5l8 W. F. LOOMIS 



well-known Henderson-Hasselbalch equation states that the concentration 

 of the H+ ion varies both with the pCOg and the bicarbonate present: 



Concentration of H+ 



HCO3 



it is clear that any change in one of these three variables will affect at least 

 one of the other two. Picture a metal triangle supported at its centre, on a 

 table, its three corners representing pH, pCOa and bicarbonate respec- 

 tively. If now one of the three corners of the triangle is held firmly against 

 the table, it will remain constant while the other two vary reciprocally like 

 a see-saw. This is the principle of a triad of experiments that we have used 

 to determine which of these three variables is the biologically active one 

 in any given situation. Experimental control of pCOg may be effected by 

 (i) exposing shallow Petri dish cultures to known concentrations of CO, 

 within a desiccator; (2) injecting closed containers with varying volumes 

 of a culture solution high in pCOo that has been previously prepared by 

 shaking it in a syringe with air containing the desired amount of COo gas; 

 and (3) bubbling the experimental culture continuously with air from a 

 tank containing the desired concentration of COg. In my laboratory in 

 Greenwich I have a series of gas tanks that vary from o- 1% to 100% COg. 

 With their aid, almost any desired pCO., can be easily and rapidly obtained. 



Today, then, tissue pCO., represents an old-but-new variable of some 

 complexity. Small wonder that most modern textbooks either ignore the 

 subject or else dangerously oversimplify it. Many people, for example, have 

 been taught that carbonic acid is a weak acid when in fact it is as strong an 

 acid as citric, formic or nitrous acid! [7]. The reason for this widespread 

 misconception is the little appreciated fact that 99-9% of the CO2 dis- 

 solved in water does not hydrate to HoCOg but remains as free gaseous 

 CO2 [8]. The o-i% of carbonic acid that does form is a surprisingly 

 strong acid. Taken together, these two facts combine to make gaseous CO2 

 equivalent to a weak acid, a simplification that may be legitimate in certain 

 situations but not in others. 



Summarizing the physical-chemical facts then, we can say that pCOa 

 is a universal biological variable generated by all respiring cells. Being fat- 

 soluble, COo can easily pass through fatty cell membranes and hence 

 unify a cellular aggregate into one overall "field of force". Highly reactive 

 chemically, it enters into many cellular reactions as a direct participant as 

 well as specifically affecting intracellular pH. Present as a gas within the 

 alveoli of the lung, as a dissolved gas within the tissues and finally as a 

 solid within the matrix of a bone or shell, five separate steps are needed to 

 connect all forms of this one metabolite, the final complexity arising when 

 part of this inorganic chain is catalyzed by the zinc-containing enzyme 

 carbonic anhydrase and so is subject to all the variables that affect the 



