But why would CO2 build up in a healthy fit diver diving on open circuit if it wasn’t happening on land considering there using higher ppO2. I’m really having trouble understanding this
There is a number of factors making CO2 a problem while diving.
The first one is that an unexperienced diver makes unneeded muscular efforts for keeping position and trim and for moving.
The dead space in the regulator has also been explained.
The third one is absolute pressure. The deeper you are, the higher will be the partial pressure of the CO2 in your lungs for the same quantity (in mass or moles), so the stimulus for short breathing increases with depth.
And finally there is a psychological factor. Many suggest to "relax". It is a wise suggestion, as when relaxed, your brain works slower, consuming less oxygen hence producing less CO2. Under stress, our brain can produce up to 1/5 of the total body CO2 production, albeit weighting 1/50 of the total body weight, So our brain has a CO2 production capability which is 10 times larger than other parts of the body.
And the interaction between brain state and CO2 production is bidirectional: as CO2 starts accumulating, we feel uncomfortable, and our stress increases, causing more CO2 to be produced.
In practice, the "natural" mechanisms of our body for counter-effect a CO2 increase are not very effective: they cause your breathing rate to increase, but each cycle becomes shallower, hence the amount of gas really swapped inside your lungs is very small. The muscular effort for breathing explodes, but the CO2 continues accumulating, as the vented volume is too small, and more than half of it is lost due to dead spaces.
This negative cycle can only be interrupted through voluntary control of the respiratory cycle, if you leave your body to "breath normally", without exerting a proper control, you easily end in dyspnea.
And it is not nice. The SAC is the last of your problem, as the sensation is to die for lack of air, suffocating: you blame you regulator not giving you enough air, and instead it is your lungs which are not working.
I have seen inexperienced divers going in dyspnea, and then giving up entirely to our sport, as it is a terrible experience.
I agree than an inexperienced diver should not worry too much of SAC. This will fix itself, as he learns to move properly, to breath properly, and to control his brain properly.
And these three types of "control" are interconnected strictly, as people practising Yoga know very well...
So it is true that breathing control, alone, cannot be the solution.
Total body and brain control is the solution.
And this involves controlling your body (buoyancy, trim, propulsion, avoiding any unneeded muscular action), controlling your breathing (long, slow inhalation and exhalation and a reasonable inspiration pause), and controlling your brain (remaining relaxed and lucid also when a problem occurs, avoiding euphoria or sadness).
