376 - Multicellular Animals, Especially Man 



Amphibia, water absorption is not conspicu- 

 ous, and such animals form large quantities 

 of hypotonic urine. But in terrestrial verte- 

 brates, the average quantity of urine is much 

 reduced, and in mammals the urine often is 

 distinctly hypertonic compared to plasma. In 

 producing such a hypertonic urine the kid- 

 ney performs definite work, which requires 

 that the tubules must be supplied with ade- 

 quate oxygen. When deprived of oxygen, the 

 kidney loses its capacity to reabsorb water 

 and produces a very large volume of highly 

 dilute urine, regardless of the needs of the 

 body. 



Secretion. Certain drugs, dyestuffs, and 

 perhaps some natural wastes seem to be trans- 

 mitted from the blood in the capillaries sur- 

 rounding the nephric tubule into the nephric 

 fluid. Therefore, the total quantity of such 

 substances in the urine represents the sum 

 of two quantities: (1) the portion that filters 

 into Bowman's capsule, and (2) the portion 

 that is secreted into the tubule by the glan- 

 dular walls. In the case of phenol red — the 

 dye used by E. K. Marshall, Jr., in 1924. when 

 he and co-workers at Johns Hopkins Uni- 

 versity first demonstrated secretional activity 

 in the kidney — the concentration of the dye 

 in the urine was more than 200 times greater 

 than in the blood; and this degree of con- 

 centration cannot be accounted for by water 

 reabsorption alone. 



Secretional activity, however, probably 

 plays a relatively minor role in the function- 

 ing of a normal human kidney. The evidence 

 shows that a large proportion of the water 

 that is filtered into Bowman's capsule is re- 

 absorbed before the filtrate reaches the col- 

 lecting tubule, and this mainly accounts for 

 the high concentration of wastes in human 

 urine. 



However, the nephrons of certain fish do 

 not possess glomeruli, and in such aglomer- 

 ular kidneys secretional activity assumes a 

 very important part in the formation of the 

 urine. 



Renal Impairments. Ordinary kidney dis- 

 sease, or nephritis, involves primarily the 



glomeruli. The diseased glomeruli become 

 more freely permeable than normally, and in 

 severe cases the glomeruli show signs of dis- 

 integration. Normally the filtrate and urine 

 are free of proteins; but in nephritis, first 

 the plasma albumins, and then the more com- 

 plex globulins, appear in the urine — depend- 

 ing on the serevity of the case. In the severest 

 cases, even intact erythrocytes are found in 

 the urine. Owing to a continued loss of pro- 

 teins, the capacity of the plasma to reabsorb 

 fluid from the tissue spaces becomes im- 

 paired, and consequently edema develops 

 (p. 335). In the terminal stages of nephritis 

 the glomeruli cease to conduct blood and the 

 filtration volume drops drastically. Then 

 both water and excretory wastes accumulate 

 in the body, and death from uremic poison- 

 ing becomes inevitable in about 24 hours, 

 unless curative measures can be taken. 



In diabetes insipidus, the patient's daily 

 output of urine is 30 to 40 liters, instead of 

 the normal average of about 1.5 liters. This 

 condition represents a grave impairment of 

 the water-reabsorbing capacity of the tubules. 

 The primary cause of diabetes insipidus is 

 not localized in the kidney, however. The 

 disease represents a failure of the pituitary 

 gland to produce the antidiuretic hormone, 

 vasopressin (p. 412). Normally this hormone 

 stimulates and controls the water-reabsorbing 

 activities of the tubules. 



Chronic hypertension, or high blood pres- 

 sure, is very frequently associated with neph- 

 ritic impairments. The high pressure pro- 

 duces a very high filtration volume and in- 

 jures the glomeruli. The kidney is not neces- 

 sarily a passive factor in hypertensive cases. 

 Recent work indicates that a normal func- 

 tion of the kidney is to produce one or more 

 vasodilator substances, which tend to coun- 

 teract high blood pressure. A kidney with 

 restricted circulation seems to produce also 

 a vasoconstrictor substance — called renin — 

 which may aggravate the hypertension. 



Evolution of the Vertebrate Kidney. The 

 earliest chordate animals (p. 664) appear to 

 have arisen in the dilutely brackish waters 



