358 PRINCIPLES OF GENERAL PHYSIOLOGY 



Since the rate of filtration in the glomerulus depends on the difference between 

 the blood pressure in it and that in the tubules, it is clear that any process increasing 

 the difference will increase the rate of flow. Rise of general blood pressure, produced 

 by means to be described in Chapter XXIII., is one of these. Dilatation of the 

 arterioles of the kidney on the heart side of the glomeruli themselves is another 

 means, and, clearly, a combination of the two would be most effective. Conversely, 

 a diminution of general blood pressure, or a constriction of renal arterioles, 

 decreases the rate of flow. The kidney is, in fact, copiously supplied with vaso- 

 constrictor nerves, and to some extent with vaso-dilator nerves, so that the 

 requisite mechanism is not wanting. 



We have seen further that the cells of the tubules intervene by active processes 

 requiring the consumption of energy, so that it does not seem improbable that 

 secretory nerves may exist, similar to those of the salivary or sweat glands. 

 Histologists have described nerve fibres ending in the cells of the tubules 

 especially the work of Smirnov, 1901, one of whose figures is reproduced in 

 Fig. 474 on p. 374 of Schafer's " Essentials of Histology "). 



Certain experimental evidence has been brought forward by Rohde and Ellinger (1913) that 

 the splanchnic nerve contains fibres which inhibit the activity of the tubule cells. The chief 

 fact in support of this view seems to be that the diuretic effects of section of the renal nerves. 

 due in the first place to removal of tonic vaso-constrictor impulses, lasts for several months, 

 by which time it is supposed that the renal arterioles have recovered from the immediate 

 effect of the section. It is to be remembered that vaso-constrictor reflexes are probably hrini; 

 sent to the intact gland during the time of observation, which are the cause of a diminished 

 secretion on this side; on the side of the section, of course, they would be absent. Si me 

 other evidence, with regard to the solid constituents of the urine, seems to me to be explicable 

 by the vaso -motor change, without the necessity of assuming secretory nerves. It must be 

 confessed, however, that there are many difficulties in the way of deciding the question. 

 Asher and Pearce (1913) believe that they have evidence that there are secretory nerves to the 

 kidney contained in the vagus nerve, but the evidence that all vaso-motor action was excluded 

 is not altogether satisfactory. Some observers had previously stated that this nerve contains 

 inhibitory fibres for the secretion of urine (see Bradford, 1889, p. 395). Bradford himself \v a^ 

 unable to find any vaso-motor fibres in the nerve. 



An interesting morphological point was made out by Bradford (1889) in his 

 investigation of the nerve roots by which the renal nerves leave the spinal cord. 

 The area is a very extensive one, from the 4th thoracic to the 4th lumbar, although 

 the largest number are contained in the llth, 12th, and 13th thoracic. This 

 long area is of interest in connection with the ancestral origin of the kidney from 

 a series of segmental organs extending over a considerable number of segments. 



Diuretics. All substances, such as salts, sugar, etc., which raise the osmotic 

 pressure of the blood, bring about the passage of water from the tissues into the 

 blood and thus decrease the osmotic pressure of the colloids of the blood. The 

 pressure necessary to separate the glomerular filtrate is thus reduced, or, if it 

 remains constant, the rate of filtration is increased. In addition to this effect, a 

 salt foreign to the organism, such as sodium sulphate, excites an active process in 

 the tubules. This is shown by the experiments of Barcroft and Straub (1910), in 

 which injections of isotonic sodium chloride produced, by mere dilution of the 

 blood, a diuresis unaccompanied by any increase of oxygen consumption. The 

 diuresis produced by sodium sulphate, on the contrary, showed a considerable 

 extra consumption of oxygen. 



Urea causes diuresis by dilatation of the renal arterioles, without any consider- 

 able effect on the general blood pressure. The diuretic effect of glucose lasts 

 longer than its effect on the concentration of the blood plasma (" hydrsemic 

 plethora"), so that it seems to bring about a local dilatation of the kidney 

 arterioles, in addition to its dilution effect. 



It has been shown by Cushny (1902) that if the increased blood flow through 

 the kidney, produced by injection of 3 per cent, sodium chloride, be brought back 

 to its initial rate by an adjustable clamp on the renal artery, the diuresis ceases ; 

 so that the vascular change is the responsible factor and no specific action on the 

 cells is present. 



Certain evidence indicates that such specific diuretics as the purine derivatives, 



