The pressure exerted by the blood in the glomerular capillaries is quite sufficient to make the urine flow from the pelvis of the kidneys into the bladder, because when the ureters are tied they become distended above the ligature by the urine flowing from the pelvis, where a pressure may be produced of some forty millimetres of Hg, at which pressure the secretion stops and becomes somewhat changed in chemical composition (kreatin appearing in greater quantity).
Normally, however, the passage of the urine along the ureters is accomplished by the peristaltic motion of the ducts, wTiich goes on alternately in the two ureters, so that the urine flows into the bladder at different periods from the right and left kidney.
The ureters have a strong middle coat of encircling fibres of smooth muscle, along which a wave of contraction, lasting about one-third of a second, passes rhythmically in about 6 to 10 seconds from the pelvis of the kidney to the bladder.
Having reached the bladder, the urine cannot return into the ureters on account of the oblique way in which these ducts pass through the walls of the bladder. When the pressure in the bladder increases, the opening of the ducts becomes closed and acts as a kind of valve.