A patient hyperventilates from anxiety; fingers tingle and feel numb. Blowing off too much CO2 raises blood pH (respiratory alkalosis) — low CO2 constricts brain vessels (dizziness) and drops ionized calcium (tingling around mouth and fingers). Breathing into a paper bag (rebreathing CO2) was the old remedy; slow controlled breathing works better. The lesson: CO2, not oxygen, is the master controller of breathing — and its lack, not excess, causes these symptoms.