526 



ORGAN SYSTEMS OF MAN 



remained in the capsular filtrate, the animal 

 would shortly drain its body of the essen- 

 tials of life. Therefore, these products must 

 be selectively reabsorbed into the blood 

 while the fluid passes through the ex- 

 tremely long tubule on its way to the pelvis. 

 This is possible because, when the blood 

 leaves the glomerulus, instead of forming a 

 vein, as is customary in other organs, it 

 becomes another set of capillaries, this time 

 surrounding the tubule tliat leaves Bow- 

 man's capsule (Fig. 20-3). As the capsular 

 filtrate passes down the tubule, the valuable 

 portions, such as the amino acids, sugars, 

 salts, and so forth, are secreted back into 

 the blood again. This requires considerable 

 work on the part of the cells lining the 

 walls of the tubules, as indicated by the 

 fact that the kidney requires more oxygen 

 than the heart when calculated on equiva- 

 lent weights. The secretion is against a dif- 

 fusion gradient, which accounts for the 

 large amount of work that is necessary. If 

 the kidney is denied oxygen, reabsorption 

 stops, although filtration proceeds normally. 

 With this arrangement it is apparent that 

 the kidney functions as an organ wliich 

 selects what substances shall remain in the 

 blood and what shall be removed. If there 

 is too much sugar in the blood, for example, 

 the tubules will reabsorb some of it but 

 leave the surplus in the capsular filtrate. 

 Under these conditions the urine will show 

 sugar, which is what happens in diabetes. 

 The same is true of other substances. 



The kidney as a regulatory organ. Be- 

 cause of its ability selectively to secrete 

 substances, the kidney is a very important 

 organ in maintaining the proper composi- 

 tion of the blood and other body fluids. As 

 we have seen, the various end products 

 of metabolism are injurious if allowed to 

 accumulate. The kidneys remove just the 

 right amount of each to prevent harmful 

 effects, and yet leave enough to maintain 

 a proper balance of ions and molecules. 

 The kidney figures prominently in the re- 



tention or release of hydrogen and hydroxyl 

 ions to maintain their proper balance, and 

 thus hold the pH of the blood constant. 

 If the salt concentration of the blood should 

 rise too high or fall too low, there would be 

 a harmful movement of water which might 

 destroy cells. This too is prevented by the 

 elimination of exactly the right amount of 

 salts through the tubules of the kidneys. 



Many substances that are used in medi- 

 cine are eventually eliminated through the 

 urine. Such compounds as antibiotics, as- 

 pirin, and many others are removed from 

 the body via the kidneys. Hence testing the 

 urine is an important criterion of the effec- 

 tiveness of a drug in treating a specific dis- 

 ease. If the drug appears in the urine 

 shortly after it is given, it can be of little 

 value. Antibiotics, for example, must re- 

 main in the body long enough to have a 

 static effect on pathogenic bacteria. Fre- 

 quently when searching for a new drug, it 

 is found that the drug does the intended job 

 very well but is lost too rapidly through 

 the urine to be effective. 



The kidney is able to return a certain 

 amount of various substances to the blood 

 from the capsular filtrate, but if the amount 

 appearing in the blood is above a certain 

 critical level, which is spoken of as the 

 "threshold" level, the kidney no longer is 

 able to prevent the substance from appear- 

 ing in the urine. In diabetics (see p. 436), 

 for example, the blood sugar reaches such 

 high levels, owing to the lack of insulin, 

 that a large amount of it appears in the 

 urine. If, in man, the amount of sugar per 

 100 cc. of blood exceeds 150 mg., sugar 

 will appear in the urine. In other words, 

 that is the threshold for sugar. Other sub- 

 stances have thresholds but they would not 

 necessarily be the same as sugar. 



Blood volume is also regulated by the 

 kidney. Following a severe hemorrhage the 

 blood pressure drops, thus slowing up 

 urine production and conserving body 

 fluids. Similarly, if the blood contains too 



