Each of hemoglobin’s four heme groups can also bind to carbon monoxide (CO). If this occurs, O2 cannot bind and carbon monoxide poisoning results.

As shown in the animation, carbon monoxide association with hemoglobin is directly related to the plasma partial pressure of CO (= pCO).In this simulation, pCO is allowed to increase to from 0.0 – 0.4 mmHg while the pCO2 is maintained at 40 mmHg, which is normal.At pCO = 0.4 mmHg, the hemglobin is almost fully saturated with CO. This pressure is approximately 250 X less than the pO2 needed to fully saturate hemoglobin with O2.

These data indicate that heme has a much greater affinity for CO than for O2. Therefore, if an individual breathes in a relatively small amount of CO, it will saturate the hemoglobin and prevent O2 from binding. As a result, O2 cannot be distributed as needed to the body’s tissues.