Answer
Because the cardiopulmonary system ( cardiovascular and respiratory systems) which has been working hard to meet the increased demands of the exercising body for oxygen has finally caught up. it is able to supply enough oxygen to the muscles (and the rest of the body) at a rate to meet the increased requirements.
Work Step by Step
Oxygen enables muscles to break down both glucose and lactic acid to generate ATPs.
For about 40 minutes of vigorous exercise both glucose and fatty acids are used equally as fuel by the muscles , but after glycogen and glucose supplies decrease, fatty acids become the major fuel .
