482 A TEXTBOOK OF PHYSIOLOGY 



and 30 grammes <>f urea may be secreted per diem. To effect this 

 by resorption, 60 litres would have to be concentrated to 2 litres. 

 Such an active resorption is possible for the amount of blood flowing 

 through the kidneys is very large. It has been estimated at 300 to 

 600 litres, and even at 1,800 litres, per diem an amount ample 

 enough to allow resorption to play an active part. 



Intravenous injection of concentrated salt or sugar solution pro- 

 duces diuresis both by exciting the renal cells and by making the blood 

 more watery. The water is drawn into the blood from the tissues, 

 and the concentration of the blood thus rapidly brought back to 

 normal. The diuresis is not large because the body holds to its 

 water. The intravenous injection of isotonic and hypotonic solutions 

 both excites the renal cells and accelerates the blood-flow through 

 the kidney; the water thus introduced produces much diuresis. Urea 

 acts as a powerful diuretic, and causes vaso-dilatation of the kidney. 

 Caffeine likewise, but this acts when the renal vaso-motor nerves are 

 destroyed. Caffeine causes little diuresis in thirsty animals. 



To sum up, the balance of evidence at present available seems to 

 indicate that the water and salt content (particularly chloride) of the 

 urine are secreted by the action of the glomerular cells, and that the 

 organic constituents of the urine are added by the cell activity of the 

 tubules. The evidence in favour of a concentrating mechanism in 

 the tubules is slight, but it is most probable that the urine is the 

 product of the give and take of the renal cells, bathed as they are 

 by the contents of the tubule on one side, and by the lymph which 

 percolates from the capillaries on the other. There are diuretic 

 substances in the blood which stimulate the kidney to secrete, e.g., 

 urea; and the secretory activity depends on the amount of these 

 substances that is, on their concentration and on the volume of the 

 blood passing through the kidneys per diem. The above view is 

 strengthened by the fact that developmentally the kidney has a 

 double organ a secretory and an excretory part. 



The Passage of Urine along the Ureters. The urine collects in the 

 pelvis of the kidney, and passes thence down the ureters to the 

 bladder. The ureters are smooth-muscle tube lined by transitional 

 epithelium. The muscle is arranged in a circular outer and a longi- 

 tudinal inner layer. It is probable that ganglion cells are present 

 between the muscular layers throughout the entire length, but they 

 are particularly abundant in the upper and lower thirds. 



Under the influence of the secretory pressure, and in the erect 

 man under the influence of gravity, the urine passes into the begin- 

 nings of the ureter, which then by peristaltic movements passes the 

 urine down into the bladder. These peristaltic movements occur 

 regularlj- about every ten to twenty seconds, being more frequent 

 the greater the amount of urine, but the presence of urine in the ureter 

 does not seem to be necessary to evoke them. They proceed over 

 the ureter at a rate of about 20 to 30 millimetres per second. 



There is some doubt as to the exact nature of these movements. 

 It was held that they were myogenic in origin, because the middle 





