Blood gas tension
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension; the most common gas tensions measured are oxygen tension (PxO2), the carbon dioxide tension (PxCO2) and carbon monoxide tension (PxCO). The x factor in the symbol for each would be replaced by the source of the gas being measured; "a" meaning arterial, "A" being alveolar, "v" being venous, "c" being capillary.
Oxygen tension 
- Arterial blood oxygen tension (normal)
- Venous blood oxygen tension (normal)
Carbon dioxide tension 
- Arterial blood carbon dioxide tension
PaCO2 — Partial pressure of carbon dioxide at sea level (765 mmHg) in arterial blood is between 35 mmHg and 45 mmHg.
- Venous blood carbon dioxide tension
PvCO2 — Partial pressure of carbon dioxide at sea level in venous blood is between 40 mmHg and 50 mmHg.
Carbon Monoxide tension 
- Arterial Carbon Monoxide tension (normal)
PaCO — Partial pressure of CO at sea level (765 mmHg) in arterial blood is approximately 0.02. It can be slightly higher in smokers and people living in dense urban areas.
The partial pressure of gas in blood is significant because it is directly related to ventilation and oxygenation. When used alongside the pH balance of the blood the PaCO2 and HCO3 (and Lactate) lets the health care practitioner know what interventions need to be made, if any.
Oxygen content 
The constant, 1.36, is the amount of oxygen (ml at 1 atmosphere) bound per gram of hemoglobin. The exact value of this constant varies from 1.34 to 1.39, depending on the reference and the way it is derived. The constant 0.0031 represents the amount of oxygen dissolved in plasma. The dissolved oxygen term is generally small relative to the hemoglobin-bound oxygen, but becomes significant at very high PaO2 (as in a hyperbaric chamber) or in severe anemia.
Oxygen saturation 
This is an estimation and does not account for differences in temperature, pH and concentrations of 2,3 DPG.
See also 
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- Severinghaus, J. W. Simple, accurate equations for human blood O2 dissociation computations. J Appl Physiol. 46(3): 599-602. 1979.