Which expression represents oxygen content of pulmonary capillary blood, CcO2?

• A. CcO2 = (Hb x 1.34) + (PvO2 x 0.003)
• B. CcO2 = (Hb x 1.34 x SaO2) + (PAO2 x 0.003)
• C. CcO2 = (Hb x 1.34) + (PAO2 x 0.003)
• D. CcO2 = (Hb x 1.34 x SvO2) + (PAO2 x 0.003)

Answer: (C)

Oxygen content in pulmonary capillary blood comes from two parts: the amount bound to hemoglobin and the amount dissolved in plasma. By the time blood exits the capillary, it has equilibrated with the alveolar air, so hemoglobin is effectively fully saturated. That makes the bound portion simply Hb × 1.34 (the amount of O2 carried per gram of Hb at full saturation). The dissolved portion depends on how much oxygen is dissolved in plasma, which is governed by the alveolar oxygen tension (PAO2) multiplied by the solubility coefficient (0.003 mL O2 per dL per mmHg). So you sum these two parts: CcO2 = (Hb × 1.34) + (PAO2 × 0.003).

Using saturation factors like SaO2 or SvO2 would apply to systemic arterial or venous blood, not capillary blood, and using PvO2 would reflect venous content. The alveolar PO2 (PAO2) is the driving pressure for the dissolved portion at the capillary-alveolar interface, hence its presence in the equation.