The kidneys are bean-shaped excretory organs in vertebrates. Part of the urinary system, the kidneys filter wastes (especially urea) from the blood and excrete them, along with water, as urine. The medical field that studies the kidneys and diseases affecting the kidney is called nephrology.
In humans the kidneys are two organs located in the posterior part of the abdomen. There is one on each side of the spine just below the liver and spleen. Superior to each kidney is an adrenal gland (also called the suprarenal gland).
The kidneys are retroperitoneal, which means they lie behind the peritoneum, the lining of the abdominal cavity. They are approximately at the vertebral level T12 to L3, and the right kidney usually lies slightly lower than the left in order to accommodate the liver.
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 fat and the pararenal fat) which help to cushion it.
In a normal human adult, each kidney is about 11 cm long and about 5 cm thick, weighing 150 grams. 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 hilus, which admits the renal artery, the renal vein, nerves, and the ureter.
The outermost portion of the kidney is called the renal cortex, deep to which lies the renal medulla. The innermost portion is the pelvis. The renal pyramids, which are actually openings of the collecting ducts, can be observed in the human renal medulla. The outside is covered by the renal capsule, which is a layer of loose connective tissue.
The basic functional unit of the kidney is the nephron, of which there are more than a million in each normal adult kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood, then reabsorbing some necessary fluid and molecules while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms. The final solution is then excreted as urine.
Each renal corpuscle contains a compact bunch of interconnected capillaries called the "glomerulus", which protrudes into the Bowman's capsule. Each glomerulus is supplied with blood by an afferent (in-coming) arteriole. The capillary walls of the glomerulus have pores allowing filtration. Blood pressure provides the force for blood plasma to be filtered through the thin, porous epithelium of the glomerulus and the neighbouring Bowman's capsule wall to produce the glomerular filtrate, which enter the kidney tubule. Blood leaves the glomerulus through an efferent (out-going) arteriole.
The Bowman's capsule contains a fluid-filled space called "Bowman's space", which is separated from blood in the glomerulus by three layers:
Due to pressure, fluid in the blood is forced out of the glomerulus, through these three layers and into the Bowman's space to form the glomerular filtrate.
The filtrate is blood plasma without blood cells and large-molecule proteins. Small proteins may be present. A person's blood pass through the kidneys 350 times a day at the rate of 1.2 litres per minute, producing 125cc of glomerular filtrate per minute. Measuring the glomerular filtration rate is a diagnostic test of kidney function.
The renal tubule is continuous with Bowman's capsule. The segment that drains glomerular filtrate from the Bowman's capsule is the proximal convoluted tubule. The next portion of the tubule is the loop of Henle, which leads to the distal convoluted tubule.
The loop of Henle was named after Friedrich Gustav Jakob Henle who described it in the early 1860s. The loop of Henle maintains an osmotic gradient set up as a countercurrent exchange to filter and concentrate glomerular filtrate. Cells lining the tubule have numerous mitochondria, enabling active transport to take place by the energy supplied by ATP. Glucose, amino acids, variable ammounts of mineral ions are thus returned to the body. 97.9% of the water in the filtrate enters the convoluted tubules and collecting ducts by osmosis.
Fluid flows from the distal convoluted tubule into the collecting duct system, which consists of:
The site where the ascending loop of Henle touches the afferent arteriole, is called the juxtaglomerular apparatus. It contains macula densa and juxtaglomerular cells . Juxtaglomerular cells are the site of renin synthesis and secretion.
The kidney regulates the pH, mineral ion concentration and water composition of the blood.
Sodium ions are controlled in a homeostatic process involving aldosterone which increases sodium ion absorption in the convoluted tubule.
Any rise or drop in blood osmotic pressure due to a lack or excess of water is detected by the hypothalamus, which notifies the pituitary gland via negative feedback. The pituitary gland secretes antidiuretic hormone, otherwise known as vasopressin (or stops the secretion if water is in excess), which causes a change in the water absorption of the kidney tubule. Tissue fluid concentration thus returns to a mean of 98%.
Renal functions include the excretion of waste material from the bloodstream, secretion of hormones - particularly erythropoietin and renin and maintaining serum electrolyte, acid-base levels and osmolality.
For more info see Renal physiology.
Generally, one can live fine with just one kidney. If both kidneys do not function properly, dialysis is performed, where the blood is filtered outside of the body.
Kidney transplants are now also quite common. The first successful such transplant was announced on March 4, 1954 at Peter Bent Brigham Hospital in Boston. The surgery was performed by Dr. Joseph E. Murray, who in 1990 was awarded the Nobel Prize in Medicine. Kidney transplants can be cadaveric (from someone who has died) or from a living donor (usually a family member). There are several advantages to a kidney transplant from a living donor, including that it is generally a better match, the donor can be thoroughly tested before transplantion and that the kidney tends to have a longer life span .
|Country||Cadaveric transplants||Living donor transplants||Total transplants|
|Canada||724||388||1,112 (in 2000)|||
|France||1,991||136||2,127 (in 2003)|||
|Italy||1,489||135||1,624 (in 2003)|
|Spain||1,991||60||2,051 (in 2003)|
|United Kingdom||1,297||439||1,736 (in 2003)|||
|United States||8,670||6,468||15,138 (in 2003)|||