4M A MANUAL OF PHYSIOLOGY 



general rise of pressure is not enough to make up for the 

 constriction of the renal vessels. 



Diuretics are substances that increase the flow of urine. Some of 

 them act mainly by increasing the general blood-pressure, others 

 mainly by a direct influence on the secreting mechanism. Digitalis 

 is a representative of the first class ; urea and caffein belong to 

 the second. The action of digitalis is to strengthen the beat of 

 the heart, which is at the same time somewhat slowed, and to con- 

 strict the arterioles. Both effects contribute to the increase of 

 pressure. It is possible that in addition this drug directly stimulates 

 the renal epithelium. Caffein, when injected into the blood, affects 

 the pressure but little. It causes dilatation of the renal vessels after 

 a passing constriction, and an increase in the flow of urine after a 

 temporary diminution. The vascular dilatation is not the chief 

 reason for the diuretic effect, for the latter is still obtained when the 

 vaso-motor mechanism has been paralyzed by chloral hydrate, and 

 even after the secretion of urine has been stopped by the fall of 

 pressure consequent on section of the spinal cord. Caffein, there- 

 fore, acts directly on the renal epithelium. The action of urea, 

 potassium nitrate, and the saline diuretics is probably also a direct 

 action on the secreting structures, although some have supposed that 

 their primary effect is to cause vaso-dilatation in the kidney, and a 

 consequent local increase in the capillary pressure. 



Summary. Our knowledge of renal secretion may be thus 

 summed up : The water and salts of the urine are chiefly separ- 

 ated by the glomeruli ; the process is not a mere physical filtration, 

 but a true secretion. Substances like sugar, peptone, egg-albumin, 

 and hemoglobin when injected into the blood are excreted by the 

 gloineruli : so probably is the sugar of diabetes. Urea, uric acid, 

 and presumably the other organic constituents of normal urine, 

 with a portion of the water and salts, are excreted by the physio- 

 logical activity of the ' rodded ' epithelium of the renal tubules. 

 The rate of secretion of urine rises and falls with the pressure, and 

 still more with the velocity, of the blood in the renal vessels. No 

 secretory nerves for the kidney have been definitely found ; the 

 effects of section or stimulation of nerves on the secretion can all 

 be explained by the changes produced in the renal blood-flow. 

 Some diuretics act by increasing the blood-flow, others directly on 

 the epithelium of the tubules. 



Micturition. The urine, like the bile, is being constantly 

 formed ; although secretion varies in its rate from time to 

 time, it never ceases. Trickling along the collecting tubules, 

 the urine reaches the pelvis of the kidney, from which it is 



