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Hemoglobin (Hb), the main component of red blood cells, is a protein that carries oxygen from the lungs to the body's tissues, and carbon dioxide from the tissues to the lungs to be exhaled.
At a pressure of 100 mmHg in the lung's capillaries, 95-98% of the Hb is combined with oxygen. In the peripheral tissues, where the pressure may be as low as 20 mmHg, less than 30% of the oxygen remains combined with Hb. (See also serum hemoglobin.)
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
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