Renal pelvis

The renal pelvis is a funnel-shaped structure located within the kidney. It is situated in the hilum of the kidney, where the renal artery, vein, and ureter enter or exit the organ. The renal pelvis is the point where two or three major calyces merge and serves as the connection between the kidney and the ureter, which drains urine into the bladder. It is located within the central part of the kidney, adjacent to the renal sinus, and acts as a collecting basin for urine before it flows into the ureter.

Structure and Anatomy

The renal pelvis is a critical anatomical structure within the kidney that acts as the central collecting point for urine before it is transferred to the ureter. Below is a detailed description of its anatomy, covering its location, structure, and relationship to surrounding kidney components.

Location and Position

The renal pelvis is located in the hilum of the kidney, which is the concave, medial portion of the kidney.

• Hilum of the Kidney: The hilum is the entry and exit point for the renal artery, renal vein, lymphatics, nerves, and the ureter. The renal pelvis is positioned centrally in the hilum and is the funnel-shaped part of the kidney that connects the major calyces to the ureter.
• Position Relative to Other Structures: The renal pelvis is situated within the renal sinus, a cavity in the kidney that also contains fat, blood vessels, and nerves. It lies just posterior to the renal arteries and veins and anterior to the renal pyramids, which form part of the kidney’s functional filtration units.

Structural Composition

The renal pelvis is a funnel-shaped sac composed of connective tissue and smooth muscle, lined internally with specialized epithelium.

Funnel Shape: The renal pelvis begins as a broad, funnel-shaped structure that collects urine from the major calyces. The funnel then narrows and tapers into the ureter, allowing urine to drain into the bladder. The wide part of the renal pelvis is called the extrarenal pelvis when part of it extends outside the kidney, while the narrower portion inside the hilum is the intrarenal pelvis.

Layers of the Renal Pelvis:

• Mucosa: The inner lining of the renal pelvis is composed of transitional epithelium (urothelium). This type of epithelium is unique because it can stretch and accommodate varying volumes of urine.
• Muscularis: Surrounding the epithelium is a layer of smooth muscle, which helps propel urine from the renal pelvis into the ureter through peristaltic contractions.
• Adventitia: The outermost layer of the renal pelvis consists of fibrous connective tissue, which provides structural support and anchors the pelvis within the kidney.

Relation to the Major and Minor Calyces

The renal pelvis is closely associated with the major and minor calyces, which are the structures that collect urine from the nephrons in the renal pyramids.

• Minor Calyces: The renal pelvis receives urine from several minor calyces, which collect urine from the tips of the renal pyramids (the papillae). Each renal papilla is connected to a minor calyx, which channels the urine into the major calyces.
• Major Calyces: Two or three major calyces combine to form the renal pelvis. The major calyces are larger collecting ducts that gather urine from the minor calyces before funneling it into the renal pelvis. The precise number of major calyces can vary between individuals, but most people have two or three.

Connection to the Ureter

The renal pelvis tapers to form the ureter, the tube that carries urine from the kidney to the bladder.

• Ureteropelvic Junction: The point where the renal pelvis narrows and joins the ureter is known as the ureteropelvic junction. This is a critical area, as it marks the transition between the wide funnel of the renal pelvis and the narrow, muscular tube of the ureter.
• Peristalsis and Urine Transport: The smooth muscle within the renal pelvis is continuous with the muscle of the ureter, which generates peristaltic contractions that push urine down toward the bladder. This muscular connection ensures that urine flows smoothly from the kidney into the ureter without backflow.

Blood Supply

The renal pelvis shares its blood supply with the kidney, receiving arterial blood from branches of the renal artery.

• Renal Artery: The renal artery divides into several branches as it enters the hilum of the kidney. These branches supply oxygenated blood to the renal pelvis and surrounding structures. The blood vessels follow the course of the ureter and supply the renal pelvis through smaller segmental arteries.
• Venous Drainage: Venous blood from the renal pelvis is collected by the renal vein, which drains deoxygenated blood from the kidney and connects to the inferior vena cava. The venous drainage follows a similar pattern to the arterial supply.

Lymphatic Drainage

The renal pelvis has a network of lymphatic vessels that drain lymph from the kidney and pelvis to nearby lymph nodes.

Lymphatic Pathways: The lymphatic drainage of the renal pelvis travels to the lumbar lymph nodes and aortic lymph nodes, which are located near the abdominal aorta. These nodes filter the lymphatic fluid, removing waste products and pathogens from the renal pelvis and surrounding structures.

Nerve Supply

The renal pelvis is innervated by both sympathetic and parasympathetic fibers that regulate the function of the kidney and urinary tract.

• Sympathetic Innervation: Sympathetic nerves, primarily arising from the renal plexus, help regulate smooth muscle contractions in the renal pelvis and ureter. These nerves modulate peristalsis, the mechanism that pushes urine from the renal pelvis into the ureter.
• Parasympathetic Innervation: Parasympathetic innervation to the renal pelvis comes from the vagus nerve (cranial nerve X). The parasympathetic fibers are responsible for stimulating relaxation and maintaining the normal functioning of the smooth muscle within the pelvis.
• Pain Sensation: The renal pelvis is sensitive to distension or irritation, such as when there is an obstruction, like a kidney stone. Sensory nerve fibers relay pain signals to the brain, usually via T10-L1 spinal segments, which can result in flank pain.

Histological Features

The renal pelvis has several histological layers that are specialized for its function in urine collection and transport.

• Transitional Epithelium (Urothelium): The inner lining of the renal pelvis is made up of transitional epithelium, which is unique to the urinary tract. This epithelium is able to stretch as the renal pelvis fills with urine, allowing for expansion without damage.
• Smooth Muscle Layer: The muscularis of the renal pelvis contains smooth muscle fibers that contract in a rhythmic fashion (peristalsis) to push urine into the ureter.
• Connective Tissue: The outer layer of the renal pelvis is surrounded by fibrous connective tissue, which protects the structure and provides support.

Function

The renal pelvis plays a crucial role in the collection, storage, and transport of urine from the kidney to the ureter. Below is a detailed explanation of its functions, broken down into specific aspects of its role in the urinary system.

Collection of Urine from the Calyces

The primary function of the renal pelvis is to serve as the collection basin for urine produced by the kidney.

• Collecting Urine from the Minor Calyces: The renal pyramids, located in the renal medulla, produce urine, which drains through the renal papillae into the minor calyces. Each minor calyx collects urine from a single papilla. These minor calyces then converge to form major calyces.
• Receiving Urine from Major Calyces: The major calyces are larger collecting ducts formed by the union of minor calyces. The major calyces empty their urine into the renal pelvis, which serves as the final collection point before the urine is transported to the ureter.

Storage and Temporary Holding of Urine

The renal pelvis functions as a temporary storage site for urine before it enters the ureter.

• Accommodating Variations in Urine Flow: The renal pelvis is able to expand slightly to accommodate varying volumes of urine depending on the kidney’s output. This flexibility is due to the presence of transitional epithelium, which allows the renal pelvis to stretch without becoming damaged. The temporary storage of urine in the renal pelvis ensures that the flow of urine into the ureter is continuous and controlled, even when the kidney produces urine at different rates.
• Preventing Backflow of Urine: The renal pelvis, along with the ureter and bladder, plays an important role in preventing the backflow of urine. Backflow, or vesicoureteral reflux, can lead to infections or kidney damage. By maintaining a one-way flow of urine from the renal pelvis into the ureter, the system reduces the risk of contamination and infection.

Transporting Urine to the Ureter

A critical function of the renal pelvis is to transport urine from the kidney to the ureter, ensuring its proper flow toward the bladder.

• Tapering Into the Ureter: The renal pelvis narrows as it approaches the ureteropelvic junction, where it transitions into the ureter. This funnel-like design ensures that urine is funneled smoothly into the ureter for efficient transport.
• Peristaltic Movements: The renal pelvis is lined with smooth muscle, which helps propel urine toward the ureter through peristalsis. Peristalsis is a series of wave-like muscle contractions that occur in the smooth muscle lining of the renal pelvis and ureter. These contractions ensure that urine is actively pushed down the urinary tract, preventing stagnation and promoting efficient drainage.
• Continuous Urine Flow: Even during periods of low urine production, the renal pelvis ensures that urine continues to flow steadily into the ureter. This continuous flow is essential for maintaining healthy kidney function and preventing blockages in the urinary system.

Preventing Urine Backflow and Maintaining Unidirectional Flow

The renal pelvis, along with the ureter and bladder, ensures that urine flows in one direction—from the kidney to the bladder—without backflow.

• One-Way Valve System: The ureteropelvic junction, where the renal pelvis narrows into the ureter, acts as a functional valve. This junction helps prevent the backflow of urine from the ureter into the renal pelvis. By maintaining this unidirectional flow, the renal pelvis protects the kidney from vesicoureteral reflux, which can lead to infections and damage to kidney tissue.
• Coordinated Contractions: The smooth muscle in the renal pelvis and ureter work together in a coordinated manner. As urine accumulates in the renal pelvis, the muscle contracts to push urine forward into the ureter. This coordinated movement ensures that the urinary system remains efficient and free from urine retention or reflux.

Facilitating Peristalsis for Urine Transport

The renal pelvis plays a role in initiating and regulating peristalsis to propel urine toward the bladder.

• Initiation of Peristalsis: As the renal pelvis fills with urine, stretch receptors in the walls of the pelvis trigger peristaltic waves. These waves begin at the renal pelvis and propagate down the ureter, pushing urine toward the bladder. The smooth muscle in the renal pelvis contracts rhythmically to maintain a steady flow of urine into the ureter.
• Regulation of Flow: The frequency and strength of peristaltic waves are adjusted according to the amount of urine being produced by the kidney. When the kidney produces more urine, the renal pelvis generates stronger and more frequent peristaltic waves to accommodate the increased flow. Conversely, when urine production is lower, peristaltic activity slows down.

Adapting to Changes in Urinary Output

The renal pelvis helps the kidney adapt to variations in urine production throughout the day.

• Handling Increased Urine Volume: During periods of increased urine output, such as after consuming large amounts of fluid, the renal pelvis can expand slightly due to the transitional epithelium lining its inner walls. This expansion allows the renal pelvis to temporarily store a greater volume of urine until it is gradually moved into the ureter.
• Dealing with Decreased Urine Volume: During periods of low urine output, such as dehydration or sleep, the renal pelvis maintains peristaltic contractions at a slower rate, ensuring that small volumes of urine are still transported to the bladder. This regulation helps prevent urine retention in the kidney, which could otherwise lead to infection or stone formation.

Supporting Kidney Filtration and Excretion

While the renal pelvis does not directly filter blood or produce urine, it plays an essential role in the final stage of urine excretion.

• Urine Conduit: The renal pelvis is the conduit through which urine leaves the kidney after it has been produced by the nephrons. It ensures that filtered urine is transported away from the kidney’s filtration units and into the ureter, allowing the kidney to continuously filter blood without the risk of urine backflow or blockage.
• Final Stage in Urine Formation: The renal pelvis marks the final stage in urine formation. After urine is collected from the renal calyces, no further changes are made to its composition. At this point, the renal pelvis serves as a storage and transport vessel, ensuring that urine reaches the bladder for eventual excretion.

Clinical Significance

The renal pelvis plays a crucial role in the urinary system by collecting and transporting urine from the kidney to the ureter. Its clinical significance becomes apparent in conditions such as hydronephrosis, where an obstruction (e.g., kidney stones, tumors, or congenital anomalies) causes urine to back up, leading to swelling and pressure in the renal pelvis. This can damage kidney tissue if left untreated.

Another important condition is pyelonephritis, a bacterial infection that can spread from the bladder to the renal pelvis and kidneys, causing inflammation, fever, and pain. Early diagnosis and treatment are crucial to prevent complications.

The renal pelvis is also the site where kidney stones can form and become lodged, causing severe pain, urinary blockage, and potential kidney damage. In severe cases, surgical intervention or procedures such as lithotripsy (stone fragmentation) may be required to remove the obstruction and restore normal urine flow.