Collecting duct system
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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.
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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, water channels 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
Connecting tubules
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.
Cortical collecting ducts
The cortical collecting ducts receive filtrate from multiple connecting tubules and descend into the renal medulla to form medullary collecting ducts.
Medullary collecting ducts
Medullary collecting ducts are divided into outer and inner segments, the latter reaching more deeply into the medulla. The terminal portions of these ducts are the papillary ducts, which end at the renal papilla and empty into a minor calyx.
The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen and bicarbinate ion continues here.
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.
The principal cell mediates the collecting duct's influence on sodium and potassium balance via sodium and potassium channels located on the cell's apical membrane. Intercalated cells come α and β varieties and participate in acid-base homeostasis. In particular, α-intercalated cells secrete acid (via an apical H+-ATPase and H+/K+ exchanger) in the form of hydrogen ions and reabsorb bicarbonate (via a basolateral Cl-/HCO3- exchanger). Similarly, β-intercalated cells secrete bicarbonate (via an apical Cl-/HCO3-) and reabsorb acid (via a basal H+-ATPase). 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.