Everything about The Collecting Duct System totally explained
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 segments of the system are as follows:
| Segment |
Description |
| connecting tubule |
|
| initial collecting tubule |
Before convergence of nephrons |
| cortical collecting ducts |
|
| medullary collecting ducts |
|
| papillary ducts, also known as duct of Bellini |
|
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.
Initial collecting tubule
The initial collecting tubule is a segment with a constitution similar as the collecting duct, but before the convergence with other tubules.
Cortical collecting duct
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.
Outer segment
The outer segment of the medullary collecting duct follows the cortical collecting duct. It reaches the level of the renal medulla where the
thick ascending limb of loop of Henle borders with the [[thin ascending limb of loop of Henle
Inner segment
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. It is also called duct of Bellini.
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's 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).
Additional images
Image:Gray1133.png|Transverse section of pyramidal substance of kidney of pig, the bloodvessels of which are injected.
Image:Alpha Intercalated Cell Cartoon.jpg|Alpha intercalated cell cartoon
Further Information
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