Among the respiratory pigments we shall consider haemoglobin in some detail as this is the most familiar, widespread as well as the most efficient respiratory pigment. More efficient haemoglobins are those that are packed in cells and combine with far greater amounts of oxygen figure.
Figure: Plasma and whole blood that are brought into equilibrium with the same gas mixture have Ute same Po, and therefore, the same amount of dissolved oxygen molecules (shown as black dots).
If the amount of haemoglobin that is packed in cells was to be free in plasma, the viscosity of blood would be like syrup. Apart from this, being enclosed in cells provides a more stable environment as reaction of oxygen and haemoglobin is affected by ion concentration and organic compounds in blood.