The urinary system is similar to a plumbing system and it functions to get urine that is produced by the kidneys out of the body. Urine that is made by the kidneys collects in the renal pelvis prior to draining through a tube called ureter into the bladder. Urine is stored in the bladder until you are ready to go to the bathroom at which time it exits through the urethra.
If a drainage pipe in your house gets clogged, then water will back up into your house. Similarly, if your urinary system gets blocked off, urine will back up. This can occur at any point from the kidney to the end of the urethra:
Ureteropelvic junction obstruction (UPJ obstruction) is a blockage between the renal pelvis and the ureter.
Ureteral obstruction is a blockage at any point along the ureter.
Ureterovesical obstruction (UVJ obstruction) is a blockage between the ureter and the bladder.
Bladder outlet obstruction is an obstruction that keeps urine from flowing out of the bladder.
A urethral stricture is a blockage in urine flow through the urethra.
Any of the blockages can cause problems including kidney failure, urinary tract infections, blood in the urine, pain, stone formation, and others.