Question

Why does gaseous exchange continue in the lungs even during expiration? Give a suitable explanation.

Answer 1

Hint: The lungs are the respiratory organ where deoxygenated blood gets oxygenated. Lungs contain millions of alveoli where the gaseous exchange occurs- carbon dioxide is eliminated and fresh oxygen is taken into the blood.
Expiration is the process where we ‘exhale’ air that is rich in carbon dioxide. The alveoli and the respiratory bronchioles are the respiratory zones where gaseous exchange takes place. The rest part is the conductive zone, i.e., it only takes part in conduction of air.

Complete answer:
The gaseous exchange in the alveoli and respiratory bronchioles occurs primarily due to the differences in partial pressure. It has nothing to do with the processes of inspiration and expiration. As long as the partial pressure of oxygen (p$O_2$) of the alveolar air is greater than the p$O_2$ of the blood, and as long as the partial pressure of carbon dioxide (p$CO_2$) of the blood in greater than that of the alveolar layer, gaseous exchange will continue.
Moreover, the time span between inspiration and expiration is very less. So, most of the diffusion cannot be completed by the time we start to exhale air. The partial pressure differences still remain, and hence as we exhale air, diffusion still continues.

Note:
Another noteworthy point in this respect is that of the residual volume. The residual volume (1200 mL) is that amount of air which remains in the lungs even after forceful expiration. As we inhale the oxygen and carbon dioxide, proportions of this air also change. So, as we exhale, the little amount of oxygen present within this volume goes on diffusing in the alveoli.