Final answer:
Aerobically fit individuals have a more efficient heart, resulting in higher cardiac output and stroke volume, and maintaining a target heart rate is crucial for maximizing efficiency. Sedentary individuals tend to have lower cardiac efficiency and rely more on heart rate increases to augment cardiac output during exertion.
Step-by-step explanation:
The methods of achieving cardiac output (CO) differ between an aerobically fit individual and a sedentary one. In an aerobically fit person, regular aerobic exercise increases the heart's efficiency, resulting in a larger cardiac reserve and a greater ability to pump blood.
This is evidenced by an increase in both heart size and mass, from about 300 g in a nonathlete to 500 g in an athlete, which allows the heart to pump more efficiently at a lower heart rate (HR).
For a healthy young adult, CO can increase to about 20 liters per minute for a nonathlete and more than 30 liters per minute for an aerobically trained athlete. During exercise, the body's demand for oxygenated blood increases, leading to an increase in HR and stroke volume (SV).
However, as the HR rises to very high levels, SV begins to decrease, which can ultimately lead to a decrease in CO if the HR exceeds approximately 160 beats per minute (bpm). This underlines the importance of maintaining a target HR between 120 and 160 bpm during aerobic exercise to ensure that CO is maintained.
Comparatively, a sedentary individual will have a smaller cardiac reserve and a higher resting heart rate, resulting in a lower maximum cardiac output during exercise. Their sedentary lifestyle may lead to decreased heart muscle conditioning, translating to a less efficient pumping mechanism.
Consequently, their body might rely more on increases in HR to augment CO during physical activity, which is not as efficient or sustainable as the increased SV seen in aerobically fit individuals.