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any of the small tubules that are the excretory units of the vertebrate kidney (同)uriniferous_tubule

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/06/09 17:06:43」(JST)

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Nephron (from 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, controls levels of electrolytes and metabolites, and regulates blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.^[1] In humans, a normal kidney contains 800,000 to 1.5 million nephrons.^[2]

Structure

Two general classes of nephrons are cortical nephrons and juxtamedullary nephrons, both of which are classified according to the length of their associated Loop of Henle and location of their renal corpuscle. All nephrons have their renal corpuscles in the cortex. Cortical nephrons have their Loop of Henle in the renal medulla near its junction with the renal cortex, while the Loop of Henle of juxtamedullary nephrons is located deep in the renal medulla; they are called juxtamedullary because their renal corpuscle is located near the medulla (but still in the cortex). The nomenclature for cortical nephrons varies, with some sources distinguishing between superficial cortical nephrons and midcortical nephrons, depending on where their corpuscle is located within the cortex.^[3]

The majority of nephrons are cortical. Cortical nephrons have a shorter loop of Henle compared to juxtamedullary nephrons. The longer loop of Henle in juxtamedullary nephrons create a hyperosmolar gradient that allows for the creation of concentrated urine.^[4]

Kidney nephron drawing with labels of the following: the Bowman's capsule, proximal convoluted tubule, loop of Henle, descending limb of loop of Henle, ascending limb of loop of Henle, distal convoluted tubule, and collecting duct.

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 solutes from the blood, delivering water and small solutes to the renal tubule for modification.^{[citation needed]}

Renal corpuscle

Composed of a glomerulus and the Bowman's capsule, the renal corpuscle (or Malphigian corpuscle) is the beginning of the nephron. It is the nephron's initial filtering component.^{[citation needed]}

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 the Bowman's capsule) passes into the efferent arteriole.The diameter of efferent arteriole is comparatively less than that of afferent arteriole, increasing the hydrostatic pressure in the glomerulus. It then moves into the vasa recta, which are only found in juxtamedullary nephrons and not cortical nephrons. The vasa recta are collecting capillaries intertwined with the loop of Henle through the interstitial space, in 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.^{[citation needed]}

The Bowman's capsule, also called the glomerular capsule, surrounds the glomerulus. It is composed of a visceral inner layer formed by specialized cells called podocytes, and a parietal outer layer composed of simple squamous epithelium. Fluids from blood in the glomerulus are filtered through the visceral layer of podocytes, and the resulting glomerular filtrate is further processed along the nephron to form urine.^{[citation needed]}

Renal tubule

The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus.^[5]^{[dead link]} After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron. ^[6]

The components of the renal tubule are:

Proximal convoluted tubule (lies in cortex and lined by simple cuboidal epithelium with brushed borders which help to increase the area of absorption greatly.)
Loop of Henle (hair-pin like i.e. U-shaped and lies in medulla)
- Descending limb of loop of Henle
- Ascending limb of loop of Henle
  - The ascending limb of loop of Henle is divided into 2 segments: Lower end of ascending limb is very thin and is lined by simple squamous epithelium. The distal portion of ascending limb is thick and is lined by simple cuboidal epithelium.
  - Thin ascending limb of loop of Henle
  - Thick ascending limb of loop of Henle (enters cortex and becomes DCT-distal convoluted tubule.)
Distal convoluted tubule

Function

Secretion and reabsorption of various solutes throughout the nephron.

The nephron carries out nearly all of the kidney's functions. Most of these functions concern the reabsorption and secretion of various solutes such as ions (e.g., sodium), carbohydrates (e.g., glucose), and amino acids (e.g., glutamate). Properties of the cells that line the nephron change dramatically along its length; consequently, each segment of the nephron has highly specialized functions.^{[citation needed]}

The proximal tubule as a part of the nephron can be divided into an initial convoluted portion and a following straight (descending) portion.^[7] 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 solutes (primarily glucose and amino acids).^{[citation needed]}

The loop of Henle, also called the nephron loop or the loop of Hundley, is a U-shaped tube that extends from the proximal tubule. 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 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 concentrate the salt in the interstitium, the tissue surrounding the loop.^{[citation needed]}

Considerable differences aid in distinguishing the descending and ascending limbs of the loop of Henle. The descending limb is permeable to water and noticeably less impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium. As the filtrate descends deeper into the 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.^{[citation needed]}

Unlike the descending limb, the Thin ascending limb of loop of Henle 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 hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to the distal convoluted tubule in the renal cortex.^{[citation needed]}

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 (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. Atrial natriuretic peptide causes the distal convoluted tubule to secrete more sodium. In addition, the tubule also secretes hydronium and ammonium ions to regulate pH.^{[citation needed]}

Collecting duct system

Each distal convoluted tubule delivers its filtrate to a 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 multiplier system.^{[citation needed]}

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 low ADH allowing for diluted urine.^{[citation needed]}

Lower portions of the collecting organ are also permeable to urea, allowing some of it to enter the medulla of the kidney, thus maintaining its high concentration (which is very important for the nephron).^{[citation needed]}

Urine leaves the medullary collecting ducts through the renal papillae, emptying into the renal calyces, the renal pelvis, and finally into the urinary bladder via the ureter.^{[citation needed]}

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.^{[citation needed]}

Juxtaglomerular apparatus

The juxtaglomerular apparatus is a specialized region of the nephron responsible for production and secretion of the enzyme renin, involved in the renin-angiotensin system. This apparatus occurs near the site of contact between the thick ascending limb and the afferent arteriole. It contains three components: the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells.^{[citation needed]}

Clinical significance

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 diuretics, inhibit the ability of the nephron to retain electrolytes (and consequently water), thereby increasing the amount of urine produced. However, this clinical significance raises many concerns about treatments to such medical conditions. Because diuretics are drugs that stimulate the body's metabolism to higher intensities, primarily stimulating the heart, the clinical significance creates minor controversy considering how individuals with high blood pressure would avoid introducing medications that raise their already high blood pressure.

Additional images

Distribution of blood vessels in cortex of kidney. (Although the figure labels the efferent vessel as a vein, it is actually an arteriole.)
Glomerulus is red; Bowman's capsule is white.
Kidney tissue
Glomerulus
This images shows the cells that make up a nephron in the kidneys. The following parts are seen: Lumen of a distal convoluted tubule (DCT), Macula densa, juxtaglomerular cells, lacis cells, mesangial cells, podocytes—visceral layer of Bowman’s capsule, parietal layer of Bowman’s capsule, lumen of a glomerular capillary, and the urinary space.
This image shows the types of cells present in the glomerulus part of a kidney nephron. Podocytes, Endothelial cells, and Glomerular mesangial cell are present.

References

^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. Cite uses deprecated parameters (help)
^ Guyton, Arthur C.; Hall, John E. (2006). Textbook of Medical Physiology. Philadelphia: Elsevier Saunders. p. 310. ISBN 0-7216-0240-1.
^ Physiology at MCG 7/7ch03/7ch03p16
^ Jameson, J. Larry & Loscalzo, Joseph (2010). Harrison's Nephrology and Acid-Base Disorders. McGraw-Hill Professional. p. 3. ISBN 978-0-07-166339-7.
^ 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. Accessed on: March 6, 2007.
^ Hook, Jerry B. & Goldstein, Robin S. (1993). Toxicology of the Kidney. Raven Press. p. 8. ISBN 0-88167-885-6.
^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 743. ISBN 1-4160-2328-3.

UpToDate Contents

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1. Chapter 1A：腎機能の概要 chapter 1a introduction to renal function
2. Chapter 9B：腎臓の水分排泄と再吸収 chapter 9b renal water excretion and reabsorption
3. 利尿剤の作用機序 mechanism of action of diuretics
4. Chapter 5A：遠位尿細管と接合部分 chapter 5a distal tubule and connecting segment
5. Chapter 12B：腎臓のカリウム排泄 chapter 12b renal potassium excretion

English Journal

The function of TRPS1 in the development and differentiation of bone, kidney, and hair follicles.

Gai Z, Gui T, Muragaki Y.SourceFirst Department of Pathology, Wakayama Medical University School of Medicine, Wakayama, Japan.
Histology and histopathology." abstractLink="yes" alsec="jour" alterm="Histol Histopathol.2011 Jul;26(7):915-21.
TRPS1 is a gene involved in Tricho-rhino-phalangeal syndrome (TRPS), an autosomal dominant skeletal disorder. TRPS1 encodes a GATA-type transcription factor that has nine zinc-finger motifs. A variety of mutations in TRPS1 including deletions and insertions, have been found in patients with TRPS typ
PMID 21630221

The sexual segment of hemidactylus turcicus and the evolution of sexual segment location in squamata.

Rheubert JL, Murray CM, Siegel DS, Babin J, Sever DM.SourceDepartment of Biological Sciences, Southeastern Louisiana University, SLU 10736, Hammond, Louisiana 70402. Justin.Rheubert@selu.edu.
Journal of morphology." abstractLink="yes" alsec="jour" alterm="J Morphol.2011 Jul;272(7):802-13. doi: 10.1002/jmor.10949. Epub 2011 Apr 28.
The kidneys of the Mediterranean Gecko, Hemidactylus turcicus (Gekkonidae), were investigated using light and electron microscopy with the primary focus placed on morphology of the sexual segment of the kidney. The nephrons of male H. turcicus are composed of five distinct regions: 1) a renal corpus
PMID 21538471

Japanese Journal

Quantitative and Qualitative Urinary Cellular Patterns Correlate with Progression of Murine Glomerulonephritis

Kimura Junpei,Ichii Osamu,Otsuka Saori,Kanazawa Tomonori,Namiki Yuka,Hashimoto Yoshiharu,Kon Yasuhiro
PLoS One 6(1), e16472, 2011-01-31
… It has recently been reported that nephron constituent cells are dropped into the urine in several pathological conditions associated with renal functional deterioration. …
NAID 120002834737

小径腎癌に対する腎温存手術

小林泰之,上原慎也,渡辺豊彦 [他]
西日本泌尿器科 73(2), 51-57, 2011-02
NAID 40018277868

Related Pictures

★リンクテーブル★

リンク元

「ネフロン」

Renal tubule
Latin	tubulus renalis
Gray's	p.1223

Nephron
	Nephron of the kidney. The labelled parts are 1. Glomerulus, 2. Efferent arteriole, 3. Bowman's capsule, 4. Proximal convoluted tubule, 5. Cortical collecting duct, 6. Distal convoluted tubule, 7. Loop of Henle, 8. Duct of Bellini, 9. Peritubular capillaries, 10. Arcuate vein, 11. Arcuate artery, 12. Afferent arteriole, 13. Juxtaglomerular apparatus.
Latin	Nephroneum
Gray's	p.1221
System	Urinary system
Precursor	Metanephric blastema (intermediate mesoderm)
MeSH	Nephrons

　　[★]

英: nephron (Z)
ラ: nephronum
同: 腎単位
関: 皮質ネフロン、傍髄質ネフロン

図：SP.776
腎臓の機能的な単位
ネフロン = 腎小体 + 尿細管
ヒトのネフロンの数は片側の腎臓で約100万個といわれているが、ネフロンの数は個人差がある。
ネフロンの数は平均血圧と反比例している

皮質ネフロンと傍髄質ネフロンとの解剖的な違い

	皮質ネフロン	傍髄質ネフロン
腎小体	皮質に存在	髄質に近い皮質
直血管	×	○
ヘンレループの下行脚	○	○
ヘンレループの細い上行脚	×	○
ヘンレループの太い上行脚	○	○

皮質ネフロンと傍髄質ネフロンとの機能的な違い

ネフロンにより貢献する役割が異なる
皮質ネフロン　：不揮発酸やK分泌
傍髄質ネフロン：水分再吸収・Na再吸収

[1] Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. Cite uses deprecated parameters (help)

[guyton11-2] Guyton, Arthur C.; Hall, John E. (2006). Textbook of Medical Physiology. Philadelphia: Elsevier Saunders. p. 310. ISBN 0-7216-0240-1.

[3] Physiology at MCG 7/7ch03/7ch03p16

[4] Jameson, J. Larry & Loscalzo, Joseph (2010). Harrison's Nephrology and Acid-Base Disorders. McGraw-Hill Professional. p. 3. ISBN 978-0-07-166339-7.

[5] 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. Accessed on: March 6, 2007.

[6] Hook, Jerry B. & Goldstein, Robin S. (1993). Toxicology of the Kidney. Raven Press. p. 8. ISBN 0-88167-885-6.

[boron743-7] Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 743. ISBN 1-4160-2328-3.

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