204 Life and Health 



v 310. How the Action of the Kidneys may be modified. 

 In the kidneys, as elsewhere, the vaso-motor nerves are dis- 

 tributed to the walls of the blood vessels and modify the quan- 

 tity and the pressure of blood in these organs. Thus some 

 strong emotion, like fear or undue anxiety, increases the blood 

 pressure, drives more blood to the kidneys, and causes a larger 

 flow of watery secretion. 



In warm weather there is a relaxation of the vessels of the 

 skin, with a more than ordinary flow of blood, together with 

 active perspiration. The blood pressure in the kidneys is not 

 only diminished, but the total quantity passing through them 

 in a given time is much lessened. As a result, the secretion 

 of the kidneys is scanty, but it contains an unusual percent- 

 age of solids. 



In cold weather the cutaneous vessels contract, the blood is 

 driven to the deeper organs with increased pressure, and there 

 is a less amount of sweat, but an increased renal secretion, 

 containing a smaller proportion of solids. Thus the skin and 

 the kidneys work in harmony in their efforts to rid the body 

 of its superfluous water. 



The Renal Secretion. The function of the kidneys is to secrete a fluid 

 commonly known as the urine. The average quantity passed in twenty-four 

 hours by an adult is about three pints. Normal urine consists of about 96 

 per cent of water and four per cent of solids. The latter consist chiefly of 

 certain nitrogenous substances known as urea and uric acid, a considerable 

 quantity of mineral salts, and some coloring matter. Urea, the most impor- 

 tant and most abundant constituent of urine, contains the four elements, 

 carbon, hydrogen, oxygen, and nitrogen, but nitrogen forms one-half its 

 weight. While, therefore, the lungs expel carbon dioxide chiefly, the kid- 

 neys expel urea and other nitrogenous wastes. All these substances express 

 the result of oxidations going on in the body. The urea and uric acids 

 represent the final result of the breaking down in the body of nitrogenous 

 substances, of which albumin is the type. 



The Bladder. The bladder is an oval bag situated in the lowest part of 

 the abdome'n. When full, the bladder is pear-shaped ; when empty, it is 

 collapsed and lies low in the pelvis. The bladder collects and retains 

 the urine, which has reached it drop by drop from the kidneys through the 

 ureters, until a certain quantity accumulates. Owing to the distention of 

 the bladder, a feeling of uneasiness arises which compels the expulsion of 

 its contents. 



