For all the other commenters asking why Mindfulness and Concentration are used as technical terms - that's because they correspond to Sati and Samadhi - terms in Pali used by the Buddha.
One of the takeaways of the book mentioned above is that concentration is NOT a state of being concentrated on a single object. It's a state of unification of the mind that persists regardless of its current object. The other important information (obvious for any serious meditator) is that, once reached, it DOESN'T require lot of force to sustain. On the contrary - it's a much more energy efficient mode of functioning. That's why in the 4th Jhana (one of the most advanced states of concentration) the breath completely subsides. It wouldn't be possible if it required so much force. BTW it doesn't mean that the oxygen intake stops completely as there's still some absorption caused by the air diffusion in the lungs and through the skin (cutaneous respiration).
Again, I really recommend the book by Culadasa for anyone even remotely interested. It's amazing from theoretical point of view and even more concerning meditation practice.
