The collecting duct system of the
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
consists of a series of tubules and ducts that physically connect
nephrons to a
minor calyx or directly to the
renal pelvis. The collecting duct system is the last part of nephron and participates in
electrolyte and
fluid balance through reabsorption and excretion, processes regulated by the
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
s
aldosterone and
vasopressin (antidiuretic hormone).
There are several components of the collecting duct system, including the connecting tubules, cortical collecting ducts, and medullary collecting ducts.
Structure
Segments
The segments of the system are as follows:
Connecting tubule
With respect to the
renal corpuscle, the connecting tubule (CNT, or junctional tubule, or arcuate renal 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 (CCD). Connecting tubules of some
juxtamedullary nephrons may arch upward, forming an arcade. It is this "arcuate" feature which gives the tubule its alternate name.
The connecting tubule derives from the
metanephric blastema
The metanephrogenic blastema or metanephric blastema (or metanephric mesenchyme, or metanephric mesoderm) is one of the two embryological structures that give rise to the kidney, the other being the ureteric bud.
The metanephric blastema mostly ...
, 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.
It participates in the regulation of
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
and
electrolytes, including
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
, and
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
. The CNT is sensitive to both
isoprotenerol (more so than the cortical collecting ducts) and
antidiuretic hormone
Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travel ...
(less so than the cortical collecting ducts), the latter largely determining its function in water reabsorption.
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
Papillary (collecting) ducts are anatomical structures of the
kidneys, previously known as the ducts of
Bellini. Papillary ducts represent the most
distal portion of the
collecting duct. They receive
renal filtrate (precursor to
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellul ...
) from several
medullary collecting ducts and empty into a
minor calyx. Papillary ducts continue the work of water reabsorption and electrolyte balance initiated in the
collecting tubules.
Medullary collecting ducts converge to form a central (papillary) duct near the apex of each
renal pyramid. This "papillary duct" exits the renal pyramid at the
renal papillae. The
renal filtrate it carries drains into a
minor calyx as
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellul ...
.
The cells that comprise the duct itself are similar to rest of the collecting system. The duct is lined by a layer of
simple columnar epithelium resting on a thin
basement membrane. The epithelium is composed primarily of principal cells and α-intercalated cells. The
simple columnar epithelium of the collecting duct system transitions into
urothelium near the junction of a papillary duct and a minor calyx.
These cells work in tandem to reabsorb water, sodium, and urea and secrete acid and potassium. The amount of reabsorbtion or secretion that occurs is related to needs of the body at any given time. These processes are mediated by
hormones (
aldosterone,
vasopressin) and the
osmolarity (concentration of electrically charged chemicals) of the surrounding medulla.
Hormones regulate how
permeable the papillary duct is to water and electrolytes. In the medullary collecting duct specifically,
vasopressin upregulates
urea transporter A1. This increases the concentration of urea in the surrounding
interstitium and increases the osmolarity.
Osmolarity influences the strength of the force that pulls (reabsorbs) water from the papillary duct into the medullary interstitium. This is especially important in the papillary ducts. Osmolarity increases from the base of the renal pyramid to the apex. It is highest at the renal apex (up to 1200 mOsm). Thus the force driving the reabsorbtion of water from the collecting system is the greatest in the papillary duct.
Cells
Each component of the collecting duct system contains two cell types, ''intercalated cells'' and a segment-specific cell type:
* For the 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 channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the chan ...
s and
potassium channels located on the cell's
apical membrane.
Aldosterone determines expression of sodium channels (especially the
ENaC
The epithelial sodium channel (ENaC), (also known as amiloride-sensitive sodium channel) is a membrane-bound ion channel that is selectively permeable to sodium ions (). It is assembled as a heterotrimer composed of three homologous subunits α ...
on the collecting tubule). 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 excretion.
Vasopressin determines the expression of
aquaporin
Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a ...
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, β, and non-α non-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
Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma.
The term ''acidemia'' describe ...
and
alkalosis
Alkalosis is the result of a process reducing hydrogen ion concentration of arterial blood plasma (alkalemia). In contrast to acidemia (serum pH 7.35 or lower), alkalemia occurs when the serum pH is higher than normal (7.45 or higher). Alkalosi ...
. Damage to the α-intercalated cell's ability to secrete acid can result in
distal renal tubular acidosis
Distal renal tubular acidosis (dRTA) is the classical form of RTA, being the first described. Distal RTA is characterized by a failure of acid secretion by the alpha intercalated cells of the distal tubule and cortical collecting duct of the di ...
(RTA type I, classical RTA)(reference). The intercalated cell population is also extensively modified in response to chronic lithium treatment, including the addition of a largely uncharacterized cell type which expressed markers for both intercalated and principal cells.
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
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
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
Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travel ...
(ADH, or vasopressin).
* In the ''absence'' of ADH, water in the renal filtrate is left alone to enter the urine, promoting
diuresis
Diuresis () is increased urination (polyuria) or, in the related word senses more often intended, the physiological process that produces such an increase or the administration of medications to encourage that process. It involves extra urine pr ...
.
* When ADH is ''present'',
aquaporin
Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a ...
s allow for the reabsorption of this water, thereby inhibiting diuresis.
The collecting duct system participates in the regulation of other
electrolytes, including
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
,
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
,
hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...
s, and
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochemi ...
.
An extracellular protein called
hensin (protein) mediates the regulation of secretion of acid by alpha cells in acidosis, and secretion of bicarbonate by beta cells in alkalosis.
Collecting duct carcinoma
Carcinoma of the collecting duct is a relatively rare subtype of renal cell carcinoma (RCC), accounting for less than 1% of all RCCs. Many reported cases have occurred in younger patients, often in the third, fourth, or fifth decade of life.
[Carter ''et al.'', 1992] Collecting duct carcinomas are derived from the medulla, but many are infiltrative, and extension into the cortex is common.
Most reported cases have been high grade and advanced stage and have not responded to conventional therapies.
Most patients are symptomatic at presentation.
[Tokuda ''et al.'', 2004] Immunohistochemical and molecular analyses suggest that collecting duct RCC may resemble transitional cell carcinoma, and some patients with advanced collecting duct RCC have responded to cisplatin- or gemcitabine-based chemotherapy.
[Peyromaure ''et al.'', 2003]
See also
*
Collecting duct carcinoma
References
External links
* "Collecting Duct (Kidney)"
* – "Urinary System: kidney, medulla, collecting duct and ascending tubule"
* – "Urinary System: kidney, H&E, collecting duct and ascending tubule"
*
Types of tubules at ndif.org
{{DEFAULTSORT:Collecting Duct System
Kidney anatomy