504 EXCRETION 



that the exchange between blood and tissue lymph is mainly accom- 

 plished by nitration from the capillaries of vast quantities of liquid 

 containing the crystalloid constituents and the gases in the concen- 

 tration in which they exist in plasma. On the contrary, the dissolved 

 substances are currently and no doubt correctly assumed to move to 

 a great extent by diffusion through the capillary walls (perhaps with 

 a certain amount of active intervention of the endothelial cells) and 

 across the thin sheets of tissue lymph on their way to and from the 

 cells. In other words, they move mainly through the water and not 

 with the water. An attempt to explain the gaseous exchange between 

 the blood and the tissues as a matter purely of nitration of plasma 

 containing dissolved gases, and not at all of diffusion of the gases, 

 would lead to curious results. There is no more reason to believe that 

 urea passes from the blood to the boundary of the tubule cells by a 

 filtration process independent of diffusion, and therefore entailing the 

 irrigation of the cells with a very large amount of lymph, than there 

 is to believe that when 100 c.c. of arterial blood loses 10 c.c. of oxygen 

 in passing through the capillaries, this is accomplished by filtration 

 into the lymph spaces of 4,000 c.c. of plasma containing 0-25 c.c. of 

 oxygen in 100 c.c. (p. 252). 



As to the nature of the mechanism set in motion, and the series 

 .of events that take place as the constituents of the urine journey 

 from the interior of the bloodvessels to the lumen of the tubules, 

 we know no more than in the case of other glands. This alone 

 is clear, that the separation of the urine from the blood implies the 

 performance of a large amount of work by the kidney. A token of 

 the intensity of the metabolic effort required is the marked increase 

 in the absorption of oxygen which occurs during diuresis. In one 

 experiment the oxygen absorbed by a dog's kidneys was n per cent, 

 of what would have been used up by the entire animal under normal 

 conditions. 



The mere fact that urine differs in its quantitative composition 

 from blood-plasma is sufficient to show that work must be done in 

 its separation from the blood. Although the amount of work 

 cannot be calculated from the difference in the osmotic pressures 

 of the two liquids, a comparison of the freezing-points affords quali- 

 tative evidence of the performance of work by the kidney. For 

 average urine, the value of A is several times as great as for the 

 plasma. Blood-plasma freezes at - 0-55 to - 0-65 C. (average, 

 - 0-6, corresponding to an osmotic pressure of 5,662 mm. of mercury, 

 or about 75 metres of water). Human urine has been found to 

 freeze at -1-38 to -2iiC. (average, - i8 C., corresponding to 

 an osmotic pressure of about 17,000 mm. of mercury, or 225 metres 

 of water). For highly concentrated urines, the depression of the 

 freezing-point may be considerably greater. Even when the 

 freezing-point is found in very dilute urines to be approximately 

 the same as in the blood-plasma, work may still have been done in 

 the separation of the urine, because although the total molecular 

 concentration may be the same in the two liquids, the concentra- 

 tion of each of the substances in solution may be different. For 



