Cardiorespiratory fitness is limited by oxygen delivery to mitochondria, driven by lung diffusion, cardiac output, blood oxygen-carrying capacity, and muscle oxygen extraction.

“It's how much oxygen can you deliver to mitochondria, right? Versus the base which is how much can you uh utilize substrate efficiently. So when it comes to delivering oxygen to the mitochondria, there are really four big drivers, right? So there there's the diffusion of oxygen from the lungs into the blood. There's cardiac output, so that's heart rate and stroke volume. Uh then there's the oxygen carrying capacity of the blood, namely hemoglobin. Um and then there's the muscle's ability to extract this.”