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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. 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.
A nephron consists of an initial filtering component called the renal corpuscle (or Malpighian corpuscle), and a renal tubule that extends from the renal corpuscle.
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. 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 "glomerular filtrate". 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. 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.
Fluids become more concentrated along the tubules and ducts to form urine, which is then drained into the bladder via the ureter.
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 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 by 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.
Medical terms related to the kidneys either involve the prefixes renal or nephro-.
| Urinary system |
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Kidneys - Ureters - Urinary bladder - Urethral sphincters - Urethra |
| Endocrine system |
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Adrenal gland - Corpus luteum - Hypothalamus - Ovaries - Pancreas - Parathyroid gland - Pineal gland - Pituitary gland - Testes - Thyroid gland |
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