Haemoglobin transports oxygen molecule to all the body cells for cellular respiration. The haemoglobin pigment present in the blood gets attached to four $O_2$ molecules that are obtained from breathing. It thus forms oxyhaemoglobin and the blood becomes oxygenated. This oxygenated blood is then distributed to all the body cells by the heart. After giving away $O_2$ to the body cells, blood takes away $CO_2$ which is the end product of cellular respiration. Now the blood becomes de-oxygenated.

Since haemoglobin pigment has less affinity for $CO_2$, $CO_2$ is mainly transported in the dissolved form. This de-oxygenated blood gives $CO_2$ to lung alveoli and takes $O_2$ in return.