144 FROM FISH TO PHILOSOPHER 



pressure of this fluid and thereby draws water out of the 

 urine as the latter passes down the collecting duct. Addi- 

 tional sodium chloride is reabsorbed by the distal con- 

 voluted tubule and the collecting duct. Li the presence 

 of the antidiurectic hormone (ADH) of the pituitary 

 gland all the osmotically free water generated in the 

 tubule by the reabsorption of sodium chloride in the thin 

 segment, distal convoluted tubule and collecting duct 

 escapes by diffusion across the tubular epithehum, so 

 that the urine emerges from the collecting duct with an 

 osmotic pressure equal to that of the interstitial fluid 

 around the loop of Henle. Hence under conditions of 

 dehydration (antidiuresis) the urine flow is minimal 

 (oliguria) and its osmotic concentration maximal. 



In the absence of ADH {i.e., during hydration), the 

 permeability of the distal tubule and the collecting duct 

 to water is greatly decreased, and consequently osmoti- 

 cally free water remains unabsorbed and is excreted to 

 form a dilute urine (water diuresis). This theory has 

 two attractive features: it disposes of any ^active' trans- 

 port of water molecules by the tubule, and quaUtatively 

 the same mechanism (sodium chloride reabsorption) is 

 involved in making both a concentrated and a dilute 

 urine. In discussing the absorption of water by the skin 

 in the Amphibia, it was noted that the action of pituitary 

 extracts has been attributed to the dilatation of Spores' 

 through which water can diffuse: similarly, the dilata- 

 tion of pores' in the distal tubule and collecting duct 

 seems adequate to explain the increased reabsorption of 

 water in the mammalian nephron under the action of 

 ADH. 



As one ingests greater or lesser quantities of water, 

 minute changes in the osmotic pressure of the blood 

 work through the osmotically sensitive receptors in the 

 midbrain, the pituitary gland, and ADH secretion, to in- 

 crease or decrease the excretion of water as required, the 

 system operating so smoothly that the osmotic pressure 

 of the blood generally varies by no more than i or 2 per 



