| Collecting duct system | |
|---|---|
 Scheme of renal tubule and its vascular supply. | |
| Details | |
| Location | Kidney |
| Identifiers | |
| Latin | tubulus renalis colligens |
| MeSH | D007685 |
| FMA | 265239 |
| Anatomical terminology | |
Thecollecting duct system of thekidney consists of a series of tubules and ducts that physically connectnephrons to aminor calyx or directly to therenal pelvis. The collecting duct participates inelectrolyte andfluid balance throughreabsorption andexcretion, processes regulated by the hormonesaldosterone andvasopressin (antidiuretic hormone).
There are several components of the collecting duct system, including the connecting tubules, cortical collecting ducts, and medullary collecting ducts.
The segments of the system are as follows:
| Segment | Description |
|---|---|
| connecting tubule | Connects distal convoluted tubule to the cortical collecting duct |
| initial collecting tubule | Before convergence of nephrons |
| cortical collecting ducts | Receives filtrate from the initial collecting tubules, and descends into therenal medulla, forming medullary collecting ducts |
| medullary collecting ducts | |
| papillary ducts |
With respect to therenal corpuscle, theconnecting tubule (CNT, orjunctional tubule, orarcuate renal tubule) is the most proximal part of the collecting duct system. It is adjacent to thedistal convoluted tubule, the most distal segment of therenal tubule. Connecting tubules from several adjacent nephrons merge to form cortical collecting tubules, and these may join to form cortical collecting ducts (CCD).[1] Connecting tubules of somejuxtamedullary nephrons may arch upward, forming an arcade. It is this "arcuate" feature which gives the tubule its alternate name.
The connecting tubule derives from themetanephric blastema, but the rest of the system derives from theureteric bud.[2] Because of this, some sources group the connecting tubule as part of thenephron, 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 therenal medulla to form medullary collecting ducts.
It participates in the regulation ofwater andelectrolytes, includingsodium, andchloride.[3] The CNT is sensitive to bothisoprotenerol (more so than the cortical collecting ducts) andantidiuretic hormone (less so than the cortical collecting ducts), the latter largely determining its function in water reabsorption.
"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 thethin descending limb of loop of Henle borders with thethick ascending limb of loop of Henle[4]: 837
The inner segment is the part of the collecting duct system between the outer segment and the papillary ducts.
Papillary (collecting) ducts are anatomical structures of thekidneys, previously known as theducts ofBellini. Papillary ducts represent the mostdistal portion of thecollecting duct. They receiverenal filtrate (precursor tourine) from severalmedullary collecting ducts and empty into aminor calyx. Papillary ducts continue the work of water reabsorption and electrolyte balance initiated in thecollecting tubules.[5]
Medullary collecting ducts converge to form a central (papillary) duct near the apex of eachrenal pyramid. This "papillary duct" exits the renal pyramid at therenal papillae. Therenal filtrate it carries drains into aminor calyx asurine.[6]
The cells that comprise the duct itself are similar to rest of the collecting system. The duct is lined by a layer ofsimple columnar epithelium resting on a thinbasement membrane. The epithelium is composed primarily ofprincipal cells and α-intercalated cells.[7] Thesimple columnar epithelium of the collecting duct system transitions intourothelium near the junction of a papillary duct and a minor calyx.[6]
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 byhormones (aldosterone,vasopressin) and theosmolarity (concentration of electrically charged chemicals) of the surrounding medulla.Hormones regulate howpermeable the papillary duct is to water and electrolytes. In the medullary collecting duct specifically,vasopressin upregulatesurea transporter A1. This increases the concentration of urea in the surroundinginterstitium 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 reabsorption of water from the collecting system is the greatest in the papillary duct.[8]
Each component of the collecting duct system contains two cell types,intercalated cells and a segment-specific cell type:
The principal cell mediates the collecting duct's influence on sodium and potassium balance viasodium channels andpotassium channels located on the cell'sapical membrane.Aldosterone determines expression of sodium channels (especially theENaC on the collecting tubule). Increases in aldosterone increase expression of luminal sodium channels.[9] Aldosterone also increases the number ofNa⁺/K⁺-ATPase pumps[10]: 949 that allow increased sodium reabsorption and potassium excretion.[10]: 336 Vasopressin determines the expression ofaquaporin channels that provide a physical pathway for water to pass through the principal cells.[11] Together, aldosterone and vasopressin let the principal cell control the quantity of water that is reabsorbed.
Intercalated cells come in α, β, and non-α non-β varieties and participate inacid–base homeostasis.[12][13]
| Type of cell | Secretes | Reabsorbs |
|---|---|---|
| α-intercalated cells | acid (via an apicalH+-ATPase andH+/K+ exchanger) in the form ofhydrogen ions | bicarbonate (viaband 3, a basolateralCl−/HCO3− exchanger)[14] |
| β-intercalated cells | bicarbonate (viapendrin a specialised apicalCl−/HCO3−) | acid (via a basalH+-ATPase) |
| non-α non-β intercalated cells | acid (via an apicalH+-ATPase andH+/K+ exchanger) andbicarbonate (viapendrin)[15][16] | - |
For their contribution to acid–base homeostasis, the intercalated cells play important roles in the kidney's response toacidosis andalkalosis. Damage to the α-intercalated cell's ability to secrete acid can result indistal 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.[17][18]
The collecting duct system is the final component of the kidney to influence the body'selectrolyte and fluid balance. In humans, the system accounts for 4–5% of the kidney's reabsorption ofsodium 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 ofantidiuretic hormone (ADH, or vasopressin).
The collecting duct system participates in the regulation of otherelectrolytes, includingchloride,potassium,hydrogen ions, andbicarbonate.
An extracellular protein calledhensin (protein) mediates the regulation of secretion of acid by alpha cells in acidosis, and secretion of bicarbonate by beta cells in alkalosis.[19][20]
Carcinoma of the collecting duct is a relatively rare subtype of renal cell carcinoma (RCC), accounting for less than 1% of all RCCs.[21][22] Many reported cases have occurred in younger patients, often in the third, fourth, or fifth decade of life.[23] Collecting duct carcinomas are derived from the medulla, but many are infiltrative, and extension into the cortex is common.[24] Most reported cases have been high grade and advanced stage and have not responded to conventional therapies.[23][25] Most patients are symptomatic at presentation.[26] 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.[27][28]
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This article incorporates text in thepublic domain frompage 1223 of the 20th edition ofGray's Anatomy(1918)