228 HUMAN BIOLOGY 



excessive, results in headache, nausea, dizziness and other 

 effects attributable to an altered state of the brain. On the 

 other hand, if the water of the plasma is decreased the 

 blood becomes thickened, the blood pressure falls, fluid 

 passes out of the cells and the temperature of the body 

 rises in a fever. The constancy of the water content of the 

 plasma, therefore, is of primary importance for the normal 

 life of the organism. 



Water is being continuously lost from the body. It floats 

 away as vapor in every breath we expel. Even when we are 

 quiet it is lost through the skin at the rate of about a quart 

 a day. And it goes out through the kidneys with waste 

 products which must be kept in solution. To replace this 

 continuous loss water can be taken only periodically and 

 then it may be taken in excess of the immediate require- 

 ment. Under the circumstances how does the plasma fare? 



Experiments have shown that its consistency is kept 

 constant in spite of most exacting tests. Over five quarts 

 of water have been drunk in six hours (indeed, the volume 

 of water exceeded by one-third the estimated volume of 

 the blood in the man who performed the feat) and yet it 

 was absorbed into the body, was carried to the kidneys 

 and by them discharged, without at any time causing a 

 dilution of the blood which could be detected by studying 

 a change of its color. On the other hand, total deprivation 

 of water for as long as three days may be endured without 

 any detectable change in the concentration of the plasma 



The remarkable uniformity of the water content of the 

 blood is maintained by storage and overflow. When much 

 water is introduced into the body it is stored in muscles 

 and other organs and in the skin. Since muscles constitute 

 nearly half of the body weight only a very small accumula- 

 tion in each muscle cell results in a large reserve. And in the 

 skin is a peculiar form of tissue, with innumerable miteuo 

 spaces, in which water and the substances dissolved in it 

 (sugar and salts) can be retained. If the water intake is not 

 great or too rapid to be accommodated in these stores it 

 pours over the dam in the kidneys and is discharged from 

 the body. The kidneys must be remarkably sensitive to a 

 slight change in the concentration of the blood or there 



