TISSUE ACTIVITY AND DISSOCIATION .331 



An increase in 3° C. betxveen the values of 36 and 30 causes the 

 IlbO., to lose oxygen to the extent of about 10 per cent, of full saturation. 



(3) Carbon-dioxide. A physico-chemical factor, however, is 

 much more potent than temperature in })roducing desaturation. 

 Active tissues tend to become acid. In deahng with muscle, we 

 have seen how lactic acid is set free as the result of activity and 

 how oxygen is required before it can be replaced in the muscle 

 complex. This free lactic acid performs another service. Either 

 directly by partially diffusing into the surrounding lymph or 

 indirectly by producing alterations in the Helmholtz (polarising) 

 electric charge, it causes a potential alteration in the hydrogen ion 

 concentration of the tissue fluid. By a series of changes which 

 we have already briefly considered, and to which we shall return, 

 the net effect is to increase the tension of COg in the capillaries. 

 The molecules of COg are to be the new passengers on the erythro- 

 cytes, and because of their acidity they cause an aggregation of 

 the colloidal particles of haemoglobin and, as has already been 

 indicated, a marked desaturation. Carbon-dioxide acts as if the 

 tension of oxygen in the tissues were reduced to 10/24ths of its 

 real value. That is, haemoglobin parts with as much oxygen at a 

 tissue tension of oxygen of24< mm. Hg. as if the tension of oxygen were 

 only 10 mm. For example, blood which in the absence of CO 2 

 would be 30 per cent, desaturated is actually desaturated to the 

 extent of 78 per cent, by the presence of a CO2 tension of 40 mm. 

 Hg. The blood becomes now a carrier of CO2 from the tissues to 

 the lungs (Fig. 82, dash line curve). 



Handling of Oxygen. Consider an inland village supplied by 

 a canal coming as close as possible to the community. Internal 

 communication is effected by waterways fed from the canal. The 

 by-products of manufacture (sent elsewhere for elaboration) are 

 transported along these waterways to the canal and the same 

 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 : 



H2CO3 + Na2HP04 — NaHoPO, + NaHCOg. 



The sodium bicarbonate so formed finds its way into the blood 

 stream, where it is in equilibrium with free dissolved carbon- 

 dioxide, so that the volume of COg in solution is 1/20 of the volume 

 of combined COo. 



The result of the influx of NaHCOg is to increase the volume of 



