(N/A) In human beings,oxygen $(O_2)$ is transported by the respiratory pigment haemoglobin present in red blood cells. Haemoglobin has a very high affinity for $O_2$. It binds with $O_2$ molecules obtained from the lungs during breathing to form oxyhaemoglobin,making the blood oxygenated. This oxygenated blood is then pumped by the heart to all body cells for cellular respiration.
After delivering $O_2$ to the body cells,the blood collects carbon dioxide $(CO_2)$,which is a byproduct of cellular respiration. Since haemoglobin has a much lower affinity for $CO_2$ compared to $O_2$,most of the $CO_2$ is transported in a dissolved form in the blood plasma. This de-oxygenated blood carries $CO_2$ back to the lung alveoli,where it is released and exchanged for fresh $O_2$.