Oxygen dissolves poorly in plasma, so almost all of it rides on hemoglobin in red blood cells. CO2 is carried mostly as bicarbonate, which links breathing to the body's acid-base balance.
Oxygen and hemoglobin
Each hemoglobin molecule binds up to four oxygen molecules. About 98 percent of the oxygen in arterial blood is bound to hemoglobin; the small dissolved part is what PaO2 measures.
The dissociation curve
The curve is S-shaped: saturation stays above 90 percent until PaO2 falls to about 60 mmHg, then drops steeply. Fever, acidosis and high CO2 shift the curve to the right, so hemoglobin releases oxygen more easily to the tissues.
Carrying CO2
About 70 percent of CO2 travels as bicarbonate, about 20 percent is bound to hemoglobin and about 10 percent is dissolved in plasma. In the lungs the process reverses and the CO2 is breathed out.
Carbon monoxide
Carbon monoxide binds hemoglobin over 200 times more tightly than oxygen. A standard pulse oximeter reads it as oxygen, so SpO2 can look normal in a poisoned patient.
| Measure | What it tells you | Normal |
|---|---|---|
| SpO2 | Percent of hemoglobin carrying oxygen | 95 to 100% |
| PaO2 | Oxygen dissolved in arterial blood | 80 to 100 mmHg |
| PaCO2 | CO2 in arterial blood, set by ventilation | 35 to 45 mmHg |
| HCO3 | Bicarbonate, the main CO2 carrier and buffer | 22 to 26 mEq/L |
At the bedside
An SpO2 of 90 percent already means a PaO2 near 60 mmHg, at the edge of the steep part of the curve, so any further drop is serious. Pulse oximetry says nothing about ventilation, so use capnography or an arterial blood gas to judge CO2.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).