Nephron 215248 225962554 2008-07-16T05:56:02Z Chaldor 7312839 Reverted 1 edit by [[Special:Contributions/58.68.64.229|58.68.64.229]] identified as [[WP:VAND|vandalism]] to last revision by [[User:84.63.101.56|84.63.101.56]]. ([[WP:TW|TW]]) {{Infobox Anatomy | Name = {{PAGENAME}} | Latin = | GraySubject = 253 | GrayPage = 1221 | Image = Gray1128.png | Width = 290 | Caption = Nephron of the kidney without [[juxtaglomerular apparatus]] | Image2 = Nefron.png | Width2 = 250 | Caption2 = Nephron. Diagram is labeled in [[Polish language|Polish]], but flow can still be identified. | Precursor = [[Metanephric blastema]] | System = | Artery = | Vein = | Nerve = | Lymph = | MeshName = Nephrons | MeshNumber = | DorlandsPre = | DorlandsSuf = }} A '''nephron''' (from [[Greek language|Greek]] νεφρός (nephros) meaning "kidney") is the basic structural and functional unit of the [[kidney]]. Its chief function is to regulate the concentration of [[water]] and soluble substances like sodium salts by filtering the [[blood]], reabsorbing what is needed and excreting the rest as [[urine]]. A nephron eliminates wastes from the body, regulates [[blood volume]] and [[blood pressure|pressure]], controls levels of [[electrolyte]]s and [[metabolite]]s, and regulates blood [[pH]]. Its functions are vital to life and are regulated by the [[endocrine system]] by [[hormone]]s such as [[antidiuretic hormone]], [[aldosterone]], and [[parathyroid hormone]].<ref>{{cite book | last = Maton | first = Anthea | authorlink = | coauthors = Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright | title = Human Biology and Health | publisher = Prentice Hall | date = 1993 | location = Englewood Cliffs, New Jersey, USA | pages = | url = | doi = | id = | isbn = 0-13-981176-1}}</ref> ==Types of nephrons== Two general classes of nephrons are cortical nephrons and [[juxtamedullary nephron]]s, both of which are classified according to the location of their associated [[renal corpuscle]]. Cortical nephrons have their renal corpuscle in the superficial [[renal cortex]], while the renal corpuscles of juxtamedullary nephrons are located near the [[renal medulla]]. The nomenclature for cortical nephrons varies, with some sources distinguishing between ''superficial cortical nephrons'' and ''midcortical nephrons'';<ref>{{GeorgiaPhysiology|7/7ch03/7ch03p16}}</ref> other sources simply call all non-juxtamedullary nephrons ''superficial nephrons''.<ref name="mcgill">http://sprojects.mmi.mcgill.ca/nephrology/presentation/presentation1.htm</ref> Functionally, cortical and juxtamedullary nephrons have distinct roles. Cortical nephrons (85% of all nephrons in humans) mainly perform excretory and regulatory functions, while juxtamedullary nephrons (15% of nephrons in humans) concentrate and dilute urine.<ref name="mcgill"/> ==Anatomy== Each nephron is composed of an initial filtering component (the "[[renal corpuscle]]") and a tubule specialized for reabsorption and secretion (the "renal tubule"). The renal corpuscle filters out large solutes from the blood, delivering water and small solutes to the renal tubule for modification. ===Renal corpuscle=== Composed of a [[glomerulus (kidney)|glomerulus]] and [[Bowman's capsule]], the [[renal corpuscle]] (or ''[[Malpighian corpuscle]]'') is the beginning of the nephron. It is the nephron's initial filtering component. {| class="wikitable" | [[Glomerulus (kidney)|Glomerulus]] || The glomerulus is a [[capillary]] tuft that receives its blood supply from an afferent [[arteriole]] of the [[renal circulation]]. The glomerular blood pressure provides the driving force for water and solutes to be filtered out of the blood and into the space made by [[Bowman's capsule]]. The remainder of the blood (only approximately 1/5 of all plasma passing through the kidney is filtered through the glomerular wall into Bowman's capsule) passes into the narrower efferent arteriole. It then moves into the vasa recta, which are collecting capillaries intertwined with the convoluted tubules through the interstitial space, and which the reabsorbed substances will also enter. This then combines with efferent venules from other nephrons into the renal vein, and rejoins the main bloodstream. |- | [[Bowman's Capsule]] || Bowman's capsule (also called the glomerular capsule) surrounds the glomerulus and is composed of a visceral inner layer and a parietal outer layer, both formed by simple squamous epithelial cells. Fluids from blood in the glomerulus are collected in the Bowman's capsule (i.e., glomerular filtrate) and further processed along the nephron to form [[urine]]. |} ===Renal tubule=== <!--Renal tubule redirects here--> {{Infobox Anatomy | Name = Renal nephron | Latin = tubulus renalis | GraySubject = 253 | GrayPage = 1223 | Image = Gray1128.png | Width = 290 | Caption = Scheme of renal tubule and its vascular supply. | Image2 = | Caption2 = | Precursor = | MeshName = | MeshNumber = | DorlandsPre = t_22 | DorlandsSuf = 12830093 }} The '''kidney tubule''', also '''renal tubule''', is the portion of the [[nephron]] of the [[kidney]] containing the [[tubular fluid]] filtered through the [[glomerulus]].<ref>Ecology & Evolutionary Biology - University of Colorado at Boulder. "The Kidney Tubule I: Urine Production." URL: [http://www.colorado.edu/eeb/web_resources/cartoons/nephrex1.html http://www.colorado.edu/eeb/web_resources/cartoons/nephrex1.html]. Accessed on: March 6, 2007.</ref> After passing the tubule, the filtrate continues to the [[collecting duct system]]. The components of the kidney tubule are: * [[Proximal tubule]] * [[Loop of Henle]] ** [[Descending limb of loop of Henle]] ** [[Ascending limb of loop of Henle]] *** [[Thin ascending limb of loop of Henle]] *** [[Thick ascending limb of loop of Henle]] * [[Distal convoluted tubule]] {{-}} == Tubule component functions == The following table describes each component of a tubule in detail. {| class="wikitable" | '''Name''' || colspan="3" | '''Description''' |- |rowspan=4| [[Proximal tubule]] |rowspan=2|convoluted |S1 <ref name=boron743> {{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year= |pages= |isbn=1-4160-2328-3 |oclc= |doi=}} Page 743 </ref> |rowspan=4| The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion. Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including approximately two-thirds of the filtered salt and water and all filtered [[organic compound|organic]] solutes (primarily [[glucose]] and [[amino acid]]s). |- |rowspan=2|S2<ref name=boron743/> |- |rowspan=2|straight |- |S3<ref name=boron743/> |- | [[Loop of Henle]] | colspan="3" | The loop of Henle (sometimes known as the nephron loop) is a tube, it is often u-shaped in diagrams for simplicity but in reality it looks more like one loop of a coil (hence, 'loop'). It extends from the proximal tube and it consists of a descending limb and ascending limb. It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla, and then returns to the cortex to empty into the distal convoluted tubule. Its primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop. |- | rowspan="2"| | ''[[Descending limb of loop of Henle|Descending Limb]]'' |colspan="2" | Its descending limb is permeable to water but completely impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium.<BR>As the filtrate descends deeper into the [[Tonicity#Hypertonicity|hypertonic]] interstitium of the renal medulla, water flows freely out of the descending limb by [[osmosis]] until the tonicity of the filtrate and interstitium equilibrate. Longer descending limbs allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more hypertonic than shorter limbs. |- | ''[[Ascending limb of Henle's loop|Ascending Limb]]'' * [[Thin ascending limb of loop of Henle|thin segment]] * [[Thick ascending limb of loop of Henle|thick segment]] |colspan="2" | Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a critical feature of the [[countercurrent exchange]] mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows [[Tonicity#Hypotonicity|hypotonic]] since it has lost much of its sodium content. This hypotonic filtrate is passed to the [[distal convoluted tubule]] in the renal cortex. |- | [[Distal convoluted tubule]] | colspan="3" | The distal convoluted tubule is not similar to the proximal convoluted tubule in structure and function. Cells lining the tubule have numerous [[mitochondria]] to produce enough energy ([[adenosine triphosphate|ATP]]) for [[active transport]] to take place. Much of the ion transport taking place in the distal convoluted tubule is regulated by the [[endocrine system]]. In the presence of [[parathyroid hormone]], the distal convoluted tubule reabsorbs more calcium and excretes more phosphate. When [[aldosterone]] is present, more sodium is reabsorbed and more potassium excreted. [[Atrial natriuretic peptide]] causes the distal convoluted tubule to excrete more sodium. In addition, the tubule also secernates [[hydrogen]] and [[ammonium]] to regulate [[pH]]. |} After traveling the length of the distal convoluted tubule, only about 1% of water remains, and the remaining salt content is negligible. ===Collecting duct system=== {{main|Collecting duct system}} Each distal convoluted tubule delivers its filtrate to a [[collecting duct system|system of collecting ducts]], the first segment of which is the [[collecting tubule]]. The collecting duct system begins in the renal cortex and extends deep into the medulla. As the urine travels down the collecting duct system, it passes by the medullary interstitium which has a high sodium concentration as a result of the loop of Henle's [[countercurrent multiplier system]]. Though the collecting duct is normally impermeable to water, it becomes permeable in the presence of [[antidiuretic hormone]] (ADH). ADH affects the function of [[aquaporins]], resulting in the reabsorption of water molecules as it passes through the collecting duct. Aquaporins are membrane proteins that selectively conduct water molecules while preventing the passage of ions and other solutes. As much as three-fourths of the water from urine can be reabsorbed as it leaves the collecting duct by osmosis. Thus the levels of ADH determine whether urine will be concentrated or diluted. An increase in ADH is an indication of [[dehydration]], while water sufficiency results in low ADH allowing for diluted urine. Lower portions of the collecting duct are also permeable to [[urea]], allowing some of it to enter the medulla of the [[kidney]], thus maintaining its high ion concentration (which is very important for the nephron). Urine leaves the medullary collecting ducts through the [[renal papilla]], emptying into the [[renal calyces]], the [[renal pelvis]], and finally into the [[urinary bladder|bladder]] via the [[ureter]]. Because it has a different origin during the [[development of the urinary and reproductive organs]] than the rest of the nephron, the collecting duct is sometimes not considered a part of the nephron. Instead of originating from the metanephrogenic blastema, the collecting duct originates from the [[ureteric bud]]. ===Juxtaglomerular apparatus=== {{main|Juxtaglomerular apparatus}} The '''juxtaglomerular apparatus''' occurs near the site of contact between the thick ascending limb and the afferent arteriole. It contains three components: {| class="wikitable" | [[macula densa]] || a tightly-packed area with diverse population of cells, including the renin granular cells |- | [[juxtaglomerular cell]]s || specialized [[smooth muscle]] cells in the wall of the [[afferent arteriole]] |- | [[extraglomerular mesangial cells]] || couple to [[arterioles]] |} Juxtaglomerular cells are the site of [[renin]] synthesis and secretion and thus play a critical role in the [[renin-angiotensin system]]. ==Clinical relevance== Because of its importance in body fluid regulation, the nephron is a common target of drugs that treat high [[blood pressure]] and [[edema]]. These drugs, called [[diuretic]]s, inhibit the ability of the nephron to retain water, thereby increasing the amount of urine produced. ==References== <references/> ==Additional images== <gallery> Image:Gray1129.png|Distribution of blood vessels in cortex of kidney. Image:Gray1130.svg|[[Glomerulus]] is red; [[Bowman's capsule]] is white. </gallery> <gallery> Image:Kidney tubules.png|Kidney tissue Image:PhysiologieGlomérulaire.png|Glomerulus </gallery> ==See also== * [[Nephrology]] * [[Urology]] * [[Renal tubular acidosis]] ==External links== *[http://www.scienceaid.co.uk/biology/humans/homeostasis.html Science aid: Kidney Nephron] About the kidney nephron for teen understanding. *[http://pcwww.liv.ac.uk/~petesmif/petesmif/why%20do%20we%20need%20kidneys/index.htm Why Do We need Kidneys?] A Whistlestop tour of renal physiology for medical students. {{kidney}} {{renal physiology}} [[Category:Kidney]] [[Category:Urology]] [[ar:المصفاة الكلوية]] [[ca:Nefró]] [[cs:Nefron]] [[da:Nephron]] [[de:Nephron]] [[et:Nefron]] [[es:Nefrona]] [[fr:Rein#Néphron]] [[it:Nefrone]] [[he:נפרון]] [[mk:Нефрон]] [[nl:Nefron]] [[ja:ネフロン]] [[pl:Nefron]] [[pt:Nefrónio]] [[ru:Нефрон]] [[sk:Nefrón]] [[sl:Nefron]] [[fi:Nefroni]] [[tr:Nefron]] [[zh:腎元]]