Collecting Duct

1. Overview

The collecting duct is the final tubular component of the nephron-collecting system in the kidney, responsible for the final regulation of water, electrolyte, and acid-base balance. It receives urine from multiple distal convoluted tubules and delivers it to the renal papilla, from where it drains into the minor calyces. It plays a key role in concentrating or diluting urine under hormonal control, particularly via antidiuretic hormone (ADH) and aldosterone.

2. Location

Collecting ducts span from the renal cortex to the renal medulla and terminate at the renal papillae. They are part of the kidney's medullary rays and collecting system:

• Begin in the cortex at the convergence of multiple distal convoluted tubules.

• Travel through the renal cortex and medulla within medullary rays and renal pyramids.

• Terminate at the papillary ducts (ducts of Bellini), which open into the minor calyx at the tip of the renal papilla.

3. Structure

Histologically, collecting ducts are composed of specialized epithelial cells and increase in size as they descend toward the renal papilla:

• Simple cuboidal epithelium: In cortical collecting ducts.

• Simple columnar epithelium: In larger medullary and papillary ducts.

• Two main cell types:

  • Principal cells: Involved in sodium reabsorption and water permeability (ADH-sensitive).

  • Intercalated cells (Type A & B): Involved in acid-base balance by secreting H+ or HCO₃-.

The ducts are surrounded by interstitial tissue and vasa recta that facilitate water reabsorption via countercurrent mechanisms.

4. Function

The collecting duct has several important regulatory functions:

• Water reabsorption: Under control of ADH, aquaporin channels are inserted to allow water retention.

• Electrolyte regulation: Sodium reabsorption and potassium secretion under the influence of aldosterone.

• Acid-base balance: Intercalated cells modulate hydrogen and bicarbonate ion secretion based on systemic pH.

• Final urine concentration: Modulates osmolality of urine before it enters the renal pelvis.

5. Physiological Role(s)

The collecting duct is essential for maintaining internal fluid and electrolyte homeostasis:

• Water conservation: Concentrates urine during dehydration by increasing water reabsorption through ADH-sensitive mechanisms.

• Hormone response: Responds to circulating hormones (ADH, aldosterone, ANP) to adjust systemic volume and pressure.

• Countercurrent multiplication: Works in conjunction with the loop of Henle and vasa recta to maintain medullary osmotic gradient.

• Potassium balance: Modulates potassium secretion, especially under high intake or aldosterone stimulation.

6. Clinical Significance

Diabetes Insipidus

Impaired ADH function or response in the collecting duct leads to diabetes insipidus, resulting in the excretion of large volumes of dilute urine:

• Central DI: Lack of ADH secretion.

• Nephrogenic DI: Collecting duct unresponsive to ADH.

Syndrome of Inappropriate ADH Secretion (SIADH)

Excessive ADH activity leads to overactive water reabsorption in the collecting ducts, causing hyponatremia and water retention.

Hyperkalemia or Hypokalemia

Dysfunction in principal cells (especially with aldosterone resistance) can impair potassium secretion, leading to hyperkalemia. Conversely, excess aldosterone may cause excessive potassium loss.

Acidosis and Alkalosis

Intercalated cell dysfunction impairs acid or base secretion, contributing to renal tubular acidosis (Type I or IV) or metabolic alkalosis.

Polycystic Kidney Disease

Cyst formation often involves collecting ducts, especially in autosomal dominant polycystic kidney disease (ADPKD), leading to renal enlargement and eventual renal failure.

Diuretics

• Potassium-sparing diuretics (e.g., spironolactone, amiloride): Act on the collecting duct to block aldosterone or sodium channels.

Histopathology and Biopsy

In renal biopsies, damage or proliferation of collecting duct epithelium may be seen in conditions such as:

• Acute tubular necrosis (ATN)

• Tubulointerstitial nephritis

• Obstructive uropathy