CBSE Class 10 Answered

Diffusion is the passive movement of a substance from an area of high concentration to an area of low concentration.  At the level of the lungs, the blood typically has a lower oxygen concentration than the gas in the alveoli.  As a result, oxygen diffuses into the blood.  The respiring cell is consuming oxygen and thus has a lower oxygen concentration than the blood in the capillaries. So oxygen from blood diffuses into tissues.

The cells of all animals require oxygen for survival and, as a consequence of this respiration, they produce carbon dioxide. This carbon dioxide accumulates in high concentrations inside the cell. Its concentration inside the cell is much higher as compared to outside the cell. Due to this difference in concentration, CO₂ diffuses to the exterior and is carried to the lungs where, due to concentration difference, it is released outside the body.

Thus diffusion acts as the driving force of respiration.