VII 

 THE RESPIRATORY FUNCTIONS OF BLOOD 



In the respiratory organs proper the gases are transported 

 from one medium to another by diffusion and, for all we know, 

 diffusion is also the only mechanism for the transport into and 

 out of the ultimately respiring cells, but in between we have a 

 transport by convection in the flowing blood. The diffusion 

 distances are always short and generally only small fractions 

 of 1 mm. The convection distances can be anything from 

 less than 1 mm to many meters. 



The efficiency of the convection transport depends upon 

 several factors. One, which has been touched upon before, 

 is the anatomical arrangements governing the blood-flow. It 

 is essential that the respiratory surfaces should be supplied 

 with "venous" blood, coming directly from the respiring 

 organs, and that the arterialized blood should travel directly 

 back to these, while any mixing of arterial and venous blood 

 greatly reduces the efficiency. Another important factor is 

 the volume flow of blood which must be regulated so as to 

 correspond at any time to the requirements of any particular 

 organ and of the body as a whole. This will be briefly dis- 

 cussed when the third factor, the transport capacity of the 

 blood, which is the main theme of this chapter, has been 

 dealt with. 



The transport capacity of a fluid for a gas is determined by 

 the relation between the pressure or "tension" of the gas and 

 the corresponding quantity which the fluid will hold. In 

 pure water this relation is rectilinear and the oxygen quanti- 

 ties corresponding to the possible tensions are so small that 

 water is inadequate as a carrier except in cases of very low 

 metabolism. The absorption coefficient of water for C0 2 

 being 100% at a temperature of 15.5°C water could serve as 

 a transport medium in many cases. Supposing in an aquatic 



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