378 - Multicellular Animals, Especially Man 



into the capsular parts of the nephrons from 

 the glomeruli, but only about (i() cc of this 

 flu itl reaches the collecting tubules as urine. 

 Meanwhile the volume is reduced, mainly by 

 the reabsorption of water, but partly by the 

 reabsorption of glucose, amino acids, and 

 various inorganic salts. In many cases reab- 

 sorption acts in opposition to the normal 

 processes of osmosis, and in performing this 

 definite work, the tubules expend energy and 

 utilize oxygen. Precisely how the cells of a 

 tubule are able to force substances to move 

 against a concentration gradient is still an 

 unsolved problem. A considerable amount 

 of water is reabsorbed by the distal tubules, 

 which are subject to control by the pituitary 

 hormone, vasopressin. The distal tubules 

 likewise are dominant in the reabsorption of 

 salts, and in regulating the hydrogen ion 

 concentration of the plasma and urine. In 

 contrast, glucose is reclaimed mainly in the 

 proximal tubules, but the other functional 

 differentiations ol the various parts of the 

 nephron are not entirely clear. 



The urine is a remnant of the nephric 

 liltrate, which retains most of the metabolic 



wastes as well as excesses of the other plasma 

 components, such as inorganic salts and glu- 

 cose. To a small extent, the initial quantity 

 of certain wastes in the filtrate may be aug- 

 mented by further quantities derived from 

 the capillaries surrounding the tubule walls, 

 but this secretional activity does not appear 

 to be very important in the human kidney. 



The kidney also has an important influ- 

 ence on the acid-base balance of the blood 

 and other body fluids. By excreting extra 

 quantities of acidic or b;;sic compounds, the 

 kidney can shift the equilibrium in either an 

 alkaline or acid direction — although the mag- 

 nitude of these changes in hydrogen ion con- 

 centration is very small, by virtue of the 

 excellent buffering capacity of the body fluids. 

 Moreover, the kidney exerts a further effect 

 on the acid-base balance of the body through 

 the medium of ammonia (NH 3 ), since this 

 alkaline compound is liberated when the 

 kidney participates in the deamination of 

 amino acids (p. 342). All in all it may be said 

 that the renal organs serve to maintain a 

 high degree of constancy in the internal en- 

 vironment of the body cells. 



TEST QUESTIONS 



1. Specify the major metabolic wastes of man. 

 What organs cooperate with die kidneys in 

 eliminating these products? Explain. 



2. Make a labeled diagram to show the human 

 kidneys, the renal blood vessels, and the con- 

 nections of the kidneys with the other parts 

 of the excretory tract. 



3. Make a labeled diagram to show all parts of 

 a nephric tubule and the relations of die 

 tubule to the capillar)' circulation. 



4. What is the "nephric liltrate"? How is it 

 formed? 



5. Carefully explain why the nephric filtrate is 

 called (a) a forced filtrate; (b) a colloid fil- 

 trate. 



6. To what extent is glucose present (a) in the 

 filtrate and (b) in the urine? Carefully ex- 

 plain this difference. 



7. To what extent is urea present (a) in the 

 nephric filtrate and (b) in the urine? Care- 

 lully explain this difference. 



8. Why is reabsorption described as a selective 

 process? Explain carefully, using (a) glucose, 

 and (b) water to exemplify the discussion. 



9. In achieving reabsorption. the tubule cells 

 perform work. Carefully explain the state- 

 ment in terms of: (a) the reabsorption of 

 glucose; (b) the reabsorption ol water. 



10. Assuming that the kidneys ol a normal indi- 

 vidual produce 100 cc ol urine per hour, 

 approximately what would be: (a) the hourly 

 filtration volume; (b) the quantity ol glucose 

 reabsorbed each hour; (c) the quantity ol 

 urea present in an hourly sample ol the 

 urine? 



11. Trace a given molecule of water through the 

 kidney, assuming: 



a. it does not enter the filtrate 



b. it enters the filtrate and is reabsorbed 



c. it enters the filtrate and is not reab- 

 sorbed 



12. After the injection of a certain dye into the 



