Major Calyces

1. Overview

Major calyces (singular: calyx) are funnel-shaped structures within the kidney that serve as conduits for urine drainage. They collect urine from the minor calyces and channel it into the renal pelvis, from which urine flows into the ureter. Though not involved in filtration or reabsorption, the major calyces play a crucial role in urine transport and anatomical organization within the renal collecting system.

2. Location

The major calyces are located in the renal sinus, a cavity within the medial aspect of the kidney. Each kidney typically contains two or three major calyces, positioned between the minor calyces and the renal pelvis. Anatomically:

• Superior major calyx: Drains the upper pole of the kidney.

• Middle major calyx: May be present, draining the midsection.

• Inferior major calyx: Drains the lower pole of the kidney.

These calyces open medially into the renal pelvis, which then narrows to form the ureter at the renal hilum.

3. Structure

Each major calyx is formed by the convergence of two or more minor calyces. Structural features include:

• Mucosa: Lined by transitional epithelium (urothelium), allowing distensibility and protection against urine toxicity.

• Muscular layer: Smooth muscle fibers arranged in inner longitudinal and outer circular layers that assist in peristalsis.

• Connective tissue: Surrounds and supports the calyceal wall, continuous with the renal sinus fat and vasculature.

Major calyces vary in size but are typically 1–2 cm long and serve as visible collecting spaces in imaging and gross anatomy.

4. Function

The primary function of the major calyces is to collect and channel urine. Specific roles include:

• Urine collection: Receive urine from multiple minor calyces, each draining a renal pyramid.

• Transport conduit: Direct urine flow into the renal pelvis for passage into the ureter.

• Flow regulation: Help modulate urine flow by passive expansion and active peristaltic movement.

5. Physiological Role(s)

Although not involved in the biochemical processes of the nephron, the major calyces have important passive and mechanical roles:

• Low-pressure reservoir: Serve as temporary holding chambers for urine to prevent backflow to the nephrons.

• Peristalsis coordination: Smooth muscle contraction moves urine toward the renal pelvis and ureter in a coordinated manner.

• Maintaining sterile environment: Prevent reflux and microbial ascent by facilitating continuous, unidirectional urine flow.

6. Clinical Significance

Hydronephrosis

Obstruction in the urinary tract (e.g., at the ureteropelvic junction) can cause dilation of major calyces and other collecting structures. This condition, known as hydronephrosis, leads to:

• Backpressure on nephrons

• Renal parenchymal thinning

• Progressive renal dysfunction

Diagnosed via ultrasound or CT imaging.

Kidney Stones (Nephrolithiasis)

Calculi can form or lodge in a major calyx, particularly where minor calyces converge. Symptoms may include:

• Flank pain

• Hematuria

• Infection

Large stones in this area may require surgical removal or lithotripsy.

Calyceal Diverticulum

A rare congenital anomaly in which a portion of the major calyx becomes saccular and may retain urine or develop stones. It can lead to recurrent infections or hematuria.

Transitional Cell Carcinoma (TCC)

The urothelium lining the major calyces can develop transitional cell carcinoma, though less common than bladder TCC. Presenting symptoms include:

• Painless hematuria

• Obstructive symptoms

Diagnosis is usually via CT urogram and confirmed by biopsy.

Imaging Relevance

Major calyces are visualized in:

• Ultrasound: May show dilation or obstruction.

• Intravenous urography (IVU): Shows contrast collecting in calyces.

• CT urography: Offers detailed anatomy, useful in stone disease and tumors.