6.4 Exchange of Gases

The primary site for the exchange of gases is the alveoli. The uptake of O and the release of CO occur between the blood and tissues by simple diffusion driven by partial pressure gradient of O and CO . ­ Partial pressure is the pressure contributed by an individual gas in a mixture of ­gases. It is represented as pO for ­oxygen and pCO for carbon–dioxide.

Due to pressure gradients, O from the alveoli enters into the blood and reaches the tissues. CO enters into the blood from the tissues and reaches alveoli for elimination. As the solubility of CO is – times higher than that of O , the partial pressure of CO is much higher than that of O (Table . and Figure .

). Respiratory Pigments Haemoglobin Haemoglobin belongs to the class of conjugated protein. The iron containing pigment portion haem constitutes only % and the rest colourless protein globin belongs to histone class. Haemoglobin has a molecular weight of , daltons and contains four atoms of iron, each of which can combine with a molecule of oxygen.

Methaemoglobin If the iron component of the haem is in the ferric state, than the normal ferrous state, it is called methaemoglobin. Methaemoglobin does not bind with O2. Normally RBC contains less than % methaemoglobin. Figure .

Exchange of Gases at the Alveolus and the Tissue with blood and transport of Oxygen and Carbondioxide