ALIGNMENT CHART 339 



the total negative ions of the serum, and hkewise for the eclls. 

 These three conditions absohitely determine the relative distribu- 

 tion of eleetrolytes under given eonditions. If the eomparatively 

 low osmotic pressures of haemoglobin and serum protein are 

 ignored, an approximate value for r may be deduced, viz. : 



[Hb,] 



r = 1 



2 [A J 



As — cannot be negative, r must always be less than 1, i.e. 



2 [A J 



there will always be an unequal distribution of anions about the 

 membrane, the concentration in the cells being always less than 

 that in the serum. 



Hence the transport of respiratory carbon-dioxide in the blood 

 involves the passage of carbonate, chloride and water from plasma 

 to cells, a change in cell volume " i'," a change in the anion ratio 

 " r " and a slight change in hydrogen ion concentration ; the 

 changes which occur when the haemoglobin is reduced and the 

 carbon-dioxide is picked up occur in the reverse order when the 

 haemoglobin is oxygenated and the carbon -dioxide excreted. 

 Thus it may be seen that in determining equilibrium eonditions in 

 the blood, it is necessary to consider oxygen tension, carbon-dioxide 

 tension, j^H, r, v, the percentage of haemoglobin combined with 

 oxygen, and the percentage of carbon-dioxide free and in combina- 

 tion with base as bicarbonate. The relationship between any 

 two of these factors, when other conditions are constant, may 

 be represented by curves. 



In studying the mechanism of the gaseous interchange of blood, 

 it is convenient to begin by studying the reaction of blood in vitro, 

 under specified conditions. From such studies, curves like Fig. 82 

 may be deduced. These curves do not, however, at all represent 

 the gaseous exchange of blood in the body. They represent the 

 variation of one gaseous constituent {e.g. O^) with one other 

 condition {e.g. oxygen pressure), assuming that other conditions 

 {e.g. tensions of other gases) are constant. In the circulating blood, 

 however, in lungs and tissues, not only oxygen tension, but carbon- 

 dioxide tension, vary from point to point, and the oxygen absorp- 

 tion curve for blood in vivo is one which intersects the curves 

 representing oxygen absorption at different oxygen pressures 

 under different constant carbon-dioxide tensions of physiological 

 range. 



To define the complete equilibrium conditions of blood at any 

 point a large number of curves would be necessary, indicating the 

 relationship between oxygen and carbon-dioxide content and ten- 



