Just throwing this into the pot to demostrate how changing your breathing can have measurable changes in body/blood chemistry (and this has nothing to do with Taiji/Qigong etc):
Training for free-diving can take many forms and be done on the land.
One example is the apnea walk. This consists of a preparation "breathe-up", followed by a short (typically 1 minute) breath hold taken at rest. Without breaking the hold, the participant then initiates a walk for as far as they can, until it becomes necessary to breathe again. Athletes can do close to 400 meters in training this way.
This form of training is
good for accustoming muscles to work under anaerobic conditions, and for
tolerance to CO2 build-up in the circulation. It is also easy to gauge progress, as increasing distance can be measured.
Before competition attempt, free-divers perform preparation sequence, which usually consists of physical stretching, mental exercise and
breath exercise. It may include sequention of variable length static apnea,
special purging deep breaths, hyperventilation. Result of preparation sequence is
slower metabolism, lower heart rate and breath rate, lower level of CO2 in bloodstream and overall mental equilibrium.
Evolution didn't give us this....it is a learned technique.
Haven't even mentioned the QG word....