The human body can quickly adapt to many situations. History has proven this. Adapting to stress from the environment and physical demands of life have kept our species thriving for thousands of years.
The same is true for sports training. Adapting and coordinating muscle activity during sports training skills by repeated action allows them to become like second nature and is then easier to implement. Being able to adapt means the body is becoming more efficient and expending less energy throughout the workout.
The cardiovascular system and respiratory systems are two of the major systems of the body, which benefit from adapting to increasing exercise. The heart will pump more efficiently which will deliver more energy to the cells at a faster rate. Blood vessels become more flexible and able to handle this increased blood flow.
The body also adapts by increasing muscle mass. When muscles are pushed to fatigue, they demand more fuel and oxygen making the workout more worthwhile. Exercise helps the body adapt to increase productivity and has always been this way.