98 RESPIRATION 



In considering how the cell acquires oxygen and rids 

 itself of CO2 we must bear this fact in mind — that the 

 extent of its gaseous interchange is determined not by the 

 amount of oxygen presented to it by the blood, but by its own 

 inherent need arising out of its metabolic activity. It is 

 the cell and not the blood which sets the pace for oxida- 

 tion. The amount of oxygen which the cell utilises is 

 therefore a measure of the work which the cell is performing. 

 When the body as a whole is at rest the blood contains 

 more than sufficient oxygen for its needs. 



The problem of respiration resolves itself into two ques- 

 tions: — -First, how are the supply of oxygen and the removal 

 of CO2 effected? Secondly, how do these vary according 

 to the varying needs of the body? Consider the task which 

 the blood performs. It conveys two gases in opposite 

 directions, one of these gases being relatively insoluble 

 in aqueous solution. This double transport of gases is 

 carried out undisturbed by the many other functions which 

 the blood performs. 



THE TRANSPORT OF OXYGEN 



Haemoglobin 



Haemoglobin is a complex substance present in red blood 

 coxpuscles. It is a combination of hsematin and a protein 

 known as globin, and has a molecular weight of about 

 16,600. Its haematin component contains iron, each 

 molecule of haemoglobin containing one atom of this metal. 

 Haemoglobin possesses the property of forming with oxygen 

 a loose compound known as oxy haemoglobin. As each 

 molecule of haemoglobin combines with two atoms of oxygen, 

 it follows that in oxyhaemoglobin there are two atoms of 

 oxygen for every atom of iron. Haemoglobin must there- 

 fore be regarded teleologically as a means of utihsing the 

 oxygen-combining property of iron, the great size of the 

 haemoglobin molecule overcoming the high specific gravity 



