PRINCIPLES OF PHYSIOLOGY 



91 



as carbamino Hb, and a small amount is present as carbonic acid, but 

 most of it is transported as bicarbonate ion, HCOg". The COo produced 

 by cells dissolves in the tissue fluid to form HoCOy, but the carbonic acid 

 is neutralized to bicarbonate by the sodium and potassium ions released 

 when oxyhemoglobin is converted to hemoglobin. The chemical details 

 of these processes are very complex. Oxyhemoglobin is a stronger acid 

 than reduced hemoglobin, hence some cations are released when HbOo 

 is converted to Hb. In the process of evolution this one molecide has 

 become endowed with all the properties needed for the transport of 

 large amounts of oxygen and carbon dioxide with a change of only a few 

 hundredths of a pH unit in the blood. 



The properties of the heme pigments are such that the amount of 

 oxygen taken up by the pigment is not directly proportional to the 

 oxygen tension; a graph of the relationship gives an S-shaped curve (Fig. 

 5.4). The blood is a more effective transporter of oxygen than it would 

 be if the oxygen content were a simple linear function of oxygen tension. 

 The effect of carbon dioxide (really the change in pH brought about by 

 changes in carbon dioxide content) on the combination of oxygen with 

 the pigment is shown in Figure 5.5. The oxygen dissociation curves for 

 arterial blood, with low carbon dioxide tension, and for venous blood, 

 with high carbon dioxide tension, illustrate how much more oxygen is 

 delivered to the tissue by a given amount of blood as carbon dioxide 

 is taken up in the tissue capillaries. The properties of the heme proteins 

 of different species are quite different, and in general are adapted to 

 the amount of carbon dioxide present. This is low in water-breathing 

 animals and high in air-breathing animals. This emphasizes the point 

 that the evolution of air-breathing animals from water-breathing ones 



100 -- 



Per Cent 



°f 75 4- 



Squld 



Hemocydnin 

 Oxygenated ^^ _ _ 



25-- 



50 100 



Oxygen tension. CpO^) in mm.Hg 



Figure 5.4. The amount of oxygen combined with hemocyanin is related to the 

 oxygen tension (pOo) by an S-shaped curve (solid line). Because of this, a greater amount 

 of oxygen (A) is defivered to the tissue by a given decrease in pOg than there would be 

 (B), if the properties of hemocyanin were such that there was a linear relationship be- 

 tween the percentage of hemocyanin oxygenated and the oxygen tension (dotted line). 



