254 RESPIRATORY FUNCTION OF THE BLOOD 



gangs of labourers unload the raw material from the barges and 

 float it up to the factories. 



To take a specific instance, muscle, as a result of its activity, 

 produces carbon dioxide. This weak acid acts on the sodium 

 hydrogen phosphate of the tissue fluid according to the following 

 equation : 



H 2 CO 3 +Na 2 HPO 4 ^ NaH 2 PO 4 +NaHCO 3 . 



The sodium bicarbonate so formed finds its way into the blood 

 stream. Now in plasma, bicarbonate is in equilibrium with free 

 dissolved carbon dioxide when the volume of CO 2 in solution is 

 1/20 of the volume of CO 2 combined 



The result of the influx of NaHCO 3 is to increase the volume of 



CO free 1 



f ree CO o in order to preserve the ratio ^.r =-r-. Briefly, 



CO 2 combined 20 



the CO 2 tension in the muscle capillaries tends to increase. As 

 stated above, increase of CO 2 tension causes increased unloading 

 of oxygen from haemoglobin. 



That is to say increased activity postulates increased energy 

 usage, which renders necessary an immediately increased supply 

 of oxygen. The amount of oxygen required is liberated by the 

 desaturating action of CO 2 the main chemical product of 

 the activity. 



The amount of oxygen in the blood does not control oxidation 

 in the tissues, but the call for oxygen by the tissues controls the 

 rate of unloading of oxygen. 



Transport of carbon dioxide. 



The principle underlying the transport of carbon dioxide is 

 identical with that enunciated for oxygen. In the tissues, the 

 tension of carbon dioxide is relatively high and the gas passes to 

 the blood, is carried to the lungs and is there eliminated. Haemo- 

 globin, once freed from its load of oxygen, takes on a cargo of 

 carbon dioxide. But haemoglobin is not the sole means of trans- 

 port. Carbon dioxide is about twenty-five times as soluble as 

 oxygen under similar conditions. Relatively more CO 2 will, 

 therefore, be carried in true solution in the plasma. In addition 

 to this amount (which we have just seen is carefully regulated) a 

 considerable quantity of the gas is adsorbed to the various colloids 

 of the plasma, (i) Each gram of fibrinogen can carry 1/30 gram 

 of CO 2 . (ii) Serum proteins may adsorb a measurable quantity of 

 carbon dioxide at the lowest estimate, over 5 per cent. It is 



