How much can cardiac output increase during exercise in trained individuals?

During exercise, cardiac output, which is the amount of blood the heart pumps per minute, does indeed increase significantly, particularly in trained individuals. The correct choice indicates that cardiac output can increase by 5-6 times the resting level during vigorous physical activity.

This increase is primarily due to improvements in both the heart's stroke volume (the amount of blood ejected with each beat) and heart rate, which tend to be more pronounced in those who are well-conditioned. In well-trained athletes, the heart can pump a substantially greater volume of blood to meet the demands of the body's musculature during intense exercise.

The other options suggest higher multiples of increase, such as 10-15 fold or up to 20 fold, which are generally not observed in realistic physiological conditions for trained individuals. The physiological limits of the cardiovascular system, even in elite athletes, typically cap the increase around a 5-6 fold change, reflecting the heart's capability to adapt to sustained periods of physical exertion while maintaining efficient circulation and oxygen delivery.