[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [General pathology](https://durgical.com/en/general-pathology/) › [Kidney](https://durgical.com/en/general-pathology/areas/kidney/) › Urinary tract obstruction and hydronephrosis

# 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.

Updated Nov 12, 2026

## Key points

- Unilateral: usually above the bladder (stone, UPJ obstruction, tumour); bilateral: usually below it (prostate, urethral valves, bladder neck).
- Common causes: stones, BPH, tumours (prostate, bladder, cervix), pregnancy, strictures, neurogenic bladder, congenital anomalies.
- Earliest damage: medulla and papillae; loss of concentrating ability.
- Complete bilateral obstruction gives anuria; partial obstruction can give polyuria.
- Post-obstructive diuresis after relief can cause salt and water loss.

## 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.

## Memory aid

One side: think above the bladder; both sides: look below it.

## 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.

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## Elsewhere in DurgiPedia

- [Atrophy](https://durgical.com/en/general-pathology/atrophy/)
- [Duplicated ureter, UPJ obstruction and posterior urethral valves](https://durgical.com/en/embryology/duplicated-ureter-upj-obstruction-and-posterior-urethral-valves/)

## Connections

## Related 5

- [Kidney stones](https://durgical.com/en/general-pathology/kidney-stones/)
- [Ureteral and bladder outlet obstruction](https://durgical.com/en/general-pathology/ureteral-and-bladder-outlet-obstruction/)
- [Benign prostatic hyperplasia](https://durgical.com/en/general-pathology/benign-prostatic-hyperplasia/)
- [Chronic pyelonephritis and reflux nephropathy](https://durgical.com/en/general-pathology/chronic-pyelonephritis-and-reflux-nephropathy/)
- [Atrophy](https://durgical.com/en/general-pathology/atrophy/)

## Related conditions 8

- [Hydronephrosis and obstructive uropathy](https://durgical.com/en/conditions/hydronephrosis-and-obstructive-uropathy/)
- [Benign prostatic hyperplasia](https://durgical.com/en/conditions/benign-prostatic-hyperplasia/)
- [Posterior urethral valves](https://durgical.com/en/conditions/posterior-urethral-valves/)
- [Kidney stones (urolithiasis)](https://durgical.com/en/conditions/kidney-stones/)
- [Acute urinary retention](https://durgical.com/en/conditions/acute-urinary-retention/)
- [Neurogenic bladder](https://durgical.com/en/conditions/neurogenic-bladder/)
- [Cervical cancer](https://durgical.com/en/conditions/cervical-cancer/)
- [Prostate cancer](https://durgical.com/en/conditions/prostate-cancer/)

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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](https://medlineplus.gov/ency/article/000507.htm) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/urinary-tract-obstruction-and-hydronephrosis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/urinary-tract-obstruction-and-hydronephrosis/