The kidneys, a pair of bean-shaped organs, filter blood and produce urine to eliminate waste products. Located on either side of the spine, just below the rib cage, these organs are approximately 10-12 cm long, 5-7 cm wide, and 3-5 cm thick in adults. Each kidney contains a hollow chamber that collects and channels filtered fluid. Understanding this internal anatomy helps explain how kidneys maintain overall bodily health and fluid balance.
Identifying the Renal Pelvis
This hollow chamber within the kidney is known as the renal pelvis. It is a funnel-shaped structure located at the hilum, the concave medial indentation of the kidney where blood vessels, nerves, and the ureter enter and exit. The renal pelvis is an expanded upper end of the ureter, the tube that carries urine from the kidney to the bladder.
The renal pelvis is often described as a curved funnel, almost entirely enclosed within the kidney’s sinus, which is the deep indentation at the hilum. Its walls are lined with transitional epithelium (urothelium), a specialized tissue that can stretch and accommodate varying volumes of urine. This lining also contains connective tissue and smooth muscle, contributing to its structure and function.
Role in Urine Collection
The primary function of the renal pelvis is to gather urine produced by the kidney’s filtering units, the nephrons. After blood is filtered in the glomeruli and processed through the renal tubules, the resulting urine flows into collecting ducts. These collecting ducts drain into small, cup-shaped minor calyces, which are positioned around the tips of the renal pyramids.
Several minor calyces converge to form major calyces, typically two or three per kidney. These major calyces merge to form the single, central renal pelvis. Once urine collects, its smooth muscle walls begin a series of rhythmic, wave-like contractions called peristalsis. These contractions propel urine downward, ensuring efficient movement from the kidney into the ureter and toward the bladder.
Integration within Kidney Anatomy
The renal pelvis is centrally positioned within the kidney’s internal architecture, residing in the renal sinus, which is the cavity at the hilum. It serves as the convergence point for the major calyces, which collect urine from the minor calyces. This arrangement creates a continuous drainage system, guiding urine from the kidney’s filtering regions.
The renal pelvis directly connects to the ureter, forming the ureteropelvic junction (UPJ), which marks the beginning of the ureter’s path out of the kidney. While distinct from the renal medulla and cortex, it receives urine that has passed through these outer and inner regions of the kidney. The relationship between the renal pelvis, calyces, and ureter forms a continuous pathway, ensuring the outflow of urine from the kidney.
Impact of Obstruction
When the normal flow of urine through the renal pelvis is disrupted, it can lead to hydronephrosis, characterized by swelling of the kidney due to urine accumulation. Common causes of such obstruction include kidney stones, which are solid masses of minerals and salts that can block the flow. Strictures (narrowings within the urinary tract), blood clots, or tumors can also impede urine drainage.
An obstruction in the renal pelvis or the ureter can cause urine to back up, leading to increased pressure within the renal pelvis and calyces. This sustained pressure can stretch and distend these structures, potentially affecting the kidney’s ability to filter blood effectively over time. The accumulation of stagnant urine also raises the risk of urinary tract infections, which can further compromise kidney function.