ACIDITY 325 



as ammonia or amines, while neutral-soil plants, such as 

 ordinary garden and crop plants, prefer theirs as the nitrate 

 ion. 



Acidity has come to play a very important part in medicine. 

 How far changes in acidity are the cause and how far the result 

 of disease is a debated question, with the majority holding the 

 latter view\ 



The first problem to consider is the pH of the blood. Blood is 

 well buffered and therefore maintains a very constant pH value 

 which, normally, is about 7.4 (i.e., slightly alkaline). Difficulty 

 in measuring the pH of the blood has been the cause of much 

 uncertainty and some controversy over values. The first errors 

 enter when the blood is drawn off. The pH value determined 

 several minutes later may not be very near the value that exists 

 in the blood stream. The carbon dioxide tension of the blood 

 is higher than that of the air; therefore there is a loss of carbon 

 dioxide when blood passes through air. If the loss of carbon 

 dioxide is partially prevented by drawing the blood off under oil, 

 there are still the errors due to drop in temperature and the 

 presence of proteins. The effect of the loss of carbon dioxide 

 from the blood stream is interestingly and convincingly illus- 

 trated in the faint condition which results in human beings from 

 excessive heavy breathing. This may go so far as to cause 

 unconsciousness. It is due to an abnormally high alkaline state 

 produced by loss of carbon dioxide. 



While measurements of blood pH may involve many errors, 

 the values obtained appear to be fairly accurate, and at least 

 comparable, as the same errors creep into all determinations 

 when made by one method. It is now generally conceded that 

 for blood determinations the colorimetric method is the most 

 reliable as well as the most rapid and convenient. 



Other body fluids have their characteristic pH values. Gastric 

 juice may be very acid with a pH of 1.0 to 6.0, usually about 2.0. 

 (Lemon juice is about pH 2.) Urine is 4.8 to 8.2, with 6.0 as the 

 average (this is the pH range of phosphate buffers, which proba- 

 bly maintain the pH of the blood). Saliva may be slightly acid 

 (6.8) though the mouth is usually alkaline. Cerebrospinal fluid 

 is just above neutrality, with a pH close to that of blood — 7.3 to 

 7.4 (for unknown reasons, the cerebrospinal fluid is very poorly 

 buffered). 



