Carboxyhemoglobin is a molecule formed in red blood cells that is a stable formation of hemoglobin with carbon monoxide. It is the mechanism behind carbon monoxide poisoning. In a normal air/blood mixture, hemoglobin is an unstable molecule that is found either as a compound with oxygen or carbon dioxide. Hemoglobin has an affinity for both molecules and bonds readily with each. However, it will bond more favorably with the molecule that is the greater concentration in the solution, giving up oxygen where carbon dioxide is prevalent (such as in the capilliaries) and giving up carbon dioxide when oxygen is readily available (such as the lungs). As such, in this state, it is rather unstable and prone to change.