kidney kidney the kidneys are two reddish-brown bean-shaped organs found in vertebrates. they are located on the left and right in the retroperitoneal space, and in adult humans are about 12 centimetres (4+1⁄2 inches) in length. they receive blood from the paired renal arteries; blood exits into the paired renal veins. each kidney is attached to a ureter, a tube that carries excreted urine to the bladder. kidneysthe kidneys lie in the retroperitoneal space behind the abdomen, and act to filter blood to create urine.view of the kidneys from behind, showing their blood supply and drainage.detailssystemurinary system and endocrine systemarteryrenal arteryveinrenal veinnerverenal plexusidentifierslatinrengreeknephrosmeshd007668ta98a08.1.01.001ta23358fma7203anatomical terminology the kidney participates in the control of the volume of various body fluids, fluid osmolality, acid–base balance, various electrolyte concentrations, and removal of toxins. filtration occurs in the glomerulus: one-fifth of the blood volume that enters the kidneys is filtered. examples of substances reabsorbed are solute-free water, sodium, bicarbonate, glucose, and amino acids. examples of substances secreted are hydrogen, ammonium, potassium and uric acid. the nephron is the structural and functional unit of the kidney. each adult human kidney contains around 1 million nephrons, while a mouse kidney contains only about 12,500 nephrons. the kidneys also carry out functions independent of the nephrons. for example, they convert a precursor of vitamin d to its active form, calcitriol; and synthesize the hormones erythropoietin and renin. chronic kidney disease (ckd) has been recognized as a leading public health problem worldwide. the global estimated prevalence of ckd is 13.4%, and patients with kidney failure needing renal replacement therapy are estimated between 5 and 7 million. procedures used in the management of kidney disease include chemical and microscopic examination of the urine (urinalysis), measurement of kidney function by calculating the estimated glomerular filtration rate (egfr) using the serum creatinine; and kidney biopsy and ct scan to evaluate for abnormal anatomy. dialysis and kidney transplantation are used to treat kidney failure; one (or both sequentially) of these are almost always used when renal function drops below 15%. nephrectomy is frequently used to cure renal cell carcinoma. renal physiology is the study of kidney function. nephrology is the medical specialty which addresses diseases of kidney function: these include ckd, nephritic and nephrotic syndromes, acute kidney injury, and pyelonephritis. urology addresses diseases of kidney (and urinary tract) anatomy: these include cancer, renal cysts, kidney stones and ureteral stones, and urinary tract obstruction. the word “renal” is an adjective meaning “relating to the kidneys”, and its roots are french or late latin. whereas according to some opinions, "renal" should be replaced with "kidney" in scientific writings such as "kidney artery", other experts have advocated preserving the use of "renal" as appropriate including in "renal artery". structure image showing the human trunk with positions of the organs. the kidneys are at the vertebral level of t12 to l3. in humans, the kidneys are located high in the abdominal cavity, one on each side of the spine, and lie in a retroperitoneal position at a slightly oblique angle. the asymmetry within the abdominal cavity, caused by the position of the liver, typically results in the right kidney being slightly lower and smaller than the left, and being placed slightly more to the middle than the left kidney. the left kidney is approximately at the vertebral level t12 to l3, and the right is slightly lower. the right kidney sits just below the diaphragm and posterior to the liver. the left kidney sits below the diaphragm and posterior to the spleen. on top of each kidney is an adrenal gland. the upper parts of the kidneys are partially protected by the 11th and 12th ribs. each kidney, with its adrenal gland is surrounded by two layers of fat: the perirenal fat present between renal fascia and renal capsule and pararenal fat superior to the renal fascia. the human kidney is a bean-shaped structure with a convex and a concave border. a recessed area on the concave border is the renal hilum, where the renal artery enters the kidney and the renal vein and ureter leave. the kidney is surrounded by tough fibrous tissue, the renal capsule, which is itself surrounded by perirenal fat, renal fascia, and pararenal fat. the anterior (front) surface of these tissues is the peritoneum, while the posterior (rear) surface is the transversalis fascia. the superior pole of the right kidney is adjacent to the liver. for the left kidney, it is next to the spleen. both, therefore, move down upon inhalation. sexweight, standard reference range right kidneyleft kidney male80–160 g (2+3⁄4–5+3⁄4 oz)80–175 g (2+3⁄4–6+1⁄4 oz) female40–175 g (1+1⁄2–6+1⁄4 oz)35–190 g (1+1⁄4–6+3⁄4 oz) a danish study measured the median renal length to be 11.2 cm (4+7⁄16 in) on the left side and 10.9 cm (4+5⁄16 in) on the right side in adults. median renal volumes were 146 cm3 (8+15⁄16 cu in) on the left and 134 cm3 (8+3⁄16 cu in) on the right. gross anatomy 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. renal sinus • 15. major calyx • 16. renal papilla • 17. renal column the functional substance, or parenchyma, of the human kidney is divided into two major structures: the outer renal cortex and the inner renal medulla. grossly, these structures take the shape of eight to 18 cone-shaped renal lobes, each containing renal cortex surrounding a portion of medulla called a renal pyramid. between the renal pyramids are projections of cortex called renal columns. 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, which is located in the cortex. this 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. this becomes the ureter. at the hilum, the ureter and renal vein exit the kidney and the renal artery enters. hilar fat and lymphatic tissue with lymph nodes surround these structures. the hilar fat is contiguous with a fat-filled cavity called the renal sinus. the renal sinus collectively contains the renal pelvis and calyces and separates these structures from the renal medullary tissue. the kidneys possess no overtly moving structures. normal adult right kidney as seen on abdominal ultrasound with a pole to pole measurement of 9.34 cm. a ct scan of the abdomen showing the position of the kidneys. the left cross-section in the upper abdomen shows the liver on the left side of scan (right side of body). center: cross-section showing the kidneys below the liver and spleen. right: further cross-section through the left kidney. image showing the structures that the kidney lies near. cross-section through a cadaveric specimen showing the position of the kidneys. blood supply main article: renal circulation the kidneys receive blood from the renal arteries, left and right, which branch directly from the abdominal aorta. the kidneys receive approximately 20–25% of cardiac output in adult human. 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. blood drains from the kidneys, ultimately into the inferior vena cava. after filtration occurs, the blood moves through a small network of small veins (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 veins which exit the kidney. nerve supply the kidney and nervous system communicate via the renal plexus, whose fibers course along the renal arteries to reach each kidney. input from the sympathetic nervous system triggers vasoconstriction in the kidney, thereby reducing renal blood flow. the kidney also receives input from the parasympathetic nervous system, by way of the renal branches of the vagus nerve; the function of this is yet unclear. 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 corresponding kidney. microanatomy renal histology is the study of the microscopic structure of the kidney. distinct cell types 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 collecting duct principal cell collecting duct intercalated cell interstitial kidney cells gene and protein expression further information: bioinformatics § gene and protein expression in humans, about 20,000 protein coding genes are expressed in human cells and almost 70% of these genes are expressed in normal, adult kidneys. just over 300 genes are more specifically expressed in the kidney, with only some 50 genes being highly specific for the kidney. many of the corresponding kidney specific proteins are expressed in the cell membrane and function as transporter proteins. the highest expressed kidney specific protein is uromodulin, the most abundant protein in urine with functions that prevent calcification and growth of bacteria. specific proteins are expressed in the different compartments of the kidney with podocin and nephrin expressed in glomeruli, solute carrier family protein slc22a8 expressed in proximal tubules, calbindin expressed in distal tubules and aquaporin 2 expressed in the collecting duct cells. development main article: kidney development the mammalian kidney develops from intermediate mesoderm. kidney development, also called nephrogenesis, proceeds through a series of three successive developmental phases: the pronephros, mesonephros, and metanephros. the metanephros are primordia of the permanent kidney. function the nephron, shown here, is the functional unit of the kidneys. its parts are labelled except the (gray) connecting tubule located after the (dark red) distal convoluted tubule and before the large (gray) collecting duct (mislabeled collection duct). main article: renal physiology the kidneys excrete a variety of waste products produced by metabolism into the urine. the microscopic structural and functional unit of the kidney is the nephron. it processes the blood supplied to it via filtration, reabsorption, secretion and excretion; the consequence of those processes is the production of urine. these include the nitrogenous wastes urea, from protein catabolism, and uric acid, from nucleic acid metabolism. the ability of mammals and some birds to concentrate wastes into a volume of urine much smaller than the volume of blood from which the wastes were extracted is dependent on an elaborate countercurrent multiplication mechanism. this requires several independent nephron characteristics to operate: a tight hairpin configuration of the tubules, water and ion permeability in the descending limb of the loop, water impermeability in the ascending loop, and active ion transport out of most of the ascending limb. in addition, passive countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function. the kidney participates in whole-body homeostasis, regulating acid–base balance, electrolyte concentrations, extracellular fluid volume, and 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. formation of urine four main processes are involved in the creation of urine. filtration filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are retained while materials of smaller molecular weights are filtered from the blood to make an ultrafiltrate that eventually becomes urine. the adult human kidney generates approximately 180 liters of filtrate a day. the normal range for a twenty four hour urine volume collection is 800 to 2,000 milliliters per day. the