The Isohydric Principle

1. Multiple Buffer Systems of the Body (Fig. 19)
   • body fluids contain many buffer pairs, each of which competes for the

     same H+ ions

     • phosphate and protein buffer systems: closed systems within the body
     • bicarbonate buffer system: open system in communication with the external environment
   • only one buffer pair needs to be closely examined to understand the [H+] in the plasma space
     • this is possible because all buffer systems are linked together through H+ (isohydric principle)
     • for biomedical purposes, the bicarbonate buffer system is of primary importance
     • the hemoglobin buffer is of secondary importance
2. Accessing the Origin of Acid-Base Disturbances
   • malfunctioning respiratory system
     • insufficient CO2 removal (respiratory acidosis)
     • excessive CO2 removal (respiratory alkalosis)
   • malfunctioning metabolic systems
     • renal (improper processing of H+ or HCO3-)
     • extrarenal (excessive CO2 and H+ production)
   • compensations
     • the kidneys can compensate for dysfunctional lungs (renal system responds after a couple of days: slow)
     • the lung can compensate for dysfunctional kidneys (respiratory system responds within minutes: fast)
     • lungs and kidneys can both compensate for acid-base disturbances of extra-renal origin
   • the origin of acid-base disturbances and presence of compensations can be determined
     • measurement of bicarbonate buffer pair components and pH (quite simple)
     • interpretation of plasma electrolytes concentrations (more complicated)