Page 7, O2 Uptake, Exercise & Cor. Circ., Dr. D. Penney
There are several factors which keep saturation of arterial hemoglobin below 100%: The oxygen-hemoglobin association-dissociation curve (ODC) is "S" shaped, and the shoulder has a low slope. Consequently, a P02 higher than that normally seen in the alveoli would be required to fully saturate it. Some hemoglobin is in the form of methemoglobin, where the Fe is in the +3 state, which prevents binding O2. Some hemoglobin is binding carbon monoxide, which is made in the body, and also comes from pollution, cigarette smoke, etc. Finally, normal cardiac and pulmonary shunts (Thebesian circ., bronchial circ.) exist which cause some blood to be less than fully oxygenated.
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