**6. Traditional physiological approach**

HCO3/CO2 buffer system is the basis of this approach. Carbonic acid freely moves in the body fluid and dissociates into bicarbonate automatically when needed. Bicarbonate in the body acts as alkaline reserve. CO2, pH and HCO3 can be calculated by Hasselbalch-Henderson Equation (2) [7, 9].

$$\text{pH} = \text{pK} + \log\_{10} \left| \text{HCO}\_3^- / \text{sPCo}\_2 \right| \tag{2}$$

This equation states that not only HCO3 and CO2, but also their ratio determines the pH. In this equation, PCO2 is the respiratory component and HCO3 − is the metabolic component of the acid base imbalance. This buffer system is the largest and independent buffer system of the body and whole body acts as an open system for CO2. In traditional approach balance is determined by the influx and efflux of H+ and HCO3.
