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

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 structure ...

s to a minor calyx 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 covnvergence of the major calyces, acting as a funnel for urine flowing from the major calyces to the ureter. It has a muco ...

. The collecting duct system is the last part of nephron and participates in

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...

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 b ...

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 ...

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 a c ...

and

vasopressin 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 trave ...

(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 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 (lumen ...

, 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 ...

, 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 ...

. 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 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 then splits up to form the segmental arteries whi ...

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

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

and

s, 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 sa ...

. The CNT is sensitive to both

isoprotenerol Isoprenaline, or isoproterenol (brand name: Isoprenaline Macure), is a medication used for the treatment of bradycardia (slow heart rate), heart block, and rarely for asthma. It is a non-selective β adrenoceptor agonist that is the isopropylam ...

(more so than the cortical collecting ducts) and antidiuretic hormone (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 Within the nephron of the kidney, the descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle. Physiology The permeability is as follows: Also, the medullary interstitium is highly ...

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 an ...

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 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; bloo ...

, previously known as the ducts of Bellini. Papillary ducts represent the most distal 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 system is the last part of nephron and participates in elect ...

. 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. Cellular ...

) 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 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 system is the last part of nephron and participates in elect ...

. Medullary collecting ducts converge to form a central (papillary) duct near the apex of each

renal pyramid The renal medulla 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 then splits up to form the segmental arteries whi ...

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

renal papilla The renal medulla 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 then splits up to form the segmental arteries whi ...

e. The renal filtrate it carries drains into a minor calyx as

. 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 Transitional epithelium also known as urothelium 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 ...

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 (

) 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/L ...

(concentration of electrically charged chemicals) of the surrounding medulla. Hormones regulate how

permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...

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 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/L ...

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 chann ...

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 c ...

s 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 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 trave ...

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 Acid–base homeostasis is the homeostatic regulation of the pH of the body's extracellular fluid (ECF). The proper balance between the acids and bases (i.e. the pH) in the ECF is crucial for the normal physiology of the body—and for cellu ...

. 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'' describes ...

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 antidiuretic hormone (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'',

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 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 atmosph ...

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 biochem ...

. 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

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