Transpo of Carbon monoxide (CO) is diffusion limited because:

A i.e. High affinity of CO for haemoglobinTranspo and toxicity of carbon monoxide (CO) is limited to its diffusing capacity because hemoglobin combines with this gas so rapidlyQ that its paial pressure never has time to build up and is essentially zero in blood. The affinity of Hb for CO is 210 greater than its affinity for 02. So the paial pressure as low as 0.6mmHg of CO may be lethalQ. CO is toxic to tissue cytochromes at level that is 1000 times of lethal dose. Therefore tissue toxicity plays no role in clinical CO poisoningQ. It is treated by hyperbaric pure oxygen and 5% CO2.Carbon monoxide (CO) poisoning shifts oxyhemoglobin dissocation curve towards leftQ (i.e. reduces amount of 02 released in tissue). It reduces the amount of Hb that can carry 02 and causes anemic hypoxia.