528 THE HUMAN BODY 



the reniportal vein the tubules of the cortex, though mixed there 

 with blood from the efferent vessels of the glomeruli. On tying 

 the renal arteries of one of these animals urinary secretion ceases, 

 there being then no blood-pressure in the glomeruli to cause the 

 transudation of liquid; but if some urea be now injected into the 

 blood the epithelial cells of the tubules are stimulated to secrete, 

 and urine rich in urea is formed ; but in these circumstances it can- 

 not come from the Malpighian bodies. It would seem then that 

 urea is a special stimulant to some cells of the tubules, and that an 

 excess of it in the blood can stir them up to its elimination along 

 with some water, quite independently of any formation of trans- 

 udation urine. 



The Relation of Renal Blood-Flow to the Secretion of Urine. 

 The kidneys have probably a richer blood supply than any other 

 organs of the Body. It has been estimated that under proper 

 circumstances their own weight of blood may flow through them 

 each minute. This rich blood supply is, of course, an adaptation 

 to secure the withdrawal of waste substances from the blood at a 

 rapid rate. From the structure of the glomeruli and the fact that 

 most of the water of the urine is derived from them it is a priori 

 probable that anything tending to increase the pressure of blood 

 in them will increase the bulk of urine secreted, and anything 

 diminishing that pressure will decrease the urine. The structure 

 of the glomeruli themselves is such that the pressure of blood with 

 them tends to be higher than in the capillaries in general. Refer- 

 ence to Figure 141 shows that the vessel which drains the glom- 

 erulus, the efferent vessel e, is smaller than the afferent vessel a. 

 This means that there is a resistance to the outflow from the capsule 

 greater than that at the point of entrance. According to the rela- 

 tion between pressure and resistance (p. 364) there must be a cor- 

 respondingly greater pressure in the glomeruli than in the other 

 capillaries whose outlet is not similarly restricted. This high 

 glomerular pressure favors filtration. Experiment shows, more- 

 over, that the vigor of urine formation depends on the pressure 'of 

 blood within the capsule. The kidney is supplied with both vaso- 

 constrictor and vasodilator nerves which reach it mainly through 

 the solar plexus. When the spinal cord is cut in the neck region 

 of a dog the kidney vessels as well as those of the rest of its Body 

 dilate and blood-pressure everywhere is very low. Under these 



