CBSE Class 10 Answered

The rate of breathing increases during vigorous exercise. This is because the body needs more energy which can be released only if greater amounts of oxygen reach the cells. For this purpose, the rate of breathing increases.( This will increase the supply of oxygen to the cells).

(However, after a while this strategy is also insufficient to meet the cell's need. This is because, there is a temporary shortage of oxygen being delivered to the working muscles. So muscles begin to respire anaerobically. Lactic acid is a by-product of producing energy anaerobically. When lactic acid accumulates at high levels in the blood, it causes muscular fatigue.)