172 ELEMENTARY PHYSIOLOGY 



part of the urine is separated in the capsules, while the solid portion is excreted 

 chiefly from the capillaries surrounding the tubes. 



The ureters are the two tubes which convey the fluid excreted 

 by the kidneys into the bladder. They are about fifteen inches in 

 length. The upper end of each, as we have seen, expands to 

 form the pelvis of the kidney ; the lower extremity passes obliquely 

 through the walls of the bladder at its base. 



The bladder is a very strong oval bag, about five inches long, 

 situated in the cavity of the pelvis. Its walls contain abundant 

 involuntary muscular fibres, and it is covered externally by a layer 

 of the peritoneum. It has three openings, two of which the 

 openings of the ureters have already been mentioned. The 

 third communicates with a duct called the Urethra, by which the 

 bladder is emptied of its contents. The neck of the bladder, that 

 is, the narrower portion which gives rise to the urethra, is sur- 

 rounded by a circular (sphincter) muscle which is generally in a 

 state of contraction, and thus the passage of liquid from the 

 bladder is prevented. But at intervals this muscle relaxes, while 

 the walls of the bladder contract, and consequently its contents 

 are discharged. 



The secretion of the urinary fluid by the kidneys is continuous, 

 and it passes into the bladder, drop by drop, through the ureters. 

 It will be seen, therefore, that the bladder simply serves to contain 

 the urine so that it may be discharged at intervals from the body. 

 No obstruction is offered to the fluid as it passes from the ureters 

 into the bladder, but it cannot pass backward again into these 

 vessels readily on account of their oblique openings ; and, further, 

 because of small elevations of the mucous membrane at these 

 openings which act as valves. 



The urine consists of urea and uric acid (both nitrogenous 

 substances), other salts, and certain gaseous substances, dissolved 

 in a large quantity of water. 



The quantity and composition of the urine vary considerably 

 with circumstances, but it is estimated that the kidneys of a 

 healthy man excrete about three pounds of water a day, in which 

 is dissolved a little more than one ounce of urea, and about ten 

 grains of uric acid. 



We must therefore look upon the kidneys as great sources of 

 loss to the blood, and in this respect they are like the lungs and 

 the skin. Of these three sources of loss the kidneys only excrete 

 large quantities of nitrogenous matter. The following table will 

 help us to compare the functions of these organs : 



Sources of Loss to the Blood. 



i The lime's I Much water - 



' 1 Much carbonic acid gas. 



