Renal Pelvis

1. Overview

The renal pelvis is a funnel-shaped structure that collects urine from the kidney's calyces and channels it into the ureter. It acts as a transitional reservoir between the internal urine-collecting structures (minor and major calyces) and the ureter, facilitating efficient urine outflow. Although passive in function, its anatomical integrity is essential for uninterrupted drainage and prevention of backflow or obstruction.

2. Location

The renal pelvis is located in the renal sinus, a hollow space at the medial border of the kidney. Specific positional landmarks include:

• It lies posterior to the renal vessels, within the renal hilum.

• Superiorly: Continuous with the major calyces.

• Inferiorly: Narrows to form the ureteropelvic junction (UPJ), which marks the start of the ureter.

It is typically flattened in coronal view and varies in shape from person to person—some have an intra-renal pelvis (entirely within the kidney), while others have an extra-renal pelvis that protrudes beyond the renal hilum.

3. Structure

The renal pelvis is composed of:

• Mucosal lining: Transitional epithelium (urothelium), specialized for distension and impermeability to urine.

• Muscular wall: Thin smooth muscle layers (longitudinal and circular) to support minor peristalsis.

• Connective tissue adventitia: Anchors the pelvis within the renal sinus and supports neurovascular supply.

Urine flows from the minor calyces → major calyces → renal pelvis → ureter.

4. Function

The renal pelvis performs the following functions:

• Urine collection: Gathers urine from multiple major calyces.

• Urine funneling: Narrows at the UPJ to direct urine smoothly into the ureter.

• Passive conduit: While not highly contractile, it contributes to the unidirectional flow of urine under low pressure.

5. Physiological Role(s)

The renal pelvis contributes to overall urinary physiology in several indirect yet vital ways:

• Prevents backflow: Functions with peristaltic ureteral movements and gravity to minimize urine reflux.

• Hydrodynamic transition zone: Acts as a low-pressure reservoir where collecting duct flow meets ureteral peristalsis.

• Adaptability: Its distensible urothelium accommodates varying volumes of urine with minimal wall tension.

6. Clinical Significance

Hydronephrosis

Dilation of the renal pelvis due to obstruction of urine outflow (e.g., at the ureteropelvic junction) results in hydronephrosis. This can cause:

• Flank pain

• Progressive kidney damage

• Urinary tract infections

Causes include ureteral stones, strictures, congenital UPJ obstruction, or external compression. Diagnosed by ultrasound or CT.

Ureteropelvic Junction (UPJ) Obstruction

This condition involves narrowing or blockage at the junction of the renal pelvis and ureter. It may be congenital or acquired, leading to poor drainage and hydronephrosis. Surgical correction may be required (e.g., pyeloplasty).

Renal Pelvis Calculi

Stones can form or lodge in the renal pelvis, causing pain, obstruction, or hematuria. These are often visible on:

• Non-contrast CT

• Intravenous urography

Management may include lithotripsy, ureteroscopy, or surgical removal depending on size and symptoms.

Transitional Cell Carcinoma (TCC)

The renal pelvis is lined with urothelium, the same as the bladder and ureters. It can be the site of urothelial carcinoma, especially in smokers or those exposed to aromatic amines. Symptoms include hematuria and flank pain. Diagnosis requires imaging and often ureteroscopy with biopsy.

Imaging Significance

The renal pelvis is best visualized using:

• Ultrasound: For detecting hydronephrosis and stones.

• CT Urography: For detailed anatomical and pathological assessment.

• IV Urograms: To observe urine flow and detect filling defects (e.g., tumors, stones).

Surgical Access

During percutaneous nephrolithotomy (PCNL) or pyeloplasty, direct access to the renal pelvis is essential. Knowledge of its shape, size, and relation to calyces and vessels is crucial for safe and effective surgical planning.