Coordinates	57°37′″N39°51′″N
latin	ren
subject	253
page	1215
image2	Kidnies_lamb.jpg
caption2	Lamb kidneys
artery	renal artery
vein	renal vein
nerve	renal plexus
name	Kidney
number	A05.810.453
	five/000056330
id	Kidney }}

The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid-base balance, and regulation of blood pressure (via maintaining salt and water balance). They serve the body as a natural filter of the blood, and remove wastes which are diverted to the urinary bladder. In producing urine, the kidneys excrete wastes such as urea and ammonium; the kidneys also are responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol, erythropoietin, and the enzyme renin.

Located at the rear of the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes urine into a ureter, itself a paired structure that empties into the urinary bladder.

Renal physiology is the study of kidney function, while nephrology is the medical specialty concerned with kidney diseases. Diseases of the kidney are diverse, but individuals with kidney disease frequently display characteristic clinical features. Common clinical conditions involving the kidney include the nephritic and nephrotic syndromes, renal cysts, acute kidney injury, chronic kidney disease, urinary tract infection, nephrolithiasis, and urinary tract obstruction. Various cancers of the kidney exist; the most common adult renal cancer is renal cell carcinoma. Cancers, cysts, and some other renal conditions can be managed with removal of the kidney, or nephrectomy. When renal function, measured by glomerular filtration rate, is persistently poor, dialysis and kidney transplantation may be treatment options. Although they are not severely harmful, kidney stones can be a pain and a nuisance. The removal of kidney stones includes sound wave treatment, which breaks up the stones into smaller pieces which are then passed through the urinary tract. One common symptom of kidney stones is a sharp pain in the medial/lateral segments of the lower back.

Anatomy

Location

In humans the kidneys are located in the abdominal cavity, more specifically in the paravertebral gutter and lie in a retroperitoneal position at a slightly oblique angle. There are two, one on each side of the spine. The asymmetry within the abdominal cavity caused by the liver typically results in the right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right. The left kidney is approximately at the vertebral level T12 to L3, and the right slightly lower. The right kidney sits just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen. Resting on top of each kidney is an adrenal gland. The upper (cranial) parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia. Each adult kidney weighs between 125 and 170 grams in males and between 115 and 155 grams in females. The left kidney is typically slightly larger than the right.

Structure

[[File:KidneyStructures PioM.svg|thumb|250px|left| 1. Renal pyramid • 2. Interlobular artery • 3. Renal artery • 4. Renal vein 5. Renal hilum • 6. Renal pelvis • 7. Ureter • 8. Minor calyx • 9. Renal capsule • 10. Inferior renal capsule • 11. Superior renal capsule • 12. Interlobular vein • 13. Nephron • 14. Minor calyx • 15. Major calyx • 16. Renal papilla • 17. Renal column ]]

The kidney has a bean-shaped structure, each kidney has a convex and concave surface. The concave surface, the renal hilum, is the point at which the renal artery enters the organ, and the renal vein and ureter leave. The kidney is surrounded by tough fibrous tissue, the renal capsule, which is itself surrounded by perinephric fat, renal fascia (of Gerota) and paranephric fat. The anterior (front) border of these tissues is the peritoneum, while the posterior (rear) border is the transversalis fascia.

The superior border of the right kidney is adjacent to the liver; and the spleen, for the left border. Therefore, both move down on inhalation.

The kidney is approximately 11–14 cm in length, 6 cm wide and 4 cm thick.

The substance, or parenchyma, of the kidney is divided into two major structures: superficial is the renal cortex and deep is the renal medulla. Grossly, these structures take the shape of 8 to 18 cone-shaped renal lobes, each containing renal cortex surrounding a portion of medulla called a renal pyramid (of Malpighi). Between the renal pyramids are projections of cortex called renal columns (of Bertin). Nephrons, the urine-producing functional structures of the kidney, span the cortex and medulla. The initial filtering portion of a nephron is the renal corpuscle, located in the cortex, which is followed by a renal tubule that passes from the cortex deep into the medullary pyramids. Part of the renal cortex, a medullary ray is a collection of renal tubules that drain into a single collecting duct.

The tip, or papilla, of each pyramid empties urine into a minor calyx, minor calyces empty into major calyces, and major calyces empty into the renal pelvis, which becomes the ureter.

Blood supply

The kidneys receive blood from the renal arteries, left & right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output.

Each renal artery branches into segmental arteries, dividing further into interlobar arteries which penetrate the renal capsule and extend through the renal columns between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries that run through the boundary of the cortex and the medulla. Each arcuate artery supplies several interlobular arteries that feed into the afferent arterioles that supply the glomeruli.

The interstitum (or interstitium) is the functional space in the kidney beneath the individual filters (glomeruli) which are rich in blood vessels. The interstitum absorbs fluid recovered from urine. Various conditions can lead to scarring and congestion of this area, which can cause kidney dysfunction and failure.

After filtration occurs the blood moves through a small network of venules that converge into interlobular veins. As with the arteriole distribution the veins follow the same pattern, the interlobular provide blood to the arcuate veins then back to the interlobar veins which come to form the renal vein exiting the kidney for transfusion for blood.

===Histology===

Renal histology studies the structure of the kidney as viewed under a microscope. Various distinct cell types occur in the kidney, including:

Kidney glomerulus parietal cell

Kidney glomerulus podocyte

Kidney proximal tubule brush border cell

Loop of Henle thin segment cell

Thick ascending limb cell

Kidney distal tubule cell

Kidney collecting duct cell

Interstitial kidney cells

Innervation

The kidney and nervous system communicate via the renal plexus, whose fibers course along the renal arteries to reach the kidney. Input from the sympathetic nervous system triggers vasoconstriction in the kidney, thereby reducing renal blood flow. The kidney is not thought to receive input from the parasympathetic nervous system. Sensory input from the kidney travels to the T10-11 levels of the spinal cord and is sensed in the corresponding dermatome. Thus, pain in the flank region may be referred from the kidney.

Functions

The kidney participates in whole-body homeostasis, regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure. The kidney accomplishes these homeostatic functions both independently and in concert with other organs, particularly those of the endocrine system. Various endocrine hormones coordinate these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others.

Many of the kidney's functions are accomplished by relatively simple mechanisms of filtration, reabsorption, and secretion, which take place in the nephron. Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are filtered from the blood to make an ultrafiltrate that will eventually become urine. The kidney generates 180 liters of filtrate a day, while reabsorbing a large percentage, allowing for the generation of only approximately 2 liters of urine. Reabsorption is the transport of molecules from this ultrafiltrate and into the blood. Secretion is the reverse process, in which molecules are transported in the opposite direction, from the blood into the urine.

Excretion of wastes

The kidneys excrete a variety of waste products produced by metabolism. These include the nitrogenous wastes called "urea", from protein catabolism, as well as uric acid, from nucleic acid metabolism. Formation of urine is also the function of the kidney.

Acid-base homeostasis

Two organ systems, the kidneys and lungs, maintain acid-base homeostasis, which is the maintenance of pH around a relatively stable value. The lungs contribute to acid-base homeostasis by regulating bicarbonate (HCO₃^-) concentration. The kidneys have two very important roles in the maintaining of the acid-base balance: to reabsorb bicarbonate from and to excrete hydrogen ions into urine

Osmolality regulation

Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone (ADH), resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.

ADH binds to principal cells in the collecting duct that translocate aquaporins to the membrane allowing water to leave the normally impermeable membrane and be reabsorbed into the body by the vasa recta, thus increasing the plasma volume of the body.

There are two systems that create a hyperosmotic medulla and thus increase the body plasma volume: Urea recycling and the 'single effect.'

Urea is usually excreted as a waste product from the kidneys. However, when plasma blood volume is low and ADH is released the aquaporins that are opened are also permeable to urea. This allows urea to leave the collecting duct into the medulla creating a hyperosmotic solution that 'attracts' water. Urea can then re-enter the nephron and be excreted or recycled again depending on whether ADH is still present or not.

The 'Single effect' describes the fact that the ascending thick limb of the loop of Henle is not permeable to water but is permeable to NaCl. This allows for a countercurrent exchange system whereby the medulla becomes increasingly concentrated, but at the same time setting up an osmotic gradient for water to follow should the aquaporins of the collecting duct be opened by ADH.

Blood pressure regulation

Long-term regulation of blood pressure predominantly depends upon the kidney. This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration. Although the kidney cannot directly sense blood pressure, changes in the delivery of sodium and chloride to the distal part of the nephron alter the kidney's secretion of the enzyme renin. When the extracellular fluid compartment is expanded and blood pressure is high, the delivery of these ions is increased and renin secretion is decreased. Similarly, when the extracellular fluid compartment is contracted and blood pressure is low, sodium and chloride delivery is decreased and renin secretion is increased in response.

Renin is the first in a series of important chemical messengers that comprise the renin-angiotensin system. Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone. Each hormone acts via multiple mechanisms, but both increase the kidney's absorption of sodium chloride, thereby expanding the extracellular fluid compartment and raising blood pressure. When renin levels are elevated, the concentrations of angiotensin II and aldosterone increase, leading to increased sodium chloride reabsorption, expansion of the extracellular fluid compartment, and an increase in blood pressure. Conversely, when renin levels are low, angiotensin II and aldosterone levels decrease, contracting the extracellular fluid compartment, and decreasing blood pressure.

Hormone secretion

The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Part of the renin-angiotensin-aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels.

Development

The mammalian kidney develops from intermediate mesoderm. Kidney development, also called ''nephrogenesis'', proceeds through a series of three successive phases, each marked by the development of a more advanced pair of kidneys: the pronephros, mesonephros, and metanephros.

Evolutionary adaptation

Kidneys of various animals show evidence of evolutionary adaptation and have long been studied in ecophysiology and comparative physiology. Kidney morphology, often indexed as the relative medullary thickness, is associated with habitat aridity among species of mammals.

Etymology

Medical terms related to the kidneys commonly use terms such as ''renal'' and the prefix ''nephro-''. The adjective ''renal'', meaning related to the kidney, is from the Latin ''rēnēs'', meaning kidneys; the prefix ''nephro-'' is from the Ancient Greek word for kidney, ''nephros (νεφρός)''. For example, surgical removal of the kidney is a ''nephrectomy'', while a reduction in kidney function is called ''renal dysfunction''.

Diseases and disorders

Congenital

Congenital hydronephrosis

Congenital obstruction of urinary tract

Duplex kidneys, or double kidneys, occur in approximately 1% of the population. This occurrence normally causes no complications but can occasionally cause urine infections.

Duplicated ureter occurs in approximately one in 100 live births

Horseshoe kidney occurs in approximately one in 400 live births

Polycystic kidney disease

*Autosomal dominant polycystic kidney disease afflicts patients later in life. Approximately one in 1000 people will develop this condition

*Autosomal recessive polycystic kidney disease is far less common, but more severe, than the dominant condition. It is apparent ''in utero'' or at birth.

Renal agenesis. Failure of one kidney to form occurs in approximately one in 750 live births. Failure of both kidneys to form is invariably fatal.

Renal dysplasia

Unilateral small kidney

Multicystic dysplastic kidney occurs in approximately one in every 2400 live births

Ureteropelvic Junction Obstruction or UPJO; although most cases appear congenital, some appear to be an acquired condition

Acquired

Diabetic nephropathy

Glomerulonephritis

Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.

Interstitial nephritis

Kidney stones (nephrolithiasis) are a relatively common and particularly painful disorder.

Kidney tumors

*Wilms tumor

*Renal cell carcinoma

Lupus nephritis

Minimal change disease

In nephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.

Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.

Renal failure

* Acute renal failure

* Stage 5 Chronic Kidney Disease

Kidney Failure

Generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive. Only when the amount of functioning kidney tissue is greatly diminished does one develop chronic kidney disease. Renal replacement therapy, in the form of dialysis or kidney transplantation, is indicated when the glomerular filtration rate has fallen very low or if the renal dysfunction leads to severe symptoms.

In other animals

In the majority of vertebrates, the mesonephros persists into the adult, albeit usually fused with the more advanced metanephros; only in amniotes is the mesonephros restricted to the embryo. The kidneys of fish and amphibians are typically narrow, elongated organs, occupying a significant portion of the trunk. The collecting ducts from each cluster of nephrons usually drain into an ''archinephric duct'', which is homologous with the vas deferens of amniotes. However, the situation is not always so simple; in cartilaginous fish and some amphibians, there is also a shorter duct, similar to the amniote ureter, which drains the posterior (metanephric) parts of the kidney, and joins with the archinephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult.

In the most primitive vertebrates, the hagfish and lampreys, the kidney is unusually simple: it consists of a row of nephrons, each emptying directly into the archinephric duct. Invertebrates may possess excretory organs that are sometimes referred to as "kidneys", but, even in ''Amphioxus'', these are never homologous with the kidneys of vertebrates, and are more accurately referred to by other names, such as nephridia.

The kidneys of reptiles consist of a number of lobules arranged in a broadly linear pattern. Each lobule contains a single branch of the ureter in its centre, into which the collecting ducts empty. Reptiles have relatively few nephrons compared with other amniotes of a similar size, possibly because of their lower metabolic rate.

Birds have relatively large, elongated kidneys, each of which is divided into three or more distinct lobes. The lobes consists of several small, irregularly arranged, lobules, each centred on a branch of the ureter. Birds have small glomeruli, but about twice as many nephrons as similarly sized mammals.

The human kidney is fairly typical of that of mammals. Distinctive features of the mammalian kidney, in comparison with that of other vertebrates, include the presence of the renal pelvis and renal pyramids, and of a clearly distinguishable cortex and medulla. The latter feature is due to the presence of elongated loops of Henle; these are much shorter in birds, and not truly present in other vertebrates (although the nephron often has a short ''intermediate segment'' between the convoluted tubules). It is only in mammals that the kidney takes on its classical "kidney" shape, although there are some exceptions, such as the multilobed reniculate kidneys of cetaceans.

History

The Latin term ''renes'' is related to the English word "reins", a synonym for the kidneys in Shakespearean English (e.g. ''Merry Wives of Windsor'' 3.5), which was also the time the King James Version was translated. Kidneys were once popularly regarded as the seat of the conscience and reflection, and a number of verses in the Bible (e.g. Ps. 7:9, Rev. 2:23) state that God searches out and inspects the kidneys, or "reins", of humans. Similarly, the Talmud (''Berakhoth'' 61.a) states that one of the two kidneys counsels what is good, and the other evil.

Animal kidneys as food

The kidneys of animals can be cooked and eaten by humans (along with other offal).

Kidneys are usually grilled or sautéed, but in more complex dishes they are stewed with a sauce that will improve their flavor. In many preparations, kidneys are combined with pieces of meat or liver, as in mixed grill or ''meurav Yerushalmi''. Among the most reputed kidney dishes, the British steak and kidney pie, the Swedish ''hökarpanna'' (pork and kidney stew), the French ''rognons de veau sauce moutarde'' (veal kidneys in mustard sauce) and the Spanish ''riñones al Jerez'' (kidneys stewed in sherry sauce), deserve special mention.

See also

Artificial kidney

Holonephros

Organ donation

Organ harvesting

Pelvic kidney

World Kidney Day

References

External links

The NephCure Foundation offers educational materials on the kidney diseases/conditions Nephrotic Syndrome and FSGS

The Kidney Foundation of Canada

electron microscopic images of the kidney (Dr. Jastrow's EM-Atlas)

European Renal Genome project kidney function tutorial

Kidney Foundation of Canada kidney disease information

Renal Fellow Network: Structure & Function of Other Animals' Kidneys

Kidney Diseases

Kidney Information

Animated Presentatin on Kidney Function

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Anthony Atala, M.D., is the W.H. Boyce Professor and Director of the Wake Forest Institute for Regenerative Medicine, and Chair of the Department of Urology at the Wake Forest University School of Medicine in North Carolina. Regenerative medicine is "a practice that aims to refurbish diseased or damaged tissue using the body's own healthy cells."

Biography

Atala was born in Peru in 1958, grew up in Boca Raton, Florida. Atala attended the University of Miami and has an undergraduate degree in Psychology. He went to medical school at the University of Louisville where he also completed his residency in urology. He was a fellow at the Harvard Medical School affiliated Children's Hospital Boston from 1990-1992 where he trained under world renowned pediatric urologic surgeons Alan Retik and Hardy Hendren. He served as the Director of the Laboratory for Tissue Engineering and Cellular Therapeutics at Children's Hospital Boston. His work there involved growing human tissues and organs to replace those damaged by disease or defects. This work became important due to shortages in the organ-donor program.

Dr. Atala continued his work in Tissue engineering and Printable organs at Wake Forest University in 2004. Dr. Atala led the team that developed the first lab-grown organ, a bladder, to be implanted into a human.

Aside from his ground breaking research, Dr. Atala also tends to clinical and administrative responsibilities. He operates regularly and runs a busy clinic at the North Carolina Baptist Hospital, the teaching hospital of the Wake Forest University School of Medicine. He is also in charge of a rapidly growing urology department and has been successful in recruiting other renowned faculty to Wake Forest such as Gopal Badlani, James Yoo and Gordon McLorie.

Along with Harvard University researchers and as described in the journal Nature Biotechnology, he has announced that stem cells with enormous potential can be harvested from the amniotic fluid of pregnant women. These amniotic stem cells are pluripotent, meaning they can be manipulated to differentiate into various types of mature cells that make up nerve, muscle, bone, and other tissues while avoiding the problems of tumor formation and ethical concerns that are associated with embryonic stem cells.

With respect to the amniotic fluid stem cells ("AFS" cells), Dr. Atala said the following:

"The cells come from the fetus, which breathes and sucks in, then excretes, the amniotic fluid throughout pregnancy;"

"Like embryonic stem cells, they appear to thrive in lab dishes for years, while normal cells, called somatic cells, die after a time ;"

"They are easier to grow than human embryonic stem cells. And, unlike embryonic stem cells, they do not form a type of benign tumour called a teratoma;" and

"A bank with 100,000 specimens of the amniotic stem cells theoretically could supply 99 per cent of the US population with perfect genetic matches for transplants."

Dr. Atala's work was seized on by opponents of the Embryonic Stem Cell Research Bill (a part of the 100-Hour Plan of the Democratic Party in the 110th United States Congress) as a more moral alternative. He wrote a letter saying, ''inter alia'', "Some may be interpreting my research as a substitute for the need to pursue other forms of regenerative medicine therapies, such as those involving embryonic stem cells. I disagree with that assertion."

References

External links

The Wake Forest Institute for Regenerative Medicine

A Tissue Engineer Sows Cells and Grows Organs, Ann Parson, ''The New York Times'', 11 July 2006

Regenerative Medicine: 2012, ''The New Yorker'', 7 May 2007

Interview with Dr. A. Atala, NDT Educational

;Peer-reviewed journals

Isolation of amniotic stem cell lines with potential for therapy

Therapeutic Advances in Urology

Category:1958 births Category:Living people Category:Stem cells Category:Peruvian emigrants to the United States Category:University of Miami alumni Category:University of Louisville alumni Category:Harvard Medical School alumni Category:Developmental biologists Category:American scientists Category:Wake Forest University faculty

da:Anthony Atala ja:アンソニー・アタラ