Obstruction of the pyeloureteric junction prevents urine flowing from the kidney into the bladder.
When the ureter is blocked, this obstruction may be physical,
The obstruction however may also be functional, when the ureter is unable to carry urine from the kidney to the bladder. The main cause of functional obstruction of the pyeloureteric junction is the lack of smooth muscle. Urine transfer from the kidney to the bladder is not a passive phenomenon but is a dynamic process which depends on the presence of smooth muscles along the ureter. These muscles generate a peristaltic movement which propels the urine.
The urinary system consists of the kidney, ureter, bladder and urethra. DR Credits.