Explain Transport of Oxygen by Red Blood Cells

Oxygen combines with haemoglobin, i.e., the red pigment in red blood cells. It combines with oxygen in the lungs, transports it around the body and releases the oxygen to cells that need it to form oxyhaemoglobin. Haemoglobin, you may already know, is a protein made up of 4 subunits, each of which contains a heme moiety attached to a polypeptide chain. Heme, is a complex made up of a porphyrin and one atom of ferrous iron. Each of the 4 iron atoms can bind reversibly one oxygen molecule. The iron remains in the ferrous form and the resulting combination with oxygen is known as oxygenation. The entire process can be represented as:

Hb₄ + O₂ → Hb₄O₂

Hb₄O₂ + O₂ → Hb₄O₄

Hb₄O₄-k O₂ → Hb₄O₆

Hb₄O₆ + O₂ → Hb₄O₈

Hb₄ stands for haemoglobin and O₂ stands for oxygen.

The tension of oxygen in the arterial blood is about 90 -100 mm Hg and oxygen tension in the tissues is less than 40 mm Hg. When the arterial blood passes through the tissues, it carries oxyhaemoglobin. As the oxygen tension in the tissues is much lower, oxygen tension in the arterial blood falls. The oxyhaemoglobin in the red blood cells is exposed to low tension of oxygen, it dissociates and oxygen leaves the blood stream and enters the tissue. The equilibrium of oxyhaemoglobin and nonbonded haemoglobin at various partial pressures can be best studied through a oxygen dissociation curve.