The muscles also gets more efficient at expressing energy and removing lactase, which is an exchange requiring oxygen.

When you're exercising, your oxygen requirements increase, so you breathe faster and your your heart beats faster. This also ensures waste products are dealt with faster.

An athlete's baseline oxygen requirement as a result of training is lower than someone untrained because their body makes better use of each breath. Thus, RHR and BRPM is lower.

This is why someone who is extremely unfit will breath heavier and faster, even when resting.