SECRETION. 255 



The- normal stimulus to the epithelial cells of the convoluted tubules, 

 using the term convoluted to include the actively secreting part-, seems to 

 be the presence of urea and related substances in the blood (lymph). That 

 the elimination of the urea is not a simple act of diffusion seems to be clearly 

 shown by the fact that its percentage in the blood is much less than in the 

 urine. In some way the urea is selected from the blood and passed into the 

 lumen of the tubule, and although we have microscopic evidence that tins 

 process involves active changes in the substance of the cells, there is no ade- 

 quate theory of the nature of the force which attracts the urea from the sur- 

 rounding lymph. The whole process must be rapidly effected by the cell, 

 since there is normally no heaping up of urea in the kidney-cells ; the material 

 is eliminated into the tubules as quickly as it is received from the blood. 

 The rate of elimination increases normally with the increase in the urea in 

 the blood, as would be expected upon the assumption that the urea itself acts 

 as the physiological stimulus to the epithelial cells. 



The Blood-flow through the Kidneys. — It will be seen from the dis- 

 cussion above that, other conditions remaining the same, the secretion of the 

 kidney varies with the quantity of blood flowing through it. It is therefore 

 important at this point to refer briefly to the nature and especially the regula- 

 tion of the blood-flow through this organ, although the same subject is referred 

 to in connection with the general description of vaso-motor regulation (see 

 Circulation). It has been shown by Landergren 1 and Tiegerstedt that the 

 kidney is a very vascular organ, at least when it is in strong functional activ- 

 ity such as may be produced by the action of diuretics. They estimate that 

 in a minute's time, under the action of diuretics, an amount of blond flows 

 through the kidney equal to the weight of the organ; this is an amount from 

 four to nineteen times as great as occurs in the average supply of the other 

 organs in the systemic circulation. Taking both kidneys into account, their 

 figures show that (in strong diuresis) 5.6 per ceut. of the total quantity of 

 blood sent out of the left heart in a minute may pass through the kidneys, 

 although the combined weight of these organs makes only 0.56 per cent, of 

 that of the body. 



The nature of the supply of vaso-motor nerves to the kidney and the con- 

 ditions which bring then) into activity are fairly well known, owing to the use- 

 ful invention of the oncometer by Roy. 2 This instrument is in principle a 

 plethysmograph especially modified for use upon the kidney of the living 

 animal. It is a kidney-shaped box of thin brass made in two parts, biuged at 

 the back, and with a clasp in front to hold them together. In the interior of 

 the box thin peritoneal membrane is so fastened to each half that a layer of olive 

 oil may be placed between it ami the brass walls. There is thus formed in 

 each half a soft pad of oil upon which the kidney rests. When the kidney. 

 freed as far as possible from fat and surrounding connective tissue, but with 

 the blood-vessels and nerves entering at the hilus entirely uninjured, is laid in 



1 Skamdinavisekes Arehvufiir Physiologie, 1892, Bd. iv. 8. 241. 



2 See Cohnheim and Roy: Virchou/s Arehiv, 1883, Bd. scii. S. -1-4. 



