Urinary tract obstruction and hydronephrosis
- Lesion
A block anywhere from the calyces to the urethra dilates the system above it and slowly destroys the kidney if not relieved.
What it is
Obstruction raises pressure and stasis upstream. Hydronephrosis is dilation of the pelvis and calyces with atrophy of the parenchyma. Filtration continues for a while, so urine backs into the interstitium and lymphatics; the papillae are compressed and the medulla is damaged first, which is why concentrating ability is lost early. Stasis favours infection and stones. Relief early allows recovery; prolonged complete obstruction causes irreversible atrophy.
Structure
- Early: dilated pelvis and calyces, blunted papillae, swollen kidney.
- Late: thin rim of cortex around a sac of urine; tubular atrophy, interstitial fibrosis, glomeruli relatively preserved until late.
- Superimposed acute pyelonephritis is common.
Step by step
- A block raises pressure in the pelvis and tubules.
- Filtration continues; filtrate leaks into the interstitium and returns via lymph and veins.
- Pressure compresses medullary vessels, so the papillae and medulla become ischemic.
- Tubules lose concentrating ability and then atrophy, with interstitial inflammation and fibrosis.
- GFR falls; if prolonged, the cortex thins to a rim.
- Stasis invites infection and stones, which worsen the obstruction.
Clinical notes
- Acute obstruction hurts (renal colic, distended bladder); chronic partial obstruction may be silent until kidney function falls.
- In children, UPJ obstruction and posterior urethral valves are leading causes; in older men, prostatic enlargement; in women, cervical cancer and pregnancy.
Test yourself
Think of the answer, then tap the question to see it.
Why is the ability to concentrate urine lost early in obstruction?
Rising pressure compresses the medullary vasculature and papillae first, injuring the concentrating segments.
What usually causes bilateral hydronephrosis in an older man?
Benign prostatic hyperplasia obstructing the bladder outlet.
Why can partial obstruction cause polyuria?
Damaged tubules cannot concentrate urine, so large volumes of dilute urine are passed despite reduced GFR.
Elsewhere in DurgiPedia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Obstructive uropathy: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.