Final answer:
In a healthy young adult, when heart rate increases above 160 bpm, cardiac output begins to decrease due to a reduced stroke volume as the ventricles have less time to fill with blood.
Step-by-step explanation:
The question concerns the effects of heart rate (HR) on cardiac output (CO) in healthy young adults. When HR increases above 160 beats per minute (bpm), the cardiac output actually begins to decrease. This decrease in CO is due to a shorter diastole period, which means less time for the ventricles to fill with blood, leading to a reduced stroke volume (SV). Initially, as HR increases, SV also increases, and CO remains stable or may rise. But, when HR exceeds 160 bpm, the decrease in SV is no longer compensated by the increase in HR, resulting in a reduced CO. Individuals are advised to maintain their HR within the target range of 120 to 160 bpm during aerobic exercises to ensure maximal benefits to heart and lung function.