The kidneys are a pair of reddish-brown, bean-shaped organs that filter waste products and excess fluid from the blood to produce urine. Structurally, these organs are divided into two main regions: the superior pole (upper end) and the inferior pole, known as the lower pole. Understanding the lower pole is important because its unique physical characteristics influence both the natural passage of urine and the approach to various medical interventions. This article defines the lower pole and explains its relevance in clinical medicine.
Anatomical Definition and Placement
The kidney is positioned in the retroperitoneal space behind the abdominal lining (peritoneum), situated on either side of the spine between the twelfth thoracic and third lumbar vertebrae. The lower pole represents the rounded, tapered bottom end of the organ. The upper poles are typically oriented slightly more inward and toward the back, while the lower poles point slightly more outward and toward the front.
The right kidney tends to sit marginally lower than the left one, due to the large size of the liver resting above it. Unlike the upper pole, which is immediately adjacent to the adrenal (suprarenal) gland, the lower pole rests against muscles of the posterior abdominal wall, such as the psoas and quadratus lumborum. This anatomical positioning, particularly its outward and downward slope, influences how substances move through and out of the kidney’s internal collecting system.
Lower Pole’s Significance in Kidney Stone Management
The lower pole is the most frequent location for kidney stones to settle. The collecting system within the kidney consists of minor calyces, which collect urine and merge to form major calyces, leading to the renal pelvis. In the lower pole, the calyces often have a downward orientation, which works against the natural clearance of stones.
The angle between the lower pole calyx and the renal pelvis, known as the infundibulopelvic angle, is often acute or narrow. This tight angle, combined with gravity, makes it difficult for stone fragments to ascend out of the collecting cup and into the ureter. This anatomical challenge directly impacts treatment efficacy for stones in this location.
Extracorporeal Shock Wave Lithotripsy (ESWL), which breaks stones into small fragments, has a significantly lower success rate for clearing fragments from the lower pole compared to other kidney areas. For larger lower pole stones, typically exceeding two centimeters, percutaneous nephrolithotomy (PCNL) is often the preferred treatment, as it offers a much higher stone-free rate. The anatomy of this region dictates the choice of procedure to ensure complete stone removal.
Other Common Clinical Findings
Beyond kidney stones, the lower pole is a common site for other clinical findings. The most common are simple renal cysts, which are fluid-filled sacs originating from the kidney’s surface. These cysts are prevalent, with up to half of people over 50 having at least one, and they are usually benign and asymptomatic.
Simple cysts found in the lower pole rarely require intervention unless they become infected or grow large enough to cause pain or obstruct urine flow. Imaging studies may also incidentally reveal solid masses or tumors in this area. These masses require careful evaluation, often using the Bosniak classification system, to determine if they are benign growths, such as angiomyolipomas, or potentially malignant tumors.
Surgical Access and Procedures
The lower pole is utilized as the entry point for minimally invasive surgical and diagnostic procedures. For procedures like percutaneous nephrolithotomy (PCNL), which involves accessing the kidney through a small incision, the lower pole is the favored access site. This preference is due to safety considerations related to surrounding structures.
Accessing the upper pole carries a higher risk of injuring the pleura, which can lead to a collapsed lung. By targeting the lower pole, surgeons can typically gain subcostal access, meaning below the ribs, minimizing the risk of thoracic complications. The lower pole is also the standard location for performing a percutaneous renal biopsy. Biopsies are preferentially performed here to avoid major blood vessels located near the center of the kidney.