In the O2 content equation, which term represents the oxygen bound to hemoglobin?

In the O2 content equation, which term represents the oxygen bound to hemoglobin?

• A. 0.003 × PaO2
• B. 1.39 × Hb × SaO2
• C. 0.003 × Hb
• D. Hb × PaO2

Answer: (B)

Oxygen content in blood comes from two parts: what’s dissolved in plasma and what’s bound to hemoglobin. The portion bound to hemoglobin depends on how much hemoglobin is present and how saturated it is with oxygen, multiplied by the oxygen-carrying capacity per gram of hemoglobin. That’s why the term representing hemoglobin-bound O2 is the one with Hb × SaO2 times the carrying capacity (about 1.34 mL O2 per gram Hb; in the given equation shown as 1.39 × Hb × SaO2). The dissolved part, 0.003 × PaO2, accounts for only a small amount of O2 and does not involve Hb or saturation. The other forms—0.003 × Hb or Hb × PaO2—don’t reflect the saturation-dependent binding of O2 to Hb, so they don’t represent the Hb-bound portion.

Oxygen content in blood comes from two parts: what’s dissolved in plasma and what’s bound to hemoglobin. The portion bound to hemoglobin depends on how much hemoglobin is present and how saturated it is with oxygen, multiplied by the oxygen-carrying capacity per gram of hemoglobin. That’s why the term representing hemoglobin-bound O2 is the one with Hb × SaO2 times the carrying capacity (about 1.34 mL O2 per gram Hb; in the given equation shown as 1.39 × Hb × SaO2). The dissolved part, 0.003 × PaO2, accounts for only a small amount of O2 and does not involve Hb or saturation. The other forms—0.003 × Hb or Hb × PaO2—don’t reflect the saturation-dependent binding of O2 to Hb, so they don’t represent the Hb-bound portion.