Kidney
The kidneys are complex organs that have numerous of biological roles. Their primary role is to maintain the homeostatic balance of bodily fluids. They primarily do this by filtering and secreting metabolites (such as urea) and minerals from the blood and excreting them, along with water, as urine. Because the kidneys are poised to sense plasma concentrations of compounds such as sodium, potassium, hydrogen ion, oxygen, and glucose, they are important regulators of blood pressure, glucose metabolism, and erythropoeisis. 
The medical field that studies the kidneys and diseases of the kidney is called nephrology.  The prefix nephro- meaning kidney is from the Ancient Greek word nephros (); the adjective renal meaning related to the kidney is from Latin renes, meaning kidneys.
In humans, the kidneys are located in the posterior part of the abdomen. There is one on each side of the spine; the right kidney sits just below the liver, the left below the diaphragm and adjacent to the spleen. Above each kidney is an adrenal gland (also called the suprarenal gland).  The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one while the left kidney is located slightly more medial.
The kidneys are retroperitoneal. They are approximately at the vertebral level T12 to L3. The upper parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney is surrounded by two layers of fat (the perirenal and pararenal fat) which help to cushion it. Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis, can occur.

# Anatomy
In a normal human adult, each kidney is about 10 cm long, 5.5 cm in width and about 3 cm thick, weighing 150 grams. Together, kidneys weigh about 0.5% of a person's total body weight.  The kidneys are "bean-shaped" organs, and have a concave side facing inwards (medially). On this medial aspect of each kidney is an opening, called the hilum, which admits the renal artery, the renal vein, nerves, and the ureter.
The outer portion of the kidney is called the renal cortex, which sits directly beneath the kidney's loose connective tissue/fibrous capsule. Deep to the cortex lies the renal medulla, which is divided into 10-20 renal pyramids in humans. Each pyramid together with the associated overlying cortex forms a renal lobe. The tip of each pyramid (called a papilla) empties into a calyx, and the calices empty into the renal pelvis. The pelvis transmits urine to the urinary bladder via the ureter. People are born with two kidneys but are able to live with only one.
The poles are the highest and lowest points of the kidney. Since the kidneys are located on different heights, the upper pole of the right kidney is at the same level as the hilum of the left kidney. This also happens to be at the same level as the transpyloric plane.

## Blood supply
Each kidney receives its blood supply from the renal artery, two of which branch from the abdominal aorta. Upon entering the hilum of the kidney, the renal artery divides into smaller interlobar arteries situated between the renal papillae. At the outer medulla, the interlobar arteries branch into arcuate arteries, which course along the border between the renal medulla and cortex, giving off still smaller branches, the cortical radial arteries (sometimes called interlobular arteries). Branching off these cortical arteries are the afferent arterioles supplying the glomerular capillaries, which drain into efferent arterioles. Efferent arterioles divide into peritubular capillaries that provide an extensive blood supply to the cortex. Blood from these capillaries collects in renal venules and leaves the kidney via the renal vein. Efferent arterioles of glomeruli closest to the medulla (those that belong to juxtamedullary nephrons) send branches into the medulla, forming the vasa recta. Blood supply is intimately  linked to blood pressure.

## Innervation
The kidney is innervated by the renal and ureteric nerve, which arises from the renal plexus.  It is sympathetic, parasympathetic and visceral afferent.  The renal plexus, in turn, is innervated by thoracic splanchnic nerves, especially the caudal ones.

## Nephron
The basic functional unit of the kidney is the nephron, of which there are more than a million within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and solute within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood under pressure, and then reabsorbing some necessary fluid and molecules back into the blood while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms established in the nephrons and associated collecting ducts.

## Collecting duct system
The fluid flows from the nephron into the collecting duct system. This segment of the nephron is crucial to the process of water conservation by the organism. In the presence of antidiuretic hormone (ADH; also called vasopressin), these ducts become permeable to water and facilitate its reabsorption, thus concentrating the urine and reducing its volume. Conversely, when the organism must eliminate excess water, such as after excess fluid drinking, the production of ADH is decreased and the collecting tubule becomes less permeable to water, rendering urine dilute and abundant. Failure of the organism to decrease ADH production appropriately, a condition known as syndrome of inappropriate ADH (SIADH), may lead to water retention and dangerous dilution of body fluids, which in turn may cause severe neurological damage.  Failure to produce ADH (or inability of the collecting ducts to respond to it) may cause excessive urination, called diabetes insipidus (DI).
A second major function of the collecting duct system is the maintenance of acid-base homeostasis.
After being processed along the collecting tubules and ducts, the fluid, now called urine, is drained into the bladder via the ureter, to be finally excluded from the organism.

# Functions

## Excretion of waste products
The kidneys excrete a variety of waste products produced by metabolism, including the nitrogenous wastes: urea (from protein catabolism) and uric acid (from nucleic acid metabolism) and water.

## Homeostasis
The kidney is one of the major organs involved in whole-body homeostasis. Among its homeostatic functions are acid-base balance, regulation of electrolyte concentrations, control of blood volume, and regulation of blood pressure. The kidneys accomplish these homeostatic functions independently and through coordination with other organs, particularly those of the endocrine system. The kidney communicates with these organs through hormones secreted into the bloodstream.

### Acid-base balance
The kidneys regulate the pH, by eliminating H+ ions concentration called augmentation mineral ion concentration, and water composition of the blood.
By exchanging hydronium ions and hydroxyl ions, the blood plasma is maintained by the kidney at a slightly alkaline pH of 7.4. Urine, on the other hand, is acidic at pH 5 or alkaline at pH 8.
The pH is maintained through four main protein transporters: NHE3 (a sodium-hydrogen exchanger), V-type H-ATPase (an isoform of the hydrogen ATPase), NBC1 (a sodium-bicarbonate cotransporter) and AE1 (an anion exchanger which exchanges chloride for bicarbonate). Due to the polar alignment of cells in the renal epithelia NHE3 and the H-ATPase are exposed to the lumen (which is essentially outside the body), on the apical side of the cells, and are responsible for excreting hydrogen ions (or protons). Conversely, NBC1 and AE1 are on the basolateral side of the cells, and allow bicarbonate ions to move back into the extracellular fluid and thus are returned to the blood plasma.

### Blood pressure
Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion reabsorption in the distal convoluted tubules.
When blood pressure becomes low, a proteolytic enzyme called Renin is secreted by cells of the juxtaglomerular apparatus (part of the distal convoluted tubule) which are sensitive to pressure. Renin acts on a blood protein, angiotensinogen, converting it to angiotensin I (10 amino acids). Angiotensin I is then converted by the Angiotensin-converting enzyme (ACE) in the lung capillaries to Angiotensin II (8 amino acids), which stimulates the secretion of Aldosterone by the adrenal cortex, which then affects the renal tubules.
Aldosterone stimulates an increase in the reabsorption of sodium ions from the kidney tubules which causes an increase in the volume of water that is reabsorbed from the tubule. This increase in water reabsorption increases the volume of blood which ultimately raises the blood pressure.

### Plasma volume
Any significant rise or drop in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. A rise in osmolality causes the gland to secrete antidiuretic hormone, 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.

## Hormone secretion
The kidneys secrete a variety of hormones, including erythropoietin, urodilatin, renin and vitamin D.

# Embryology
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. (The plural forms of these terms end in -oi.)

## Pronephros
During approximately day 22 of human gestation, the paired pronephroi appear towards the cranial end of the intermediate mesoderm. In this region, epithelial cells arrange themselves in a series of tubules called nephrotomes and join laterally with the pronephric duct, which does not reach the outside of the embryo. Thus the pronephros is considered nonfunctional in mammals because it cannot excrete waste from the embryo.

## Mesonephros
Each pronephric duct grows towards the tail of the embryo, and in doing so induces intermediate mesoderm in the thoracolumbar area to become epithelial tubules called mesonephric tubules. Each mesonephric tubule receives a blood supply from a branch of the aorta, ending in a capillary tuft analogous to the glomerulus of the definitive nephron. The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the mesonephric duct or Wolffian duct. The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the cloaca. The mammalian mesonephros is similar to the kidneys of aquatic amphibians and fishes.

## Metanephros
During the fifth week of gestation, the mesonephric duct develops an outpouching, the ureteric bud, near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, the metanephric duct, later forms the ureter. As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the collecting duct system of the kidney. It also forms the major and minor calyces and the renal pelvis.
The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the renal tubules. As the renal tubules grow, they come into contact and join with connecting tubules of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive glomerulus.

# Terms
- renal capsule: The membranous covering of the kidney.
- cortex: The outer layer over the internal medulla. It contains blood vessels, glomeruli (which are the kidneys' "filters") and urine tubes and is supported by a fibrous matrix.
- hilus: The opening in the middle of the concave medial border for nerves and blood vessels to pass into the renal sinus.
- renal column: The structures which support the cortex. They consist of lines of blood vessels and urinary tubes and a fibrous material.
- renal sinus: The cavity which houses the renal pyramids.
- calyces: The recesses in the internal medulla which hold the pyramids. They are used to subdivide the sections of the kidney. (singular - calyx)
- papillae: The small conical projections along the wall of the renal sinus. They have openings through which urine passes into the calyces. (singular - papilla)
- renal pyramids: The conical segments within the internal medulla. They contain the secreting apparatus and tubules and are also called malpighian pyramids.
- renal artery: Two renal arteries come from the aorta, each connecting to a kidney. The artery divides into five branches, each of which leads to a ball of capillaries. The arteries supply (unfiltered) blood to the kidneys. The left kidney receives about 60% of the renal bloodflow.
- renal vein: The filtered blood returns to circulation through the renal veins which join into the inferior vena cava.
- renal pelvis: Basically just a funnel, the renal pelvis accepts the urine and channels it out of the hilus into the ureter.
- ureter: A narrow tube 40 cm long and 4 mm in diameter. Passing from the renal pelvis out of the hilus and down to the bladder. The ureter carries urine from the kidneys to the bladder by means of peristalsis.
- renal lobe: Each pyramid together with the associated overlying cortex forms a renal lobe

# Diseases and disorders

## Congenital
- Congenital hydronephrosis
- Congenital obstruction of urinary tract
- Duplicated ureter
- Horseshoe kidney
- Polycystic kidney disease
- Renal dysplasia
- Unilateral small kidney
- Multicystic dysplastic kidney

## 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 are a relatively common and particularly painful disorder.
- Kidney tumors
Wilms tumor
Renal cell carcinoma
- 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
Chronic renal failure
- Acute renal failure
- Chronic renal failure

# The failing kidney
Generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive, possibly due to the nature of the prehistoric human diet. Only when the amount of functioning kidney tissue is greatly diminished will chronic renal failure develop. If the glomerular filtration rate (a measure of renal function) has fallen very low (end-stage renal failure), or if the renal dysfunction leads to severe symptoms, then renal replacement therapy is indicated, either dialysis or renal transplantation.

# Medical terminology
- Medical terms related to the kidneys involve the prefixes renal- and nephro-.
- Surgical removal of the kidney is a nephrectomy, while a radical nephrectomy is removal of the kidney, its surrounding tissue, lymph nodes, and potentially the adrenal gland. A radical nephrectomy is performed for the removal of the cancers.

# Histology
Human cell types found in the kidney include:
- 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
Cortical collecting duct cell
Medullary collecting duct cell
- Cortical collecting duct cell
- Medullary collecting duct cell
- Interstitial kidney cell, which do not participate in the filtration process.

# World Kidney Day
World Kidney Day is observed on the second Thursday of March every year.
It was held for the first time in 2006, to increase awareness of kidney disease and educate persons at risk regarding the importance of prevention and early detection. 
 It is a joint initiative by the International Society of Nephrology (ISF) and International Federation of Kidney Foundations (IFKF).
The next World Kidney Day will be held on 13 March 2008. In 2007, it was held on 8th March.

# Histopathological Findings in Kidney Diseases