renal pelvis

The renal pelvis is part of the upper urinary collecting system, located within (or partially outside, depending on anatomic variant) the renal sinus at the hilum of the kidney. Urine formed in the nephrons drains through the collecting ducts into minor calyces, which merge into major calyces, which then converge into the renal pelvis. The renal pelvis narrows into the ureteropelvic junction (UPJ) before continuing as the ureter, which transports urine to the bladder.

Histologically, the renal pelvis is lined by transitional epithelium (urothelium), the same epithelium lining the ureters, bladder, and proximal urethra, making it susceptible to urothelial (transitional cell) carcinoma—clinically important because these cancers can be multifocal and require surveillance of the entire urothelial tract.

Clinical significance: - Ureteropelvic junction (UPJ) obstruction: a common congenital anomaly causing hydronephrosis in children, due to abnormal smooth muscle development or extrinsic compression (e.g., aberrant crossing vessel), leading to impaired urine drainage and pelvicaliceal dilation. - Nephrolithiasis: stones often form in the renal pelvis and can cause obstruction, presenting with colicky flank pain radiating to the groin, hematuria, and hydronephrosis. - Pyelonephritis: ascending infection from the bladder can involve the renal pelvis (pyelitis) and renal parenchyma, presenting with fever, flank pain, and costovertebral angle tenderness. - Hydronephrosis: any obstruction distal to the renal pelvis (stones, tumors, BPH, pregnancy) leads to dilation of the pelvis and calyces, visualized on ultrasound or CT. - Transitional cell carcinoma of the renal pelvis: presents with painless hematuria; associated with smoking, analgesic abuse (phenacetin), and occupational exposures (aromatic amines).

Imaging: The renal pelvis is well visualized on CT urography, retrograde pyelography, and ultrasound, which are used to assess for obstruction, stones, or filling defects (tumors).