OSMOTIC PRESSURE 15 



work are expended.* But that such comparisons of the osmotic pres- 

 sure of blood and urine are fallacious as an indication of the work of 

 the kidney is evidenced, not alone by the results of the above calcula- 

 tions, but also by the fact that under certain circumstances (as after 

 copious diuresis) the osmotic pressure of the urine may be considerably 

 lower than that of the blood. That opposite relationships should exist 

 indicates that differences in osmotic pressure between blood and urine 

 can signify little if anything regarding the work done by the kidney. 



For some time after the application of osmotic pressure measurements 

 to the study of biological problems, it was thought that determination 

 of A in urine might be of clinical value as a criterion of renal efficiency, 

 especially in one kidney as compared with the other. For this purpose 

 A was determined in samples of urine removed from each ureter by 

 catheterization. The tests of renal efficiency based on the rate of excre- 

 tion of potassium iodide, phenolphthalein, etc., have however been found 

 of much greater value. 



3. Is the formation of lymph purely a physical process? The osmotic 

 pressure of normal lymph is nearly always somewhat below that of 

 blood serum, although occasionally it has been found to be a trifle 

 higher. Physical processes, such as filtration, might therefore suffice 

 to account for its formation under most conditions. But when we con- 

 sider the excessive production of lymph that occurs 'as a result of cel- 

 lular activity or following the injection of certain substances, called 

 "lymphagogues," it is not so easy to explain the .production in such 

 terms, although some interesting attempts have been made to do so by 

 those that are wedded to the mechanistic view. For example, the very 

 marked increase in lymph flow which occurs as a result of muscular 

 exercise or glandular activity has been attributed to the fact that dur- 

 ing such processes large molecules become broken down into small ones 

 in the cell protoplasm, so that the osmotic pressure is raised and water 

 is attracted into the the cell until the latter becomes distended and a 

 process of filtration into the neighboring lymph spaces occurs (see 

 page 119). 



There are several other physiologic processes of secretion and excre- 

 tion which might be considered in the present relationship, but the above 

 instances will suffice to illustrate the general principle upon which all of 

 them have to be considered. 



"Osmotic pressure corresponding to A = -0.6 C. equals 5,662 mm. Hg (75 m. of H 2 O), and 

 that corresponding to A = -1-8 C. equals 16,986 mm. Hg (225 m. H 2 O). The difference is there- 

 fore equal to a column of water 150 m. high. According to these calculations it would appear that 

 the kidney in producing the average daily output of 1500 c.c. urine performs 225 kilogrammeters of 

 work in comparison with the 14,000 kilogrammeters which the heart is computed to perform in the 

 same time (page 212). 



