O2 transport: Quantitative aspects

Total O₂ content is expressed by the following equation:

• O₂ content (CaO₂) = ([SaO₂ x Hgb x 1.31 ml/dl blood]) + ([0.003 ml/dl blood per mmHg x PO₂)

where Hgb is hemoglobin concentration and SaO₂ is hemoglobin saturation at the given PO₂.

Oxygen transport is dependent on both respiratory and circulatory function. Total O₂ delivery (DO₂) to tissues is the product of arterial O₂ content and cardiac output (QT).

• DO₂ = CaO₂ x QT

Note that arterial O₂ content is dependent on PaO₂ as well as hemoglobin concentration. As a result, deficiencies in O₂ delivery may be due to a low PaO₂, a low hemoglobin concentration, or an inadequate cardiac output.

The Fick equation expresses the relationship between O₂ consumption, O₂ content, and cardiac output:

• O₂ consumption = VO₂ = QT x (CaO₂ – CvO₂)

Oxyhaemoglobin Dissociation Curve

Consequently, the arteriovenous difference is a good measure of overall adequacy of O₂ delivery. With a normal O₂ consumption of approximately 250 ml/min and cardiac output of 5000 ml/min the normal arteriovenous difference is calculated to be about 5 mlO₂/dl blood. The normal extraction ratio is approximately 25%, thus the body normally consumes only ~25% of the O₂ carried on hemoglobin. When O₂ demand exceeds supply, the extraction fraction exceeds 25%, and conversely, if O₂ supply exceeds demand, the extraction fraction falls below 25%.

When DO₂(oxygen delivery) is moderately reduced, VO₂usually remains normal because of increased O₂extraction (meaning mixed venous O₂saturation decreases). With further reductions in the DO₂, a critical point is reached beyond which VO₂becomes directly proportional to DO₂. This state of supply-dependentO₂is typically associated with progressive lactic acidosis caused by cellular hypoxia.

For good measure, remember the oxygen-hemoglobin dissociation curve: A rightward shift (increasing P50) lowersO₂affinity, displacesO₂from hemoglobin, and makes moreO₂available to tissues. A leftward shift (decreasing P50) increaseshemoglobin’saffinityforO₂reducing its availability to tissues. Normal P50 in adults is 26.6 mmHg.