While breathing human beings inhale oxygen. Four molecules of O₂ combine with haemoglobin (Hb-respiratory pigment) present in the blood. Hb has great affinity for oxygen. On combining of oxygen with haemoglobin, the blood becomes oxygenated and it is then pumped by the heart to all body cells. After giving oxygen to the blood cells, the blood picks up CO₂ released during cellular respiration, becomes deoxygenated since Hb has low affinity for CO₂ so it is transported in dissolved form and CO₂ is exhaled out of the body from the lung alveoli.