In the context of erythrocytes and hemoglobin, how does carbon monoxide (CO) affect a smoker's blood oxygen-carrying capacity?

• A. CO binds to hemoglobin with high affinity, forming carboxyhemoglobin and reducing the amount of hemoglobin available to carry $O_2$
• B. CO is rapidly converted to $C_{O_2}$ in erythrocytes, which directly increases hemoglobin saturation with $O_2$
• C. CO binds primarily to the heme iron and increases hemoglobin's ability to carry $O_2$, raising blood oxygen content
• D. CO binds to plasma proteins rather than hemoglobin, so it has minimal effect on $O_2$ transport by erythrocytes

Answer: (A)

Verified step by step guidance

1. Understand that hemoglobin in erythrocytes carries oxygen (O_2) by binding it to the iron in the heme groups.
2. Recognize that carbon monoxide (CO) has a much higher affinity for hemoglobin than oxygen does, meaning CO binds more strongly to the heme iron than O_2.
3. When CO binds to hemoglobin, it forms carboxyhemoglobin, which reduces the number of available binding sites for oxygen on hemoglobin molecules.
4. This binding decreases the overall oxygen-carrying capacity of the blood because fewer hemoglobin molecules are free to transport oxygen to tissues.
5. Additionally, the presence of CO shifts the oxygen-hemoglobin dissociation curve to the left, making it harder for oxygen to be released from hemoglobin to the tissues.