TRANSPORT OF THE RESPIRATORY GASES 99 



adopted accessory air breathing and in consequence the CO 2 

 tension at the respiratory epitheHum will be higher than in 

 aquatic breathing. This decrease in sensitivity of the haemo- 

 globin to CO 2 tension must have been a necessary physiological 

 adaptation which preceded the adoption of the land habit. A 

 similar decrease in sensitivity to CO 2 occurs in the respiratory 

 centres of air breathers. 



(b) TRANSPORT OF CARBON DIOXIDE 



Blood pigments serve additional functions to those of oxygen 

 transfer for they are important in the transport of carbon 

 dioxide, both directly and as buffers, and in some instances they 

 serve to maintain the colloid osmotic pressure of the blood. The 

 amount of CO 2 dissolved in the blood far exceeds that which 

 results from ordinary physical solution which would be no more 

 than 2-5 vols. %. Arterial blood contains 45-50% and venous 

 blood 55-60 vols. % CO2. Only about 10 vols. % are therefore 

 given off in the lungs. Although most carbon dioxide is carried 

 as sodium bicarbonate, about one-third of the total transported 

 and liberated is in the form of HHbCOg, in which the CO2 is 

 directly combined with amino groups of the haemoglobin 

 molecules. This carbamino compound reacts reversibly with 

 oxygen, so that CO2 is liberated when oxygen is taken up. This 

 may take place rapidly and without any marked change in pH. 

 When the COg from tissue cells has diffused into the blood it 

 dissolves by a slow reaction and form HCO3 ions. Some of the 

 CO2 diffuses into the red blood cells and there forms HgCOg by a 

 rapid process catalysed by the enzyme carbonic anhydrase. This 

 carbonic acid subsequently dissociates into bicarbonate and 

 hydrogen ions. The latter are buffered extremely efficiently by 

 the haemoglobin. Thus if a quarter million molecules of 

 CO2 are introduced into the blood only one hydrogen ion 

 remains. The increased production of hydrogen ions en- 

 courages the dissociation of oxyhaemoglobin to reduced 

 haemoglobin and oxygen, which is given up to the cells. 

 Most of the bicarbonate ions diffuse out of the red blood 

 cells into the plasma and prevent the accumulation of an excess 



H 



