366 HAND-BOOK OF PHYSIOLOGY. 



The greater the blood-pressure in the arterial system generally, and 

 consequently in the renal arteries, the greater, cceteris paribus, will be 

 the blood-pressure in the glomeruli, and the greater the quantity of urine 

 separated; but even without increase of the general blood-pressure, if the 

 renal arteries be locally dilated, the pressure in the glomeruli will be 

 increased and with it the secretion of urine. On the other hand, if the 

 local blood-pressure be diminished, the amount of fluid will be lessened. 

 All the numerous causes, therefore, which increase the blood-pressure (p. 

 152) will, as a rule, secondarily increase the secretion of urine. Of these 

 the heart's action is amongst the most important. When its contractions 

 are increased in force, increased diuresis is the result. Similarly, causes 

 which lower the blood-pressure, e.g., enfeebled action of the heart, great 

 loss of blood, etc., will diminish the activity of the secretion of urine. 



The close connection between the blood-pressure generally and the 

 nervous system has been before considered, and it will be clear, therefore, 

 that the amount of urine secreted depends greatly upon the influence of 

 the nervous system. Thus, division of the spinal cord, by producing 

 general vascular dilatation, causes a great diminution of blood-pressure, 

 and so diminishes the amount of water passed; since the local dilatation 

 in the renal arteries is not sufficient to counteract the general diminution 

 of pressure. Stimulation of the cut cord produces, strangely enough, 

 the same results i.e., a diminution in the amount of the urine passed, 

 but in a different way, viz., by constricting the arteries generally, and, 

 among others, the renal arteries; the diminution of blood-pressure result- 

 ing from the local resistance in the renal arteries being more potent to 

 diminish blood-pressure in the glomeruli than the general increase of 

 blood-pressure is to increase it. Section of the renal nerves or of any 

 others which produce local dilatation without greatly diminishing the 

 general blood-pressure will cause an increase in the quantity of fluid 



The fact that in summer or in hot weather the urine is diminished 

 may be attributed partly to the copious elimination of water by the skin 

 in the form of sweat which occurs in summer, as contrasted with the 

 greatly diminished functional activity of the skin in winter, but also to 

 the dilated condition of the vessels of the skin causing a decrease in the 

 general blood-pressure. Thus we see that in regard to the elimination 

 of water from the system, the skin and kidneys perform similar functions, 

 and are capable to some extent of acting vicariously, one for the other. 

 Their relative activities are inversely proportional to each other. 



The intimate connection between the condition of the kidney and the 

 blood-pressure has been exceedingly well shown by the introduction of an 

 instrument called the Oncometer, recently introduced by Roy, which is a 

 modification of the plethysmograph (Fig. 138). By means of this appa- 

 ratus any alteration in the volume of the kidney is communicated to an 



