Question

The primary site of exchange of gases in humans is:

• A. Bronchi
• B. Trachea
• C. Bronchioles
• D. Alveoli

Answer 1

The correct option is D Alveoli

Alveoli are the primary sites of the exchange of gases. The exchange of gases also occurs between blood and tissues. The alveoli are the thin membrane bound sacs surrounded by capilaries arising from the pulmonary artery. The carbon dioxide from the deoxygenated blood in the capillaries diffuses into the alveolar sacs. Simultaneously oxygen from the inhaled air in the alveolar sacs diffuses into the blood present in the capillaries.
The oxygenated blood travels to the heart and is pumped to the entire body via arteries. In the organs, oxygen from the blood diffuses into the tissues and carbon dioxide from the tissues is picked up by the blood.
$O_2$ and $C{O}_2$ are exchanged in these sites by simple diffusion, mainly based on pressure/concentration gradient. The solubility of the gases as well as the thickness of the membranes involved in diffusion, are also some important factors that can affect the rate of diffusion.