The usual problem for breath-holding is not lack of oxygen, but surplus of CO2. It is the latter that creates the panicked urge to breathe. Free divers and st

The usual problem for breath-holding is not lack of oxygen, but surplus of CO2. It is the latter that creates the panicked urge to breathe. Free divers and stunt magicians therefore hyperventilate prior to diving, or getting sealed inside an ice block, or whatever. The disadvantage to this is that they then have no natural physiological gauge to determine when they are actually running low on oxygen, as the CO2 signal is delayed past the point where unconsciousness due to hypoxia occurs. With artificial RBCs, you could perhaps load them up with an oxygen-storage protein like myoglobin instead of just oxygen-transporting hemoglobin, and use bicarbonate concentration as a trigger to release O2 and bind CO2--borrowing from both whales and crocodilians. It might also be useful to develop an artificial lung or gill that employs perfluorodecalin flowing through narrow tubes of gas-permeable membrane to transport gases.