VOL. 4 (1950) 



BODY SIZE AND TISSUE RESPIRATION 



255 



TABLE II 



COMPARISON OF THE COMPOSITION OF SERUM AND SERUM SUBSTITUTE 



In accordance with common usage one P is taken as 1.8 equivalent. 



(expressed as S) per 100 ml = 0.7 to i.o milliequivalent per litre^^' ^"; somewhat higher 

 figures are given by Guillaumin'^. For dog, ox, goat, and horse figures between 3 and 

 4 mg S per 100 ml are reported'^^' ''*. The serum substitute, being primarily intended for 

 use with animal tissues, copies the sulphate concentration of animal serum. If a substi- 

 tute for human serum is required half of the MgS04 should be replaced by an equivalent 

 amount of MgClg solution. 



Owing to the danger of bacterial infection the solutions containing organic sub- 

 stances should be freshly prepared before use. A composite stock solution containing 

 solutions 1-5 in the proportion stated and 3 parts of solution 6 is stable; the use of this 

 mixture shortens the procedure for preparing the full medium. 



Phosphate saline without Ca, and loio in bicarbonate and CO^ {Medium II). Serum 

 and the saline serum substitute may be inconvenient in the manometric measurement 

 of respiration because they must be kept in equilibrium with gas mixtures containing 

 about 5% CO2. The measurement of the oxygen uptake is simpler and more accurate if 

 the CO2 pressure of the gas phase can be kept near zero by absorbing the gas with alkali. 

 A reduction of the CO2 pressure necessitates an equivalent reduction in the bicarbonate 

 concentration if p^ is to remain within the physiological range. The following two types 

 of media with low bicarbonate and CO, concentrations have been in use : 



Type A. The greater part of the bicarbonate-COg buffer system is replaced by a 

 phosphate buffer of the same pn and approximately equivalent concentration. As a high 

 concentration of phosphate is incompatible with the physiological concentration of 

 calcium ions the latter are usually omitted from such media. Ca-free phosphate salines 

 are especially valuble as a medium for minced tissues and homogenates, as they give 

 higher and steadier rates than calcium containing media"^' ^^' ^> ^^> ^^' ^^. 



Type B. The bicarbonate content is reduced to about one-tenth of the physiological 

 value, with no change in the other constituents^^' ^^. Such a medium has the advantage 

 of having a physiological concentration of calcium, but its buffering capacity is much 

 below that of the media of Type A. The pn is not precisely defined but indicator tests 

 References p. 26/—26g. 



