Transport of the Respiratory Gases 



After the oxygen has been absorbed at the respiratory surface it 

 must pass to the cells where the oxidative processes take place. 

 Diffusion of oxygen in water and in the body fluids of an animal 

 is a relatively slow process (Table 2). It can be calculated that 

 to provide oxygen at the rate of 100 ccs/kg/hour by diffusion 

 alone is only possible in a spherical animal of about 1 mm. 

 diameter. This is one aspect of the decrease in surface/volume 

 ratio with increasing size. The surface area for oxygen intake 

 decreases but the distances to be traversed increase in larger 

 organisms. The need for a system of transportation within the 

 body capable of carrying large volumes of oxygen from the 

 respiratory organs to the tissues is clear. The physical process 

 involved is basically one of convection. 



(a) THE TRANSPORT OF OXYGEN 



In many animals, including very nearly all vertebrates, trans- 

 port of O 2 is assisted by the presence of a respiratory pigment in 

 the blood. Among vertebrates this is haemoglobin which is con- 

 tained within the red blood corpuscles. The effect of packing 

 such large molecules into corpuscles is to decrease the viscosity 

 of the blood to a level below that which it would be if all 

 the haemoglobin were in solution. Mammalian red blood 

 cells are non-nucleated and, except in camels, circular and bi- 

 concave. The absence of nuclei increases the space for haemo- 

 globin but probably decreases the life of the corpuscle. Most 

 other vertebrates have nucleated, elliptical, bi-convex cor- 

 puscles which are generally larger than the non-nucleated ones 

 of mammals. Red blood corpuscles of mammals are small, e.g. 



