42 GENERAL BIOCHEMISTRY 



large quantity of hydrogen ion quickly combines with the small 

 amount of acetate ion, leaving much hydrogen ion, and the pH 

 changes markedly. When the salt of the acid has been included, how- 

 ever, a large supply of anion is available, and the system is capable of 

 accepting protons in quantity. Thus a mixttire of weak acid (or base) 

 and its salt resists change in either direction. 



The life processes involve proton transfers. Hence pH could vary 

 widely and fatally since many critical materials break down at ex- 

 tremes of pH. Such changes are prevented by naturally occurring 

 buffer systems of many kinds, but all function by the general mechan- 

 ism illustrated above. 



Gases in Solution 



The discussion of the natural transport of gases has already empha- 

 sized that gas molecules are dispersed by diffusion in liquids. The 

 resulting solutions then participate in gas exchanges and the like. 

 Quantitatively, rates of diffusion depend upon concentration gra- 

 dients which are concentration differences in different parts of the 

 system. Such differences in concentration depend partly upon the 

 amount of gas dissolving, which depends in turn upon the pressure 

 of the gas in contact with the solution. 



For dilute solutions the concentration of gas dissolved is directly 

 proportional to the pressure of that particular gas exerted on the 

 solution. At higher pressures more gas dissolves, and biological circu- 

 latory and diffusion systems can transfer more of it since higher con- 

 centration gradients become possible. 



The effect of high altitudes on human beings may be cited as an 

 example of the process. People ascending high moimtains or flying in 

 planes commonly show symptoms of oxygen deficiency, although there 

 are individual variations in the nature and severity of the manifesta- 

 tions. The basic cause arises from a progressive decrease in air pressure 

 with increasing altitude. And since the composition of air changes 

 but little with altitude, the pressure of oxygen decreases, diminishing 

 the amount dissolving in the blood. Thus the transfer rates decline 

 and the respiring cells run short, particularly in the brain and in the 

 voluntary muscles during exercise. To some extent the human 

 physiology can compensate for the low oxygen pressure, given time for 

 adaptation. Persons with normal hemoglobin synthesis accumulate 

 extra red blood cells, and although each cell transports less oxygen 

 than it did at sea level, the increased cell count helps bring up oxygen 

 transfer. Individuals born and reared at exceptionally high altitudes 

 have an additional tissue adaptation whose nature is not yet under- 



