Human muscle cells use anaerobic respiration during vigorous exercise． （b）An athlete runs a race which lasts 60 seconds． Fig． 4.1 shows the heart rate and breathing rate of the athlete during the race and as he recovers after the race． Fig． 4.1 (i) The rate of breathing increases during the race in response to a change detected by the body. Identify the stimulus in this response and state the organ that detects the stimulus. stimulus ......................................................................................................................... organ ............................................................................................................................. (ii) Using the information in Fig. 4.1, calculate the rate of decrease in heart rate from 125 seconds to 260 seconds as the athlete recovers. Space for working. ............................. beats per minute per second (iii) Explain the results shown in Fig. 4.1 from 60 seconds until 420 seconds. ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . ....................................................................................................................................... . .................................................................................................................................

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