Therefore, each hemoglobin molecule can bind four oxygen molecules. In a healthy adult, the concentration of hemoglobin in blood is 150 g/L. Importantly, 1.34 ml of O2 can bind 1 g of hemoglobin. Each liter of blood can therefore carry 3 ml of dissolved CO2 and 201 ml hemoglobin-bound O2. The hemoglobin bound O2 does not contribute to arterial PO2.

By contrast, CO2 is transported by the blood in three different forms. Since CO2 is very soluble in water, about 10% is transported as a dissolved gas. Another 60% is transported in the form of bicarbonate. Carbonic acid will also form, but at a ratio of 1:20 with bicarbonate. This represents the primary pH buffering system in the human body. Another 30% of CO2 will be carried by amine residues on hemoglobin and plasma proteins, with hemoglobin carrying the most CO2 by far.

Oxygen therefore requires a special mechanism to...
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