EXCRETION 273 



takes more of the nature of ordinary endothelium, such 

 as lines a lymph space, and, when irritated, responds like 

 the latter by the production of connective tissue. 



The volume of urine produced by the kidney depends 

 upon the amount of blood passing through it in a given 

 time, and the blood-pressure in the renal capillaries. 

 Care must be taken to distinguish between a high general 

 blood-pressure and a high renal pressure, for the latter 

 may occur independently of the former. If, for example, 

 the cutaneous vessels become contracted from exposure 

 to cold, the vessels of the abdominal organs, including 

 those of the kidney, become overfilled, and the renal 

 pressure rises, with the result that more water is excreted 

 in the urine ; and yet this may happen without there 

 being any rise of general blood-pressure. All that has 

 taken place has been a redistribution of blood in the 

 body (see p. 167). It is in this way that exposure to cold 

 increases the volume of the urine, whilst warmth, hot-air 

 baths, and a mild climate all of which tend to ' de- 

 termine ' blood to the surface lessen the pressure in 

 the kidney, and are therefore useful in nephritis and 

 renal congestion. If, again, the control exercised by 

 the vasomotor nerves over the renal bloodvessels is 

 relaxed, more blood goes to the kidney, the pressure in 

 the glomeruli rises, and more water is excreted. This is 

 the explanation of the polyuria of nervousness and 

 hysteria. On the other hand, constriction of the renal 

 vessels, with consequent diminution of the urine, occurs 

 in asphyxia and in the convulsions of epilepsy and 

 strychnine-poisoning. 



In all these cases we are dealing simply with an altera- 



18 



