Respiration and VO2 max

I know that cardio respiratory tests indicate fitness levels through measuring or  estimating your VO2max. And it was quiet interesting when you see a  VO2max  test in action. You could see that when the workload increased so did respiration and heart rate in a linear fashion. What I found interesting was when the work load increased to a point there was a change in gradient which became much steeper. This was explained to me as a ventilatory threshold.

I know that  VO2max  is the maximum uptake of O2 by the body and this is normally tested in a submaximal test where the data is been extrapolated In order to predict  VO2max . So why does the heart rate and breathing rate suddenly change gradient when the workload is increased over a certain point and why is this relevant to say playing sport?

I also know that if the aerobic system is at capacity and the work load increases again the body can utilise the anerobic system using the muscle glycogen to perform contractions to compensate for any shortfall. But in nutrition I learned that anaerobic energy systems are only used for short bursts of work. So why does the heart and breathing rate increase when the body is pushed almost beyond its “aerobic threshold”… this might be the way the body tries to buffer between competing energy systems …. Hmm

I also now know that a product of muscle contraction is CO₂. Excess CO₂ raises the PH level in the blood, As our breathing is driven by the need to remove this CO₂ from the blood to maintain a stable PH, I can understand why our breathing rate increases as the workload is increased – it needs to get rid of more CO2 as the work increases to maintain a stable PH in the blood however it doesn’t explain why all of a sudden heart rate and breathing rate sharply increases, as it’s been established that as the body does more work the higher breathing and heart rate increase and this is in a straight line.

So what?

There must be other factors at work here, things that aren’t observable. I think I will have a look at what are the responses to elevated PH levels in the blood as this may have a bearing on how the lungs and heart try to compensate for workloads over this aerobic threshold.

Also wonder if this has implications and therefore training considerations for sports that utilise both the aerobic system and the anaerobic system? Can you increase this threshold or is this a individual static value that can’t be modified like height?

So what next?

I don’t understand how adaptations are made by increasing cardiovascular and respiratory capacity happen.