Collecting duct system

Segments

The segments of the system are as follows:

Connecting tubule

With respect to the renal corpuscle, the "connecting tubule" is the most proximal part of the collecting duct system. It is adjacent to the distal convoluted tubule, the most distal segment of the renal tubule. Connecting tubules from several adjacent nephrons merge to form cortical collecting tubules, and these may join to form cortical collecting ducts. Connecting tubules of some juxtamedullary nephrons may arch upward, forming an arcade.

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.

The initial collecting tubule is a segment with a constitution similar as the collecting duct, but before the convergence with other tubules.

The "cortical collecting ducts" receive filtrate from multiple initial collecting tubules and descend into the renal medulla to form medullary collecting ducts.

Medullary collecting duct

"Medullary collecting ducts" are divided into outer and inner segments, the latter reaching more deeply into the medulla. The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ion continues here. Urea passively transports out of duct here and creates 500mOsm gradient.

The outer segment of the medullary collecting duct follows the cortical collecting duct. It reaches the level of the renal medulla where the thin descending limb of loop of Henle borders with the thick ascending limb of loop of Henle

The inner segment is the part of the collecting duct system between the outer segment and the papillary ducts.

Papillary duct

The terminal portions of the medullary collecting ducts are the "papillary ducts", which end at the renal papilla and empty into a minor calyx.

Cells

Each component of the collecting duct system contains two cell types, intercalated cells and a segment-specific cell type:

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 expression of sodium channels. Increases in aldosterone increase expression of luminal sodium channels. Aldosterone also increases the number of Na⁺/K⁺-ATPase pumps that allow increased sodium reabsorption and potassium secretion. Vasopressin determines the expression of aquaporin channels that provide a physical pathway for water to pass through the principal cells. Together, aldosterone and vasopressin let the principal cell control the quantity of water that is reabsorbed.

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)(reference).

Function

The collecting duct system is the final 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, in particular, the outer medullary and cortical collecting ducts, are largely impermeable to water without the presence of antidiuretic hormone (ADH, or vasopressin).

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