THE KIDNEY 45 



mented by either tubular reabsorption or tubular excre- 

 tion? The answers to this question can be obtained only 

 by measuring the initial process of filtration: that is, by 

 determining the volimie of water filtered per unit time 

 in any individual under a given set of circumstances. 

 (This is, of course, far greater than the rate of excretion 

 of water as urine because most of the filtered water is 

 reabsorbed by the tubules.) Given a method of deter- 

 mining the rate of filtration of water, then simultaneous 

 measurements of the rate of excretion of any other sub- 

 stance permits one to determine whether it is reabsorbed 

 from this filtrate, or, alternately, added to the filtrate by 

 tubular excretion and, in either instance, precisely to 

 what extent. A hundred years and more elapsed in the 

 study of renal physiology before this method became 

 available, and indeed before the problem was even 

 phrased in this simple, quantitative way. When the an- 

 swer came, it came not primarily from studies of renal 

 function in man but from studies in the comparative 

 physiology of the kidney in a large variety of animals. 



The development of renal physiology as a science 

 might be said to have started in ancient times, since the 

 physicians of Egypt, Babylonia, Greece, and Rome had 

 acquired certain elementary notions of cause and effect 

 in such sequences as the obvious increase in urine flow 

 (diuresis) that results from the ingestion of large quan- 

 tities of water, the decrease in flow (oliguria) that re- 

 sults from water deprivation, and the discoloration or 

 odor of the urine that follows the ingestion of certain 

 foods. However, the physicians of ancient times had no 

 knowledge of the fine anatomy of the kidney and less 

 knowledge of physics and chemistry, and consequently 

 their observations lacked any proper interpretation. 



Modem renal physiology (like all physiology) begins 

 properly with William Harvey's demonstration, in 1628, 

 that the heart is a pump that keeps the blood in steady 

 circulation around the body through the arteries and 

 veins. In estabUshing the reality of this circulation, Har- 



