Final answer:
When transitioning from resting to exercising, the cardiac cycle time shortens, and heart rate increases. Initially, cardiac output increases with heart rate until a certain point. For optimized cardiovascular health, maintaining a target heart rate range of 120-160 bpm during exercise is recommended to ensure that the heart and lungs benefit maximally.
Step-by-step explanation:
When an individual transitions from a resting heart rate to an exercising heart rate, the cardiac cycle time (R-R interval) decreases, and the heart rate (HR) increases. The R-R interval is the time between two consecutive R waves on an electrocardiogram, which corresponds to one cardiac cycle. During rest, this interval is longer as the heart beats at a slower pace; however, exercise induces an increase in HR and a reduction in the R-R interval. This physiological response is due to the activation of the sympathetic nervous system and the cardioaccelerator centers, which increase the HR to supply more oxygenated blood to the muscles
In a healthy young individual, below a certain threshold of HR (about 120 bpm), the stroke volume (SV) remains high despite less filling time, which leads to an increase in cardiac output (CO). Between 120 and 160 bpm, CO is maintained because the HR increase compensates for the decrease in SV. Above 160 bpm, CO may actually decrease because the heart cannot compensate for the rapid rate with sufficient blood volume per beat. Maintaining the target heart rate during exercise ensures that CO is optimized for cardiovascular health. The target HR range for aerobic workouts is between 120 and 160 bpm, which is critical for maintaining the health of both the heart and lungs.