Why do we need haemoglobin?

The primary function of hemoglobin (Hb) is to transport oxygen. Since oxygen is not very soluble in water (the major constituent of blood), an oxygen transport protein must be used to allow oxygen to be 'soluble'. Hemoglobin (Hb) is the oxygen transport protein used in the blood of vertebrates.

When haemoglobin level decreases, it leads to anemia, hypoxia (deficiency of oxygen in tissue).