Collecting duct system

The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter. It participates in electrolyte and fluid balance through reabsorption and excretion, processes regulated by the hormones aldosterone and antidiuretic hormone.

Anatomically, there are several components of the collecting duct system, including the connecting tubules, cortical collecting ducts, and medullary collecting ducts.

Function
The collecting duct system is the last component of the kidney to influence the body's electrolyte and fluid balance. In humans, the system accounts for 4-5% of the kidney's reabsorption of sodium and 5% of the kidney's reabsorption of water. At times of extreme dehydration, over 24% of the filtered water may be reabsorbed in the collecting duct system.

The wide variation in water reabsorption levels for the collecting duct system reflects its dependence on hormonal activation. The collecting ducts, particularly the outer medullary and cortical collecting ducts, are largely impermeable to water without the presence of antidiuretic hormone (ADH, or vasopressin).
 * In the absence of ADH, water in the renal filtrate is left alone to enter the urine, promoting diuresis.
 * When ADH is present, aquaporins allow for the reabsorption of this water, thereby inhibiting diuresis.

The collecting duct system participates in the regulation of other electrolytes, including chloride, potassium, hydrogen ions, and bicarbonate.

Anatomy
The parts of the system are as follows:

The connecting tubule derives from the metanephric blastema, but the rest of the system derives from the ureteric bud. Because of this, some sources group the connecting tubule as part of the nephron, rather than grouping it with the collecting duct system.

Cell types
Each component of the collecting duct system contains two cell types, intercalated cells and a segment-specific cell type:
 * For the connecting tubules, this specific cell type is the connecting tubule cell
 * For the collecting ducts, it is the principal cell. The inner medullary collecting ducts contain an additional cell type, called the inner medullary collecting duct cell.

Principal cells
The principal cell mediates the collecting duct's influence on sodium and potassium balance via sodium channels and potassium channels located on the cell's apical membrane. Aldosterone determines if the sodium channels transport ions.

Intercalated cells
Intercalated cells come in α and β varieties and participate in acid-base homeostasis.

For their contribution to acid-base homeostasis, the intercalated cells play important roles in the kidney's response to acidosis and alkalosis. Damage to the α-intercalated cell's ability to secrete acid can result in distal renal tubular acidosis (RTA type I, classical RTA).