Kidney

Kidney

Overview
The kidneys, organs
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 with several functions, serve essential regulatory roles in most animals, including vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s and some invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s. They are essential in the urinary system
Urinary system
The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder and the urethra.-Kidney:...

 and also serve homeostatic
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

 functions such as the regulation of electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s, maintenance of acid–base balance, and regulation of blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 (via maintaining salt and water balance). They serve the body as a natural filter of the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

, and remove wastes which are diverted to the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

.
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Encyclopedia
The kidneys, organs
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

 with several functions, serve essential regulatory roles in most animals, including vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s and some invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s. They are essential in the urinary system
Urinary system
The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder and the urethra.-Kidney:...

 and also serve homeostatic
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

 functions such as the regulation of electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s, maintenance of acid–base balance, and regulation of blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 (via maintaining salt and water balance). They serve the body as a natural filter of the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

, and remove wastes which are diverted to the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

. In producing urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

, the kidneys excrete wastes such as urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 and ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

, and they are also responsible for the reabsorption of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s. The kidneys also produce hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

s including calcitriol
Calcitriol
Calcitriol , also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3, is the hormonally active form of vitamin D with three hydroxyl groups...

, erythropoietin
Erythropoietin
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...

, and the enzyme renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

.

Located at the rear of the abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

 in the retroperitoneum
Retroperitoneum
The retroperitoneal space is the anatomical space in the abdominal cavity behind the peritoneum. It has no specific delineating anatomical structures...

, the kidneys receive blood from the paired renal arteries
Renal artery
The renal arteries normally arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood. Each is directed across the crus of the diaphragm, so as to form nearly a right angle with the aorta....

, and drain into the paired renal vein
Renal vein
The renal veins are veins that drain the kidney. They connect the kidney to the inferior vena cava.It is usually singular to each kidney, except in the condition "multiple renal veins".It also divides into 2 divisions upon entering the kidney:...

s. Each kidney excretes urine into a ureter
Ureter
In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....

, itself a paired structure that empties into the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

.

Renal physiology
Renal physiology
Renal physiology is the study of the physiology of the kidney. This encompasses all functions of the kidney, including reabsorption of glucose, amino acids, and other small molecules; regulation of sodium, potassium, and other electrolytes; regulation of fluid balance and blood pressure;...

 is the study of kidney function, while nephrology
Nephrology
Nephrology is a branch of internal medicine and pediatrics dealing with the study of the function and diseases of the kidney.-Scope of the specialty:...

 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
Nephritic syndrome
Nephritic syndrome is a collection of signs associated with disorders affecting the kidneys, more specifically glomerular disorders. It is characterized by having small pores in the podocytes of the glomerulus, large enough to permit proteins and red blood cells to pass into the urine...

 and nephrotic syndrome
Nephrotic syndrome
Nephrotic syndrome is a nonspecific disorder in which the kidneys are damaged, causing them to leak large amounts of protein from the blood into the urine....

s, renal cyst
Renal cyst
A renal cyst is a fluid collection in the kidney. There are several types based on the Bosniak classification. The majority are benign, simple cysts that can be monitored and not intervened upon...

s, acute kidney injury, chronic kidney disease, urinary tract infection
Urinary tract infection
A urinary tract infection is a bacterial infection that affects any part of the urinary tract. Symptoms include frequent feeling and/or need to urinate, pain during urination, and cloudy urine. The main causal agent is Escherichia coli...

, nephrolithiasis, and urinary tract obstruction. Various cancers of the kidney exist; the most common adult renal cancer is renal cell carcinoma
Renal cell carcinoma
Renal cell carcinoma is a kidney cancer that originates in the lining of the proximal convoluted tubule, the very small tubes in the kidney that filter the blood and remove waste products. RCC is the most common type of kidney cancer in adults, responsible for approximately 80% of cases...

. Cancers, cysts, and some other renal conditions can be managed with removal of the kidney, or nephrectomy
Nephrectomy
Nephrectomy is the surgical removal of a kidney.-History:The first successful nephrectomy was performed by the German surgeon Gustav Simon on August 2, 1869 in Heidelberg. Simon practiced the operation beforehand in animal experiments...

. When renal function, measured by glomerular filtration rate, is persistently poor, dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...

 and kidney transplantation
Kidney transplantation
Kidney transplantation or renal transplantation is the organ transplant of a kidney into a patient with end-stage renal disease. Kidney transplantation is typically classified as deceased-donor or living-donor transplantation depending on the source of the donor organ...

 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 to break 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.

Location



In humans the kidneys are located in the abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

, 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
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 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
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

, the left below the diaphragm and posterior to the spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

. Resting on top of each kidney is an adrenal gland
Adrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...

. The upper (cranial) parts of the kidneys are partially protected by the eleventh and twelfth rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

s, and each whole kidney and adrenal gland are surrounded by two layers of fat (the perirenal and pararenal fat) and the renal fascia
Renal fascia
The renal fascia is a layer of connective tissue encapsulating the kidneys. The deeper layers below the renal fascia are, in order, the perinephric fat , renal capsule, and finally the parenchyma of the renal cortex...

. 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



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
Renal artery
The renal arteries normally arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood. Each is directed across the crus of the diaphragm, so as to form nearly a right angle with the aorta....

 enters the organ, and the renal vein
Renal vein
The renal veins are veins that drain the kidney. They connect the kidney to the inferior vena cava.It is usually singular to each kidney, except in the condition "multiple renal veins".It also divides into 2 divisions upon entering the kidney:...

 and ureter
Ureter
In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....

 leave. The kidney is surrounded by tough fibrous tissue, the renal capsule
Renal capsule
The renal capsule is a tough fibrous layer surrounding the kidney and covered in a thick layer of perinephric adipose tissue. It provides some protection from trauma and damage....

, which is itself surrounded by perinephric fat, renal fascia
Renal fascia
The renal fascia is a layer of connective tissue encapsulating the kidneys. The deeper layers below the renal fascia are, in order, the perinephric fat , renal capsule, and finally the parenchyma of the renal cortex...

 (of Gerota
Dimitrie Gerota
Dimitrie D. Gerota , Romanian anatomist, physician, radiologist, urologist, and an associated member of the Romanian Academy from 1916.-Biography:...

) and paranephric fat. The anterior (front) border of these tissues is the peritoneum
Peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

, while the posterior (rear) border is the transversalis fascia
Transversalis fascia
The transversalis fascia is a thin aponeurotic membrane which lies between the inner surface of the Transversus abdominis and the extraperitoneal fascia....

.

The superior border of the right kidney is adjacent to the liver; and the spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...

, for the left kidney. 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
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...

, of the kidney is divided into two major structures: superficial is the renal cortex
Renal cortex
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections that extend down between the pyramids. It contains the renal corpuscles and the renal tubules except for parts of...

 and deep is the renal medulla
Renal medulla
The renal medulla is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the arcuate arterioles. The arcuate arterioles each in turn branch into...

. Grossly, these structures take the shape of 8 to 18 cone-shaped renal lobe
Renal lobe
The renal lobe is a portion of a kidney consisting of a renal pyramid and the renal cortex above it. It is visible without a microscope, though it is easier to see in humans than in other animals....

s, each containing renal cortex surrounding a portion of medulla called a renal pyramid (of Malpighi
Marcello Malpighi
Marcello Malpighi was an Italian doctor, who gave his name to several physiological features, like the Malpighian tubule system.-Early years:...

). Between the renal pyramids are projections of cortex called renal column
Renal column
The renal column is a medullary extension of the renal cortex in between the renal pyramids...

s (of Bertin
Exupere Joseph Bertin
Exupere Joseph Bertin was a French anatomist born in Ille-et-Vilaine, Brittany. He was the father of cardiologist René-Joseph-Hyacinthe Bertin ....

). Nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

s, the urine-producing functional structures of the kidney, span the cortex and medulla. The initial filtering portion of a nephron is the renal corpuscle
Renal corpuscle
In the kidney, a renal corpuscle is the initial blood-filtering component of a nephron. It consists of two structures: a glomerulus and a Bowman's capsule. The glomerulus is a small tuft of capillaries containing two cell types. Endothelial cells, which have large fenestrae, are not covered by...

, 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
Medullary ray (anatomy)
In anatomy, the medullary ray is the middle part of the cortical lobule or renal lobule, consisting of a group of straight tubes to the collecting ducts.Their name is potentially misleading -- the "medullary" refers to their destination, not their location...

 is a collection of renal tubules that drain into a single collecting duct.

The tip, or papilla
Renal papilla
In the kidney, the renal papilla is the location where the medullary pyramids empty urine into the minor calyx. Histologically it is marked by medullary collecting ducts converging to form a duct of Bellini to channel the fluid. Transitional epithelium begins to be seen.-Role in disease:Some...

, of each pyramid empties urine into a minor calyx
Minor calyx
The minor calyx, in the kidney, surrounds the apex of the renal pyramids. Urine formed in the kidney passes through a papilla at the apex into the minor calyx then into the major calyx....

; minor calyces empty into major calyces, and major calyces empty into the renal pelvis
Renal pelvis
The renal pelvis or pyelum is the funnel-like dilated proximal part of the ureter in the kidney.In humans, the renal pelvis is the point of convergence of two or three major calyces...

, which becomes the ureter.

Blood supply


The kidneys receive blood from the renal arteries
Renal artery
The renal arteries normally arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood. Each is directed across the crus of the diaphragm, so as to form nearly a right angle with the aorta....

, left and right, which branch directly from the abdominal aorta
Abdominal aorta
The abdominal aorta is the largest artery in the abdominal cavity. As part of the aorta, it is a direct continuation of the descending aorta .-Path:...

. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...

.

Each renal artery branches into segmental arteries, dividing further into interlobar arteries
Interlobar arteries
The interlobar arteries are vessels of the renal circulation which supply the renal lobes.-External links: - "Renal Vasculature: Efferent Arterioles & Peritubular Capillaries" - "Urinary System: neonatal kidney, vasculature"*...

 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
Glomerulus
A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...

.

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
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

. Various conditions can lead to scarring and congestion
Congestion
Congestion generally means excessive crowding.Congestion may refer to:* congestion in heart failure, a term to describe low cardiac output seen in heart failure.* Nasal congestion, the blockage of nasal passages due to swollen membranes...

 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
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

 studies the structure of the kidney as viewed under a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. Various distinct cell type
Cell type
A cell type is a distinct morphological or functional form of cell. When a cell switches state from one cell type to another, it undergoes cellular differentiation. A list of distinct cell types in the adult human body may include several hundred distinct types.-References:...

s 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
    Kidney collecting duct cell
    A kidney collecting duct cell can be of two different cell types:*principal cells*Intercalated cells...

  • Interstitial kidney cells

Innervation


The kidney and nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 communicate via the renal plexus
Renal plexus
The renal plexus is formed by filaments from the celiac plexus, the aorticorenal ganglion, and the aortic plexus .It is joined also by the least splanchnic nerve....

, whose fibers course along the renal arteries to reach the kidney. Input from the sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...

 triggers vasoconstriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

 in the kidney, thereby reducing renal blood flow
Renal blood flow
In the physiology of the kidney, renal blood flow is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 22% of cardiac output, amounting to 1.1 L/min in a 70-kg adult male...

. The kidney is not thought to receive input from the parasympathetic nervous system
Parasympathetic nervous system
The parasympathetic nervous system is one of the two main divisions of the autonomic nervous system . The ANS is responsible for regulation of internal organs and glands, which occurs unconsciously...

. Sensory input from the kidney travels to the T10-11 levels of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 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
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

, regulating acid-base balance, electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

 concentrations, extracellular fluid volume, and regulation of blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. The kidney accomplishes these homeostatic functions both independently and in concert with other organs, particularly those of the endocrine system
Endocrine system
In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...

. Various endocrine hormones coordinate these endocrine functions; these include renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

, angiotensin II, aldosterone
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

, antidiuretic hormone, and atrial natriuretic peptide
Atrial natriuretic peptide
Atrial natriuretic peptide , atrial natriuretic factor , atrial natriuretic hormone , or atriopeptin, is a powerful vasodilator, and a protein hormone secreted by heart muscle cells. It is involved in the homeostatic control of body water, sodium, potassium and fat...

, 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
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

. Filtration, which takes place at the renal corpuscle
Renal corpuscle
In the kidney, a renal corpuscle is the initial blood-filtering component of a nephron. It consists of two structures: a glomerulus and a Bowman's capsule. The glomerulus is a small tuft of capillaries containing two cell types. Endothelial cells, which have large fenestrae, are not covered by...

, is the process by which cells and large proteins are filtered from the blood to make an ultrafiltrate that eventually becomes 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
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. These include the nitrogenous wastes called "urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

", from protein catabolism
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

, as well as uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...

, from nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 around a relatively stable value. The lungs contribute to acid-base homeostasis by regulating bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 (HCO3-) concentration.
The kidneys have two very important roles in maintaining the acid-base balance: to reabsorb bicarbonate from urine, and to excrete hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 ions into urine

Osmolality regulation


Any significant rise in plasma osmolality
Plasma osmolality
-Measured osmolality :Osmolality can be measured on an analytical instrument called an osmometer. It works on the method of depression of freezing point.Plasma osmolality is affected by changes in water content...

 is detected by the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

, 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
Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...

 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
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 predominantly depends upon the kidney. This primarily occurs through maintenance of the extracellular fluid
Extracellular fluid
Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...

 compartment, the size of which depends on the plasma sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 concentration. Although the kidney cannot directly sense blood pressure, changes in the delivery of sodium and chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

 to the distal part of the nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....

 alter the kidney's secretion of the enzyme renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

. 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
Renin-angiotensin system
The renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....

. Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

. 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
Erythropoietin
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...

, and the enzyme renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

. Erythropoietin
Erythropoietin
Erythropoietin, or its alternatives erythropoetin or erthropoyetin or EPO, is a glycoprotein hormone that controls erythropoiesis, or red blood cell production...

 is released in response to hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

 (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis
Erythropoiesis
Erythropoiesis is the process by which red blood cells are produced. It is stimulated by decreased O2 in circulation, which is detected by the kidneys, which then secrete the hormone erythropoietin...

 (production of red blood cells) in the bone marrow
Bone marrow
Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

. Calcitriol
Calcitriol
Calcitriol , also called 1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3, is the hormonally active form of vitamin D with three hydroxyl groups...

, the activated form of vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

, promotes intestinal absorption of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 and the renal reabsorption
Reabsorption
In physiology, reabsorption or tubular reabsorption is the flow of glomerular filtrate from the proximal tubule of the nephron into the peritubular capillaries, or from the urine into the blood...

 of phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

. Part of the renin-angiotensin-aldosterone system, renin is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 involved in the regulation of aldosterone
Aldosterone
Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

 levels.

Development


The mammalian kidney develops from intermediate mesoderm
Intermediate mesoderm
Intermediate mesenchyme or intermediate mesoderm is a type of mesoderm that is located between the paraxial mesoderm and the lateral plate.It develops into the part of the urogenital system * forms of urogenital system...

. Kidney development
Kidney development
Kidney development, or nephrogenesis, describes the embryologic origins of the kidney, a major organ in the urinary system. It is often considered in the broader context of the development of the urinary and reproductive organs.-Phases:...

, 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
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

 and have long been studied in ecophysiology
Ecophysiology
Ecophysiology or environmental physiology is a biological discipline which studies the adaptation of organism's physiology to environmental conditions...

 and comparative physiology
Comparative physiology
Comparative physiology is a subdiscipline of physiology that studies and exploits the diversity of functional characteristics of various kinds of organisms. It is closely related to evolutionary physiology and environmental physiology. Many universities offer undergraduate courses that cover...

. 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
Adjective
In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....

 renal, meaning related to the kidney, is from the Latin rēnēs, meaning kidneys; the prefix nephro- is from the Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 word for kidney, nephros (νεφρός). For example, surgical removal of the kidney is a nephrectomy
Nephrectomy
Nephrectomy is the surgical removal of a kidney.-History:The first successful nephrectomy was performed by the German surgeon Gustav Simon on August 2, 1869 in Heidelberg. Simon practiced the operation beforehand in animal experiments...

, while a reduction in kidney function is called renal dysfunction.

Congenital

  • Congenital hydronephrosis
    Hydronephrosis
    Hydronephrosis is distension and dilation of the renal pelvis calyces, usually caused by obstruction of the free flow of urine from the kidney, leading to progressive atrophy of the kidney...

  • 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
    Duplicated ureter
    Duplicated ureter is a congenital condition in which the ureteric bud, the embryological origin of the ureter, splits , resulting in two ureters draining a single kidney. It is the most common renal abnormality, occurring in approximately 1% of the population...

     occurs in approximately one in 100 live births
  • Horseshoe kidney
    Horseshoe kidney
    Horseshoe kidney, also known as renal fusion or super kidney, is a congenital disorder affecting about 1 in 400 people. In this disorder, the patient's kidneys fuse together to form a horseshoe-shape during development in the womb...

     occurs in approximately one in 400 live births
  • Polycystic kidney disease
    Polycystic kidney disease
    Autosomal dominant polycystic kidney disease is an inherited systemic disorder that predominantly affects the kidneys, but may affect other organs including the liver, pancreas, brain, and arterial blood vessels...

    • 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
    Renal agenesis
    Renal agenesis is a unilateral or bilateral medical condition in which one or both fetal kidneys fail to develop leading to oligohydramnios, resulting in a 40-fold increase in perinatal mortality.It can be associated with RET or UPK3A.-Bilateral:...

    . 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
    Multicystic dysplastic kidney
    Multicystic dysplastic kidney is a condition that results from the malformation of the kidney during fetal development. The kidney consists of irregular cysts of varying sizes and has no function....

     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
    Diabetic nephropathy
    Diabetic nephropathy , also known as Kimmelstiel-Wilson syndrome, or nodular diabetic glomerulosclerosis and intercapillary glomerulonephritis, is a progressive kidney disease caused by angiopathy of capillaries in the kidney glomeruli. It is characterized by nephrotic syndrome and diffuse...

  • Glomerulonephritis
    Glomerulonephritis
    Glomerulonephritis, also known as glomerular nephritis, abbreviated GN, is a renal disease characterized by inflammation of the glomeruli, or small blood vessels in the kidneys...

  • Hydronephrosis
    Hydronephrosis
    Hydronephrosis is distension and dilation of the renal pelvis calyces, usually caused by obstruction of the free flow of urine from the kidney, leading to progressive atrophy of the kidney...

     is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.
  • Interstitial nephritis
    Interstitial nephritis
    Interstitial nephritis is a form of nephritis affecting the interstitium of the kidneys surrounding the tubules...


  • Kidney stone
    Kidney stone
    A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...

    s (nephrolithiasis) are a relatively common and particularly painful disorder.
  • Kidney tumors
    • Wilms tumor
    • Renal cell carcinoma
      Renal cell carcinoma
      Renal cell carcinoma is a kidney cancer that originates in the lining of the proximal convoluted tubule, the very small tubes in the kidney that filter the blood and remove waste products. RCC is the most common type of kidney cancer in adults, responsible for approximately 80% of cases...

  • Lupus nephritis
    Lupus nephritis
    Lupus nephritis is an inflammation of the kidney caused by systemic lupus erythematosus , a disease of the immune system. Apart from the kidneys, SLE can also damage the skin, joints, nervous system and virtually any organ or system in the body....

  • Minimal change disease
    Minimal change disease
    Minimal Change Disease is a disease of the kidney that causes nephrotic syndrome and usually affects children .-Epidemiology:...

  • In nephrotic syndrome
    Nephrotic syndrome
    Nephrotic syndrome is a nonspecific disorder in which the kidneys are damaged, causing them to leak large amounts of protein from the blood into the urine....

    , the glomerulus has been damaged so that a large amount of protein
    Protein
    Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

     in the blood enters the urine
    Urine
    Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

    . Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.
  • Pyelonephritis
    Pyelonephritis
    Pyelonephritis is an ascending urinary tract infection that has reached the pyelum or pelvis of the kidney. It is a form of nephritis that is also referred to as pyelitis...

     is infection of the kidneys and is frequently caused by complication of a urinary tract infection
    Urinary tract infection
    A urinary tract infection is a bacterial infection that affects any part of the urinary tract. Symptoms include frequent feeling and/or need to urinate, pain during urination, and cloudy urine. The main causal agent is Escherichia coli...

    .
  • Renal failure
    Renal failure
    Renal failure or kidney failure describes a medical condition in which the kidneys fail to adequately filter toxins and waste products from the blood...

    • Acute renal failure
      Acute renal failure
      Acute kidney injury , previously called acute renal failure , is a rapid loss of kidney function. Its causes are numerous and include low blood volume from any cause, exposure to substances harmful to the kidney, and obstruction of the urinary tract...

    • 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
Renal replacement therapy
Renal replacement therapy is a term used to encompass life-supporting treatments for renal failure.It includes:*hemodialysis,*peritoneal dialysis,*hemofiltration and*renal transplantation.These treatments will not cure chronic kidney disease...

, in the form of dialysis
Dialysis
In medicine, dialysis is a process for removing waste and excess water from the blood, and is primarily used to provide an artificial replacement for lost kidney function in people with renal failure...

 or kidney transplantation
Kidney transplantation
Kidney transplantation or renal transplantation is the organ transplant of a kidney into a patient with end-stage renal disease. Kidney transplantation is typically classified as deceased-donor or living-donor transplantation depending on the source of the donor organ...

, 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
Mesonephros
The mesonephros is one of three excretory organs that develop in vertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included in the Wolffian body after Caspar Friedrich Wolff who described it in 1759...

 persists into the adult, albeit usually fused with the more advanced metanephros; only in amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s is the mesonephros restricted to the embryo. The kidneys of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 and amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s 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
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 with the vas deferens
Vas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....

 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
Bladder
Bladder usually refers to an anatomical hollow organBladder may also refer to:-Biology:* Urinary bladder in humans** Urinary bladder ** Bladder control; see Urinary incontinence** Artificial urinary bladder, in humans...

 or cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...

. 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
Hagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

 and lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, 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 reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s 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.

Bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s 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 mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. 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
Loop of Henle
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...

; 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 kidney
Reniculate kidney
The reniculate kidney is a multilobed kidney most commonly found in marine mammals, with the exception of the sirenians. Kidneys of this morphology have increased surface area for removing toxins from the body more efficiently than a non-lobed kidney....

s 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
Conscience
Conscience is an aptitude, faculty, intuition or judgment of the intellect that distinguishes right from wrong. Moral judgement may derive from values or norms...

 and reflection, and a number of verses in the Bible
Bible
The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

 (e.g. Ps. 7:9, Rev. 2:23) state that God searches out and inspects the kidneys, or "reins", of humans. Similarly, the Talmud
Talmud
The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

 (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
Cooking
Cooking is the process of preparing food by use of heat. Cooking techniques and ingredients vary widely across the world, reflecting unique environmental, economic, and cultural traditions. Cooks themselves also vary widely in skill and training...

 and eaten by humans (along with other offal
Offal
Offal , also called, especially in the United States, variety meats or organ meats, refers to the internal organs and entrails of a butchered animal. The word does not refer to a particular list of edible organs, which varies by culture and region, but includes most internal organs other than...

).

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
Mixed grill
Many regional cuisines feature a mixed grill, a meal consisting of a traditional assortment of grilled meats. Versions of the mixed grill include:...

 or meurav Yerushalmi. Among the most reputed kidney dishes, the British
British cuisine
English cuisine encompasses the cooking styles, traditions and recipes associated with England. It has distinctive attributes of its own, but also shares much with wider British cuisine, largely due to the importation of ingredients and ideas from places such as North America, China, and India...

 steak and kidney pie
Steak and kidney pie
Steak and kidney pie is a savoury pie that is filled principally with a mixture of diced beef, diced kidney , fried onion, and brown gravy...

, the Swedish hökarpanna (pork and kidney stew), the French
French cuisine
French cuisine is a style of food preparation originating from France that has developed from centuries of social change. In the Middle Ages, Guillaume Tirel , a court chef, authored Le Viandier, one of the earliest recipe collections of Medieval France...

 rognons de veau sauce moutarde (veal kidneys in mustard
Mustard (condiment)
Mustard is a condiment made from the seeds of a mustard plant...

 sauce) and the Spanish
Spanish cuisine
Spanish cuisine consists of a variety of dishes, which stem from differences in geography, culture and climate. It is heavily influenced by seafood available from the waters that surround the country, and reflects the country's deep maritime roots...

 riñones al Jerez (kidneys stewed in sherry
Sherry
Sherry is a fortified wine made from white grapes that are grown near the town of Jerez , Spain. In Spanish, it is called vino de Jerez....

 sauce), deserve special mention.

See also

  • Artificial kidney
    Artificial kidney
    Artificial kidney is often a synonym for hemodialysis, but may also, more generally, refer to renal replacement therapies that are in use and/or in development...

  • Holonephros
    Holonephros
    The holonephros is the kidney of the larvae of cyclostomes and some amphibians. The entire mass of nephrogenic tissue gives rise to this kidney, which is usually of simple form with a single tubule in each segment.-External links:*...

  • Organ donation
    Organ donation
    Organ donation is the donation of biological tissue or an organ of the human body, from a living or dead person to a living recipient in need of a transplantation. Transplantable organs and tissues are removed in a surgical procedure following a determination, based on the donor's medical and...

  • Organ harvesting
    Organ harvesting
    Organ harvesting refers to the removal, preservation and use of human organs and tissue from the bodies of the recently deceased to be used in surgical transplants on the living...

  • Pelvic kidney
    Pelvic kidney
    If a kidney does not ascend as it should in normal childhood development it remains in the pelvic area and is called a pelvic kidney, ectopic kidney or a pancake kidney...

  • World Kidney Day
    World kidney day
    World Kidney Day is a global health awareness campaign focusing on the importance of the kidneys and reducing the frequency and impact of kidney disease and its associated health problems worldwide....


External links