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PH-RN05

Stones and urinary obstruction

How crystals form stones when urine becomes too concentrated in stone-forming salts, why a stone moving down the ureter causes intense colicky pain, and how blocked urine flow swells the kidney, reduces its function and invites infection.

By the end, you can

  • Trace the path of urine from the renal pelvis to the bladder and name the ureter’s narrowest points.
  • Explain how supersaturation, low inhibitors and urine pH lead to stone formation, and name the main stone types.
  • Explain why a ureteric stone causes colicky loin-to-groin pain and blood in the urine.
  • Explain how obstruction causes hydronephrosis and falling kidney function.
  • Explain why an infected, obstructed kidney is an emergency.
  • Recognise the appearance of hydronephrosis and of stones in the urinary tract, classify obstruction by level, speed and completeness, and explain how analgesia, medical expulsive therapy, drainage by stent or nephrostomy, lithotripsy and ureteroscopy act on the mechanism.

What it is

Urinary stones (urolithiasis) are hard crystal masses that form in the urinary tract, usually in the kidney. Urinary obstruction is blockage of urine flow anywhere from the renal pelvis to the urethra, by a stone or another cause such as an enlarged prostate or a tumour. Obstruction that dilates the renal pelvis and calyces is hydronephrosis.

3 types to tell apart
Calcium stones
Calcium oxalate (the majority) and calcium phosphate; linked to high urinary calcium or oxalate and low citrate.
Uric acid stones
Form in persistently acidic urine, especially with gout or high cell turnover; not visible on plain X-ray.
Struvite and cystine stones
Struvite forms with urease-producing bacteria and can fill the pelvis (staghorn); cystine stones are from an inherited transport defect.

US spelling: tumour → tumor.

Normal function

Urine flows from kidney to bladder

Urine made by the nephrons drains into the calyces and the , then down the to the . Waves of smooth muscle contraction (peristalsis) push it along, a few each minute.

The ureter is narrow, a few millimetres across, and narrowest at three points: where the pelvis joins the ureter, where the ureter crosses the iliac vessels at the pelvic brim, and where it enters the bladder wall obliquely, which also acts as a valve against reflux.

Urine normally holds calcium, oxalate, phosphate and urate in solution, helped by its volume and by inhibitors of crystal growth such as citrate and magnesium.

Point to it on the model