Hydronephrosis describes the situation where the urine collecting system of the kidney is dilated. This may be a normal variant or it may be due to an underlying illness or medical condition.
Normally, the kidney filters waste products from blood and disposes of it in the urine. The urine drains into individual calyces (single=calyx) that form the renal pelvis. This empties into the ureter, a tube that connects the kidney to the bladder. The urethra is the tube that empties the bladder.
While obstruction or blockage is the most frequent cause of hydronephrosis, it may be due to problems that occur congenitally in a fetus (prenatal) or may be a physiologic response to pregnancy. A large percentage of pregnant women develop hydronephrosis or hydroureter. Experts think this is, in part, because of the effects of progesterone on the ureters, which decreases their tone.
Technically, hydronephrosis specifically describes dilation and swelling of the kidney, while the term hydroureter is used to describe swelling of the ureter. Hydronephrosis may be unilateral involving just one kidney or bilateral involving both.
A complication of hydronephrosis that is not physiologic is decreased kidney function. The increased pressure of extra fluid within the kidney decreases the blood filtration rate and may cause structural damage to kidney cells. This decrease in function is often reversible if the underlying condition is corrected but if the duration is prolonged, the damage is often permanent.