Basic function of cardiac output

The delivery of sufficient oxygen to the tissues is the basic function of cardiac output. At rest, the heart will pump out 5 Liters/min to satisfy the oxygen demand of the body’s tissues.

1)What happens to cardiac output and cardiac function during intense exercise? Why?

2)What component of the nervous system is responsible for initiating this response?

3)Why do athletes have a much greater maximum cardiac output and cardiac efficiency than non-athletes?