Apologizing in advance if this is not the right topic/forum/website, but:

As a pilot I understand that Pressure Altitude vs. Temperature = Density Altitude.  I also clearly understand what density altitude does to aircraft performance.

As an athlete I understand that an increase in pressure altitude means a decrease in oxygen and the body's ability to perform at its peak.  No surprise I can't run as fast or as far at 6000' MSL than at Sea Level.

The question I can't answer.. but I know someone out there can.. is this:  Is the body's respiratory system affected by density altitude or does the body only recognize pressure altitude?    

I was pondering this the other day while climbing a very steep mountain on my bike at 3000' MSL and 100F.  My lungs and heart felt more like they would at 6000' MSL at that moment.  Could have been having an off-day or could have been overheating...but still thinking about the question and I can't figure it out.