Carbon dioxide transport in blood occurs primarily in three forms: dissolved (about 5%); as the bicarbonate anion (about 90%); and as carbamino compounds (about 5%).
Bicarbonate is created by the reaction of carbon dioxide with water to form carbonic acid, which dissociates into hydrogen and bicarbonate. The reaction of carbon dioxide and water in red blood cells is catalyzed by carbonic anhydrase.
Carbamino compounds are formed via the reaction of carbon dioxide with the terminal amino groups of blood proteins, including hemoglobin.
The carbon dioxide-hemoglobin dissociation curve
The various forms of carbon dioxide in the blood create an equilibrium between dissolved carbon dioxide and chemically-bound carbon dioxide, including that bound to hemoglobin.
Unlike the sigmoid-shaped oxyhemoglobin dissociation curve, the carbon dioxide-hemoglobin dissociation curve is more linear. The amount of carbon dioxide in the blood at any given level of carbon dioxide tension depends on the degree of hemoglobin oxygenation—constituting the Haldane effect. As oxygen is unloaded from hemoglobin in peripheral tissues, the hemoglobin more avidly binds carbon dioxide.
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