As exercise intensity increases there comes a break point where blood lactate levels will start to rise (production starts to exceed clearance). This is often referred to as the lactate threshold i think (LT). If exercise intensity continues to increase a second and often more obvious that increase in lactate accumulation is seen.
If an increase in acid overwhelms the body's acid-base control systems, the blood will become acidic. As blood pH drops (becomes more acidic), the parts of the brain that regulate breathing are stimulated to produce faster and deeper breathing (respiratory compensation). hope this helped