Which factor is more important for the respiration during exercise?

When maratoner is running the track, we assume, his physical ability is depend on respiration capacity.

Between CO2 output and O2 input, which one is more important role on the exercise ability?

I'd say oxygen input for two reasons:

 

1. Aerobic activities, such as marathoning, by definition, require oxygen and, when a marathoner does not receive enough oxygen, the metabolism becomes partially anaerobic and produces toxic by-products that result in fatigue.

 

2. Unlike oxygen intake, carbon dioxide output has multiple routes -- both carried by hemoglobin and dissolved in the plasma -- so it is not hemoglobin-limited the way oxygen input.

I'm not sure they can be isolated since ingress and egress comes to a pinch-point at the lung sac membranes largely driven by concentration gradients so they would be in competition right?

Which one is the major factor of the Oxygen shortage in general case?

Here are examples.

*Lug oxygen absorption capacity is not so big.

*The oxygen carrier, blood, circulation capacity is small by cardiac capacity or circulation system structure.

*Heat transfer limitation from the lug or tissue, temperature rising, effects the oxygen absorption capacity.

*Total blood oxygen absorption capacity is not so big.

*muscle tissue to blood vessel oxygen transfer capacity is not so big.

*others

Hm, could be tested by having them run with pure oxygen, possibly also with increased levels of CO2 to impede CO2 transfer from the lungs.

Hm, could be tested by having them run with pure oxygen, possibly also with increased levels of CO2 to impede CO2 transfer from the lungs.

Maybe so, but one would risk oxygen narcosis.

Both have to be balanced out. As the exchange is by diffusion a proper gradient for both gases (in opposite directions) have to be maintained. Impeding CO2 transfer would lead to hypercapnia.

Then, how can we easily detect D.O. in the blood, any good instrument? And, are there any CO2 concentration analyzer in the blood without expelling it from our body?

If you are concerned about the factor that cause greater increase in the breathing rate through the intervention of nervous system, then it is the concentration of carbon dioxide because oxygen doesn't have any direct influence on sensory centers of brain, It is the concentration of carbon dioxide that initiates nervous response that increases the rate of respiration.