2.	D – Capillary wall – exchange of O2 and CO2 takes place here
3.	B – Red blood cell –  transport of CO2 mainly.
4.	C – Arterial capillary – passes oxygen to tissues

Transport of gases in alveoli takes place by: