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 the two organs that are located in the posterior part of the abdomen, on either side of the spine just below the liver and spleen on the right and left sides of the body respectively. 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, due to the size of the liver.
The upper parts of the kidneys are protected somewhat 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 outer portion of the kidney is called the renal cortex, the next portion is called the renal medulla, at the center of the kidney is the pelvis. 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 made 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 substances (especially electrolytes) in the body by filtering it all out first, reabsorbing what should be kept and converting the rest into urine for excretion. They use countercurrent exchange mechanisms.
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:
- a single-cell capillary endothelium in the glomerulus
- a proteinaceous layer of basement membrane
- a single-cell epithelial lining of Bowman's capsule (these cells are called podocytes)
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 mnute. 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 connecting tubule
- the cortical collecting duct
- the medullary collecting duct .
The site where the ascending loop of Henle touches the afferent arteriole, is called the juxtaglomerular apparatus. It contains macula densa and juxtaglomerular cell s. Juxtaglomerular cells are the site of renin synthesis and secretion.
Homeostatic function of the kidney
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.
As for water, the rise (drop) in blood osmotic pressure due to a lack (excess) of water is detected by the hypothalamus. By negative feedback the pituitary gland is notified. The gland secrets antidiuretic hormone, otherwise known as vasopressin (or stops the secretion if water is in excess) and results in a change in the water absorption of the kidney tubule. Tissue fluid concentration thus returns to a mean of 98%.
- 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 pyramid.
- 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 )
- pyramids: The conical segments within the internal medulla. They contain the secreting apparatus and tubules and are also called malpighian pyramid s.
- 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 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.
Diseases and disorders
Congenital diseases of the kidneys
- Congenital hydronephrosis
- renal dysplasia
- Congenital obstruction of urinary tract
- horseshoe kidney
- duplicated ureter
Acquired diseases of the kidneys
- Kidney stones are a relatively common and particularly painful disorder.
- Pyelonephritis is infection of the kidneys and is frequently caused by complication of a urinary tract infection.
- Azotemia is a toxic condition characterized by abnormal and dangerously high levels of urea, creatinine, various body waste compounds, and other nitrogen-rich compounds in the blood.
- Hydronephrosis is the enlargement of one or both of the kidneys caused by obstruction of the flow of urine.
- 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.
- kidney tumors
- Renal failure (acute and chronic)
- Diabetic nephropathy
- Lupus nephritis
Dialysis and kidney transplants
Generally, one can live fine with just one kidney. If both kidneys don't 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 transplation and that the kidney tends to have a longer life span  .
Kidney Transplant Statistics
|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)|||
Medical terms related to the kidneys either involve the prefixes renal or nephro-.