The collecting duct system of the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

consists of a series of tubules and ducts that physically connect

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structu ...

s to a

minor calyx The renal calyces ( calyx) are conduits in the kidney through which urine passes. The minor calyces form a cup-shaped drain around the apex of the renal pyramids. Urine formed in the kidney passes through a renal papilla at the apex into the m ...

or directly to the

renal pelvis The renal pelvis or pelvis of the kidney is the funnel-like dilated part of the ureter in the kidney. It is formed by the convergence of the major calyces, acting as a funnel for urine flowing from the major calyces to the ureter. It has a mucous ...

. The collecting duct participates in

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

and

fluid balance Fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body ...

through

reabsorption In renal physiology, reabsorption, more specifically tubular reabsorption, is the process by which the nephron removes water and solutes from the tubular fluid (pre-urine) and returns them to the circulating blood. It is called ''reabsorption' ...

and

excretion Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, Kidney (vertebrates), kidneys, and skin. This is in contrast with secretion, where the substa ...

, processes regulated by the hormones

aldosterone Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays ...

and

vasopressin Mammalian 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 ...

(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 A renal corpuscle (or Malpighian body) is the blood-filtering component of the nephron of the kidney. It consists of a glomerulus - a tuft of capillaries composed of endothelial cells - and a glomerular capsule known as Bowman's capsule. Str ...

, 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 distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule. Physiology It is partly responsible for the regulation of potassium, sodium, calcium, and pH. On its apical surface (lum ...

, the most distal segment of the

renal tubule The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structure ...

. 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 d ...

, but the rest of the system derives from the

ureteric bud The ureteric bud, also known as the metanephric diverticulum, is a protrusion from the mesonephric duct during the development of the urinary and reproductive organs. It later develops into a conduit for urine drainage from the kidneys, which, in c ...

. Because of this, some sources group the connecting tubule as part of the

, 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 The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which ...

to form medullary collecting ducts. It participates in the regulation of

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and

s, including

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

, and

chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

. The CNT is sensitive to both

isoprotenerol Isoprenaline, also known as isoproterenol and sold under the brand name Isuprel among others, is a sympathomimetic medication which is used in the treatment of acute bradycardia (slow heart rate), heart block, and rarely for asthma, among other ...

(more so than the cortical collecting ducts) and

antidiuretic hormone Mammalian 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 ...

(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 Thin may refer to: * ''Thin'' (film), a 2006 documentary about eating disorders * Thin, a web server based on Mongrel * Thin (name), including a list of people with the name * Mal language, also known as Thin See also * * * Body shape * Em ...

borders with the

thick ascending limb of loop of Henle Within the nephron of the kidney, the ascending limb of the loop of Henle is a segment of the heterogenous loop of Henle downstream of the descending limb, after the sharp bend of the loop. This part of the renal tubule is divided into a thin and ...

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 In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retro ...

, previously known as the ducts of Bellini. Papillary ducts represent the most

distal Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

portion of the

collecting duct The collecting duct system of the kidney 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 participates in electrolyte and fluid balance through rea ...

. They receive renal filtrate (precursor to

urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

) from several medullary collecting ducts and empty into a

. 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 The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which ...

. This "papillary duct" exits the renal pyramid at the

renal papilla In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retr ...

e. The renal filtrate it carries drains into a

. 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 Simple columnar epithelium is a single layer of columnar epithelial cells which are tall and slender with oval-shaped nuclei located in the basal region, attached to the basement membrane. In humans, simple columnar epithelium lines most organs ...

resting on a thin

basement membrane The basement membrane, also known as base membrane, is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between epithelial tis ...

. The epithelium is composed primarily of principal cells and α-intercalated cells. The

of the collecting duct system transitions into

urothelium Transitional epithelium is a type of stratified epithelium. Transitional epithelium is a type of tissue that changes shape in response to stretching (stretchable epithelium). The transitional epithelium usually appears cuboidal when relaxed and ...

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 reabsorption or secretion that occurs is related to needs of the body at any given time. These processes are mediated by

hormones A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physiology and behavior. Hormones a ...

(

) and the

osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/ ...

(concentration of electrically charged chemicals) of the surrounding medulla.

Hormones A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physiology and behavior. Hormones a ...

regulate how permeable the papillary duct is to water and electrolytes. In the medullary collecting duct specifically,

upregulates urea transporter A1. This increases the concentration of urea in the surrounding

interstitium In anatomy, the interstitium is a contiguous fluid-filled space existing between a structural barrier, such as a cell membrane or the skin, and internal structures, such as organs, including muscles and the circulatory system. The fluid in this s ...

and increases the osmolarity.

Osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/ ...

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 reabsorption 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 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 channel Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell (biology), cell's cell membrane, membrane. They belong to the Cation channel superfamily, superfamily of cation channels. Classific ...

s and

potassium channel Potassium channels are the most widely distributed type of ion channel found in virtually all organisms. They form potassium-selective pores that span cell membranes. Potassium channels are found in most cell types and control a wide variety of ...

s located on the cell's

apical membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extra ...

Aldosterone Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays ...

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 Mammalian 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 ...

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 ...

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 Acid–base homeostasis is the homeostasis, homeostatic regulation of the pH of the Body fluid, body's extracellular fluid (ECF). The proper #Acid–base balance, balance between the acids and Base (chemistry), bases (i.e. the pH) in the ECF is cr ...

. For their contribution to acid–base homeostasis, the intercalated cells play important roles in the kidney's response to

acidosis Acidosis is a biological process producing hydrogen ions and increasing their concentration in blood or body fluids. pH is the negative log of hydrogen ion concentration and so it is decreased by a process of acidosis. Acidemia The term ac ...

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). 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). 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

and fluid balance. In humans, the system accounts for 4–5% of the kidney's reabsorption of

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

(ADH, or vasopressin). * In the ''absence'' of ADH, water in the renal filtrate is left alone to enter the urine, promoting

diuresis Diuresis () is the excretion of urine, especially when excessive (polyuria). The term collectively denotes the physiologic processes underpinning increased urine production by the kidneys during maintenance of fluid balance. In healthy people, ...

. * When ADH is ''present'',

s allow for the reabsorption of this water, thereby inhibiting diuresis. The collecting duct system participates in the regulation of other

s, including

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

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 particl ...

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 bioche ...

. An extracellular protein called

hensin (protein) is a Japanese term for live-action films or television programs that make heavy use of practical special effects. Credited to special effects director Eiji Tsuburaya, ''tokusatsu'' mainly refers to science fiction film, science fiction, War fi ...

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

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