Obstruction presents two types of problems. First, it can cause the urinary tract to become filled up with an excess amount of fluid. Second, when urine does not flow normally, kidney filtering functions are compromised and the kidney begins to lose its ability to process the waste material in the blood. This is not an immediate event but occurs gradually, over several weeks. Problems with the kidneys and the urinary tract can generate a great deal of pain.
Obstruction causes pain by filling the tubes of the urinary tract with too much fluid.
Pressure on the walls of the renal pelvis or the ureter cause a classic form of aching and relentless pain called colic. Patients with kidney stones or some other reason for rapid obstruction of the ureters or other parts of the urinary tract in the upper abdomen can experience a tremendous amount of discomfort that is controlled only with pain medication. Women who have passed kidney stones and who have also given birth without the benefit of anesthesia will often remark that passage of a kidney stone is more painful than having a baby. In contrast to rapid obstruction of the urinary tract, gradual obstruction rarely causes pain. Tumors that slowly obstruct the urinary tract, particularly one of the ureters, can do so without ever causing pain, because the obstruction occurs so gradually.