oxygen tension
Summary
Oxygen tension is the partial pressure of oxygen dissolved in a gas or fluid, written PO2 and measured in mmHg. It is the driving force for oxygen diffusion and is distinct from oxygen content, which is dominated by haemoglobin-bound oxygen.
Detail
The distinction between tension and content is repeatedly examined. Arterial PO2 reflects only dissolved oxygen, about 0.003 mL per dL per mmHg, whereas content equals 1.34 times haemoglobin times saturation plus that small dissolved term. This is why in anaemia the PaO2 and saturation are normal while content is low, and why in carbon monoxide poisoning PaO2 is normal and pulse oximetry is falsely reassuring while content and delivery are severely reduced. Normal alveolar PO2 is about 100 mmHg and mixed venous about 40 mmHg. The alveolar gas equation, PAO2 = FiO2 x (Patm - PH2O) - PaCO2/R, allows calculation of the A-a gradient, which distinguishes hypoventilation and low inspired oxygen (normal gradient) from shunt, V/Q mismatch, and diffusion limitation (raised gradient); only shunt fails to correct with supplemental oxygen. Oxygen tension also governs bacterial ecology, favouring obligate anaerobes such as Clostridium and Bacteroides in devitalized tissue, and drives hypoxic pulmonary vasoconstriction, which is opposite in direction to the systemic vascular response to hypoxia.
Sources
- West's Respiratory Physiology
- First Aid for the USMLE Step 1
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