The kidneys are two bean-shaped organs found on the left and right sides of the body in vertebrates. They are located at the back of the abdominal cavity in the retroperitoneal space. They are about in length.They receive blood from the paired renal arteries; blood exits into the paired renal veins. Each kidney has an excretory tube, the ureter, which ends at the bladder. Microscopically, the basic structural and functional unit of the kidney is the nephron: its structure resembles the letter "S" turned on its side. The nephron and its associated peritubular capillary carry out four basic functions: filtration, reabsorption, secretion, and excretion. Filtration occurs in the glomerulus: all blood components are filtered except blood cells and blood proteins. 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 potassium and uric acid. Examples of substances excreted are urea, ammonium, and uric acid. The kidneys also carry out functions not part of the nephron. For example, they convert a precursor to produce calcitriol, and synthesize the hormones erythropoietin and renin. Renal physiology is the study of kidney function. Nephrology is the medical speciality which addresses kidney function diseases. These include nephritic and nephrotic syndromes, acute kidney injury, chronic kidney disease including dialysis management, and urinary tract infection. Urology addresses abnormal kidney (and urinary tract) anatomy, such as renal cysts, kidney stones, urinary tract obstruction, renal carcinoma, and renal transplantation.
Location, showing kidneys at the 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,Bålens ytanatomy (Superficial anatomy of the trunk). Anca Dragomir, Mats Hjortberg and Godfried M. Romans. Section for human anatomy at the Department of Medical Biology, Uppsala University, Sweden. and the right is slightly lower. The right kidney sits just below the diaphragm and posterior to the liver. The left 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 perinephric fat present between renal fascia and renal capsule and paranephric fat superior to the renal fascia.
Structure[[File:KidneyStructures PioM.svg|thumb|300px|right| 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 kidney is a bean-shaped structure with a and a 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 ( adipose capsule), renal fascia, and pararenal fat ( paranephric body). 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. In adult males, the kidney weighs between 125 and 170 grams. In females the weight of the kidney is between 115 and 155 grams. A Danish study measured the median renal length to be on the left side and on the right side in adults. Median renal volumes were 146 cm3 on the left and 134 cm3 on the right. The substance, or parenchyma, of the 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 (of Malpighi). Between the renal pyramids are projections of cortex called renal columns (or Bertin 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 surrounds 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.Clapp, WL. "Renal Anatomy". In: Zhou XJ, Laszik Z, Nadasdy T, D'Agati VD, Silva FG, eds. Silva's Diagnostic Renal Pathology. New York: Cambridge University Press; 2009. The kidneys possess no overtly moving structures
Blood supply, showing renal arteries and veins.]] The renal circulation supplies the blood to the kidneys via the renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output. Each renal artery branches into segmental arteries, dividing further into interlobar arteries, which penetrate the renal capsule and extend through the renal columns between the renal pyramids. The interlobar arteries then supply blood to the arcuate arteries that run through the boundary of the cortex and the medulla. Each arcuate artery supplies several interlobular arteries that feed into the afferent arterioles that supply the glomeruli. The medullary interstitium is the functional space in the kidney beneath the individual filters (glomeruli), which are rich in blood vessels. The interstitium absorbs fluid recovered from urine. Various conditions can lead to scarring and 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. The table below shows the path that blood takes when it travels through the glomerulus, traveling "down" the arteries and "up" the veins. However, this model is greatly simplified for clarity and symmetry. Some of the other paths and complications are described at the bottom of the table. The interlobar artery and vein (not to be confused with interlobular) are between two renal lobes, also known as the renal column (cortex region between two pyramids).
- Note 1: The renal artery also provides a branch to the inferior suprarenal artery to supply the adrenal gland.
- Note 2: Each renal artery partitions into an anterior and posterior branch. The anterior branch further divides into the superior (apical), anterosuperior, anteroinferior and inferior segmental arteries. The posterior branch continues as the posterior segmental artery.
- Note 3: Also called the cortical radiate arteries. The interlobular artery also supplies to the stellate veins.
- Note 4: The efferent arterioles do not directly drain into the interlobular vein, but rather they go to the peritubular capillaries first. The efferent arterioles of the juxtamedullary nephron drain into the vasa recta.
Nerve supplyThe 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 (cranial nerve X); 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.
Renal clearanceRenal clearance is the volume of plasma from which the substance is completely cleared from the blood per unit time. (U'x)}}
- is the clearance of X (normally in units of mL/min.
- is the urine concentration of X.
- is the plasma concentration of X.
- is the urine flow rate.
Mathematical modellingThe kidney is a very complex organ and numerical modelling has been used to better understand kidney function at several scales, including fluid uptake and secretion.
Clinical significanceKidney disease, is kidney disease or damage to a kidney. Nephrosis is non-inflammatory nephropathy and nephritis is inflammatory kidney disease. Nephrology is the speciality that deals with kidney function and disease. Medical terms related to the kidneys commonly use terms such as renal and the prefix nephro-. The adjective renal, meaning related to the kidney, is from the Latin rēnēs, meaning kidneys; the prefix nephro- is from the Ancient Greek word for kidney, nephros (νεφρός). For example, surgical removal of the kidney is a nephrectomy, while a reduction in kidney function is called renal dysfunction.
- Diabetic nephropathy
- Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.
- Interstitial nephritis
- Kidney stones (nephrolithiasis) are a relatively common and particularly painful disorder. A chronic condition can result in scars to the kidneys. The removal of kidney stones involves ultrasound 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 to disabling pain in the middle and sides of the lower back or groin.
- Kidney tumour
- * Wilms tumor
- * Renal cell carcinoma
- Lupus nephritis
- Minimal change disease
- In nephrotic syndrome, the glomerulus has been damaged so that a large amount of protein in the blood enters the urine. Other frequent features of the nephrotic syndrome include swelling, low serum albumin, and high cholesterol.
- Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.
- Renal failure
- * Acute renal failure
- * Stage 5 Chronic Kidney Disease
- Renal artery stenosis
- Renovascular hypertension
Kidney injury and failureGenerally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive. Only when the amount of functioning kidney tissue is greatly diminished does one develop chronic kidney disease. Renal replacement therapy, in the form of dialysis or kidney transplantation, is indicated when the glomerular filtration rate has fallen very low or if the renal dysfunction leads to severe symptoms.
- Congenital hydronephrosis
- Congenital obstruction of urinary tract
- Duplex kidneys, or double kidneys, occur in approximately 1% of the population. This occurrence normally causes no complications, but can occasionally cause urine infections.
- Duplicated ureter occurs in approximately one in 100 live births
- Horseshoe kidney occurs in approximately one in 400 live births
- Nutcracker syndrome
- Polycystic kidney disease
- * Autosomal dominant polycystic kidney disease afflicts patients later in life. Approximately one in 1000 people will develop this condition
- * Autosomal recessive polycystic kidney disease is far less common, but more severe, than the dominant condition. It is apparent in utero or at birth.
- Renal agenesis. Failure of one kidney to form occurs in approximately one in 750 live births. Failure of both kidneys to form used to be fatal; however, medical advances such as amnioinfusion therapy during pregnancy and peritoneal dialysis have made it possible to stay alive until a transplant can occur.
- Renal dysplasia
- Unilateral small kidney
- Multicystic dysplastic kidney occurs in approximately one in every 2400 live births
- Ureteropelvic Junction Obstruction or UPJO; although most cases appear congenital, some appear to be an acquired condition