Editing Nephron (section)

== Functions ==
[[File:2618 Nephron Secretion Reabsorption.jpg|thumb|Fig.3) Secretion and reabsorption of various substances throughout the nephron.]]
The nephron uses four mechanisms to convert blood into urine: filtration, reabsorption, secretion, and excretion.<ref name="junqueiras" />{{rp|395–396}} These apply to numerous substances. The structure and function of the epithelial cells lining the lumen change during the course of the nephron, and have segments named by their location and which reflects their different functions.

[[File:Kidney nephron molar transport diagram.svg|thumbnail|Fig.4) Diagram outlining movement of ions in nephron, with the collecting ducts on the right.]]
[[File:Complex proximal tubule with acid base.svg|thumb|Fig.5) Proximal tubule cell showing pumps involved in acid base balance, left is the lumen of tubule]]

===Proximal tubule===
The [[proximal tubule]] as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion.<ref name=boron743>{{cite book | vauthors = Boron WF |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |page=743 |isbn=978-1-4160-2328-9 |year=2005 }}</ref> Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including 80% of glucose, more than half of the filtered salt, water and all filtered [[organic compound|organic]] solutes (primarily [[glucose]] and [[amino acid]]s).<ref name="junqueiras" />{{rp|400–401}}

===Loop of Henle===
The [[loop of Henle]] is a U-shaped tube that extends from the proximal tubule. It consists of a descending limb and an ascending limb. It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla as the descending limb, and then returns to the cortex as the ascending limb to empty into the distal convoluted tubule. The primary role of the loop of Henle is to enable an organism to produce concentrated urine, not by increasing the tubular concentration, but by rendering the interstitial fluid hypertonic.<ref name="lote" />{{rp|67}}

Considerable differences aid in distinguishing the descending and ascending limbs of the loop of Henle. The [[Descending limb of loop of Henle|descending limb]] is permeable to water and noticeably less permeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the [[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. The hypertonicity of the medulla (and therefore concentration of urine) is determined in part by the size of the loops of Henle.<ref name="lote" />{{rp|76}}

Unlike the descending limb, the [[Ascending limb of loop of Henle|thick ascending limb]] 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 [[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.<ref name="lote" />{{rp|72}}

===Distal convoluted tubule===
The [[distal convoluted tubule]] has a different structure and function to that of the proximal convoluted tubule. 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 secretes more phosphate. When [[aldosterone]] is present, more sodium is reabsorbed and more potassium secreted. Ammonia is also absorbed during the selective reabsorption. [[Atrial natriuretic peptide]] causes the distal convoluted tubule to secrete more sodium.

=== Connecting tubule ===
A part of Distal nephron. This is the final segment of the tubule before it enters the collecting duct system. Water, some salts and nitrogenous waste like urea and creatinine are passed out to collecting tubule.

=== Collecting duct system ===
{{main|Collecting duct system}}
[[File:Gray1133.png|thumbnail|Fig.6) Cross-sectional histologic preparation showing (b)small connecting tubules with simple columnar epithelium and (a) large connecting tubules with simple cuboidal epithelium.]]

Each distal convoluted tubule delivers its filtrate to a [[collecting duct system|system of collecting ducts]], the first segment of which is the [[connecting 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 exchange#In the kidney|countercurrent multiplier system]].<ref name="lote" />{{rp|67}}

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]].<ref name="mitchell">{{cite book | vauthors = Mitchell B, Sharma R |title= Embriology | edition = 2nd |year=2009|publisher=Churchill Livingstone Elsevier}}</ref>{{rp|50–51}}

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-quarters 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 a decrease in ADH allowing for diluted urine.<ref name="junqueiras" />{{rp|406}}
[[Image:Kidney Nephron Cells.png|thumb|250px|Fig.7)
Cross-sectional diagram of the juxtaglomerular apparatus and adjacent structures: 1) top, yellow - distal convoluted tubule; 2) top, brown -macula densa cuboidal cells surrounding arterioles; 3) small blue cells - juxtaglomerular cells; 4) large blue cells - mesangial cells; 5) tan - podocytes lining [[Bowman's capsule]] adjacent to capillaries, and parietal layer of capsule, 6)center - five glomerular capillaries, and the 6)bottom, purple - exiting tubule. Structures (2), (3), and (4) constitute the juxtaglomerular apparatus.]]
Lower portions of the collecting organ are also permeable to [[urea]], allowing some of it to enter the medulla, thus maintaining its high concentration (which is very important for the nephron).<ref name="lote" />{{rp|73–74}}

Urine leaves the medullary collecting ducts through the [[renal papilla]]e, emptying into the [[Minor calyx|renal calyces]], the [[renal pelvis]], and finally into the [[urinary bladder]] via the [[ureter]].<ref name="junqueiras" />{{rp|406–407}}

=== Juxtaglomerular apparatus ===
{{main|Juxtaglomerular apparatus}}
The [[juxtaglomerular apparatus]] (JGA) is a specialized region associated with the nephron, but separate from it. It produces and secretes into the circulation the enzyme [[renin]] (angiotensinogenase), which cleaves [[angiotensinogen]] and results in the ten amino acid substance angiotensin-1 (A-1).  A-1 is then converted to angiotensin-2, a potent vasoconstrictor, by removing two amino acids: this is accomplished by angiotensin converting enzyme (ACE). This sequence of events is referred to as the [[renin–angiotensin system]] (RAS) or renin-angiotensin-aldosterone system (RAAS). The JGA is located between the thick ascending limb and the afferent arteriole. It contains three components: the [[macula densa]], [[juxtaglomerular cell]]s, and [[extraglomerular mesangial cell]]s.<ref name="junqueiras" />{{rp|404}}