[eitau_1]

Can someone provide an explanation why CO2 concentrations above 1000 ppm have such a negative influence given the fact that CO2 concentration in lungs (at rest) never falls below 10000 ppm?

[fer]

It simply makes the baseline higher. If you want to go to extreme cases, check carbogen

https://en.wikipedia.org/wiki/Carbogen

[Terr_]

I'm not a doctor, but I would consider it in terms of flow and throughput, rather than,—metaphorically—the amount of water the pipe can hold.

[eitau_1]

Per Wikipedia, at rest 500 ml of inhaled air is diluted with ≥2500 ml [1] of residual air in lungs containing ≥40000 ppm (4%) of CO2 [2]. Other things being equal, increasing concentration of CO2 in ambient air 10x (500ppm -> 5000 ppm) would increase concentration of CO2 in the lungs after taking the breath by less than 2.5% [3].

I imagine it could easily be compensated by an equivalently minor increase of breathing rate or breathing depth.

[1] https://en.wikipedia.org/wiki/Gas_exchange#Alveolar_air

[2] https://en.wikipedia.org/wiki/Breathing#Composition

[3] 5/6 × 40000 ppm + 1/6 × 500 ppm = 33417 ppm; 5/6 × 40000 ppm + 1/6 × 5000 ppm = 34167 ppm; 34167 / 33417 = 1.0225