Lung Capacity Calculator Formulas:
   total lung capacity = IRV + TV + ERV + RV
   vital capacity = IRV + TV + ERV
   functional residual capacity = ERV + RV
   inspiratory capacity = TV + IRV
Where:
   IRV: Inspiratory Reserve Volume
   TV: Tidal Volume
   ERV: Exspiratory Reserve Volume
   RV: Residual Volume