Which of the following forms a toxic substance in blood by combining with haemoglobin?
The haemoglobin is the respiratory pigment present in the red blood cells. It has iron moiety associated with it. The haemoglobin has an affinity for the oxygen under normal circumstances when the partial pressure of oxygen is high. However, when the carbon monoxide is present in the alveoli of the lungs, then CO shows more affinity than oxygen and binds with the haemoglobin to form the carboxyhaemoglobin. This results in the deficiency of the oxygen in the body called carbon monoxide poisoning. Hence, the correct answer is 'Carbon monoxide'.