The are
Organs that filter wastes (such as
Urea ) from the
Blood and
Excrete them, along with
Water , as
Urine . The medical field that studies the kidneys and diseases of the kidney is called
Nephrology 1. The prefix ''nephro-'' meaning kidney is from the
Ancient Greek word ''nephros (νεφρός)''; the
Adjective ''renal'' meaning related to the kidney is from
Latin ''rēnēs'', meaning kidneys.
In
Human s, the kidneys are located in the
Posterior part of the
Abdomen . There is one on each side of the
Spine ; the right kidney sits just below the
Liver , the left below the
Diaphragm and adjacent to the
Spleen . Above each kidney is an
Adrenal Gland (also called the ''suprarenal gland''). The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one while the left kidney is located slightly more medial.
The kidneys are
Retroperitoneal . They are approximately at the
Vertebra l level T12 to L3. The upper parts of the kidneys are partially protected by the eleventh and twelfth
Rib s, and each whole kidney is surrounded by two layers of fat (the perirenal and pararenal fat) which help to cushion it. Congenital absence of one or both kidneys, known as unilateral or bilateral
Renal Agenesis , can occur.
s.]]
In a normal
Human adult, each kidney is about 10 cm long, 5.5 cm in width and about 3 cm thick, weighing 150 grams.
2 Together, kidneys weigh about 0.5% of a person's total body weight . 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
Hilum , which admits the renal
Artery , the renal
Vein ,
Nerve s, and the
Ureter .
The outer portion of the kidney is called the
Renal Cortex , which sits directly beneath the kidney's
Loose Connective Tissue/fibrous Capsule . Deep to the cortex lies the
Renal Medulla , which is divided into 10-20
Renal Pyramid s in humans. Each pyramid together with the associated overlying cortex forms a renal lobe. The tip of each pyramid (called a ''
Papilla '') empties into a
Calyx , and the
Calices empty into the
Renal Pelvis . The pelvis transmits urine to the
Urinary Bladder via the
Ureter .
Each kidney receives its
Blood Supply from the
Renal Artery , two of which branch from the abdominal
Aorta . Upon entering the hilum of the kidney, the renal artery divides into smaller interlobar arteries situated between the renal papillae. At the outer medulla, the interlobar arteries branch into arcuate arteries, which course along the border between the renal medulla and cortex, giving off still smaller branches, the cortical radial arteries (sometimes called ''interlobular arteries''). Branching off these cortical arteries are the afferent arterioles supplying the glomerular capillaries, which drain into efferent arterioles. Efferent arterioles divide into peritubular capillaries that provide an extensive blood supply to the cortex. Blood from these capillaries collects in renal venules and leaves the kidney via the
Renal Vein . Efferent arterioles of glomeruli closest to the medulla (those that belong to
Juxtamedullary Nephron s) send branches into the medulla, forming the vasa recta. Blood supply is intimately linked to blood pressure.
2.
Efferent Vessel 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.
Afferent Vessel 13.
Nephron 14.
Minor Calyx 15.
Major Calyx 16.
Renal Papilla 17.
Renal Column ]]
See Also: Nephron
The basic functional unit of the kidney is the
Nephron , of which there are more than a million within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and solute within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and soluble matter (especially
Electrolyte s) in the body by first filtering the blood under pressure, and then reabsorbing some necessary fluid and molecules back into the blood while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and
Countertransport mechanisms established in the nephrons and associated collecting ducts.
See Also: Collecting duct system
The fluid flows from the nephron into the
Collecting Duct System . This segment of the nephron is crucial to the process of water conservation by the organism. In the presence of
Antidiuretic Hormone (ADH; also called vasopressin), these ducts become permeable to water and facilitate its reabsorption, thus concentrating the urine and reducing its volume. Conversely, when the organism must eliminate excess water, such as after excess fluid drinking, the production of ADH is decreased and the collecting tubule becomes less permeable to water, rendering urine dilute and abundant. Failure of the organism to decrease ADH production appropriately, a condition known as
Syndrome Of Inappropriate ADH (SIADH), may lead to water retention and dangerous dilution of body fluids, which in turn may cause severe neurological damage. Failure to produce ADH (or inability of the collecting ducts to respond to it) may cause excessive urination, called
Diabetes Insipidus (DI).
A second major function of the collecting duct system is the maintenance of
Acid-base Homeostasis .
After being processed along the collecting tubules and ducts, the fluid, now called
Urine , is drained into the
Bladder via the
Ureter , to be finally excluded from the organism.
See Also: Renal physiology
The kidneys excrete a variety of (from protein catabolism) and
Uric Acid (from nucleic acid metabolism) and water.
The kidney is one of the major organs involved in whole-body
Homeostasis . Among its homeostatic functions are acid-base balance, regulation of
Electrolyte concentrations, control of
Blood Volume , and regulation of
Blood Pressure . The kidneys accomplish these homeostatic functions independently and through coordination with other organs, particularly those of the
Endocrine System . The kidney communicates with these organs through hormones secreted into the bloodstream.
The kidneys regulate the
PH , by eliminating H ions concentration called augmentation mineral ion concentration, and water composition of the blood.
By exchanging
Hydronium Ions and
Hydroxyl Ions , the blood plasma is maintained by the kidney at a slightly alkaline pH of 7.4. Urine, on the other hand, is acidic at pH 5 or alkaline at pH 8.
The pH is maintained through four main -
Hydrogen exchanger), V-type H-ATPase (an
Isoform of the hydrogen
ATPase ), NBC1 (a sodium-
Bicarbonate cotransporter) and AE1 (an
Anion exchanger which exchanges chloride for bicarbonate). Due to the polar alignment of cells in the renal epithelia NHE3 and the H-ATPase are exposed to the
Lumen (which is essentially outside the body), on the
Apical side of the cells, and are responsible for excreting hydrogen ions (or protons). Conversely, NBC1 and AE1 are on the
Basolateral side of the cells, and allow bicarbonate ions to move back into the extracellular fluid and thus are returned to the blood plasma.
See Also: Renin-angiotensin system
Sodium ions are controlled in a homeostatic process involving
Aldosterone which increases sodium ion reabsorption in the distal convoluted tubules.
When blood pressure becomes low, a proteolytic enzyme called
Renin is secreted by cells of the
Juxtaglomerular Apparatus (part of the distal convoluted tubule) which are sensitive to pressure. Renin acts on a blood protein,
Angiotensinogen , converting it to angiotensin I (10 amino acids). Angiotensin I is then converted by the
Angiotensin-converting Enzyme (ACE) in the lung capillaries to Angiotensin II (8 amino acids), which stimulates the secretion of Aldosterone by the adrenal cortex, which then affects the renal tubules.
Aldosterone stimulates an increase in the reabsorption of sodium ions from the kidney tubules which causes an increase in the volume of water that is reabsorbed from the tubule. This increase in water reabsorption increases the volume of blood which ultimately raises the blood pressure.
Any significant rise or drop in
Plasma Osmolality is detected by the
Hypothalamus , which communicates directly with the
Posterior Pituitary Gland . A rise in osmolality causes the gland to secrete
Antidiuretic Hormone , resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.
The kidneys secrete a variety of
Hormones , including
Erythropoietin ,
Urodilatin ,
Renin and
Vitamin D .
In the case of starvation, in the kidneys, an amino group is removed from protein and glucose is formed in the process of gluconeogenesis.
See Also: Development of the urinary and reproductive organs
The mammalian kidney develops from
Intermediate Mesoderm . Kidney development, also called ''nephrogenesis'', proceeds through a series of three successive phases, each marked by the development of a more advanced pair of kidneys: the pronephros, mesonephros, and metanephros.
3 (The plural forms of these terms end in ''-oi''.)
See Also: Pronephros
During approximately day 22 of human
Gestation , the paired pronephroi appear towards the cranial end of the intermediate mesoderm. In this region,
Epithelial cells arrange themselves in a series of tubules called
Nephrotome s and join laterally with the
Pronephric Duct , which does not reach the outside of the embryo. Thus the pronephros is considered nonfunctional in mammals because it cannot excrete waste from the embryo.
See Also: Mesonephros
Each pronephric duct grows towards the tail of the embryo, and in doing so induces intermediate mesoderm in the thoracolumbar area to become epithelial tubules called
Mesonephric Tubule s. Each mesonephric tubule receives a blood supply from a branch of the
Aorta , ending in a capillary tuft analogous to the
Glomerulus of the definitive
Nephron . The mesonephric tubule forms a capsule around the capillary tuft, allowing for filtration of blood. This filtrate flows through the mesonephric tubule and is drained into the continuation of the pronephric duct, now called the
Mesonephric Duct or
Wolffian Duct . The nephrotomes of the pronephros degenerate while the mesonephric duct extends towards the most caudal end of the embryo, ultimately attaching to the
Cloaca . The mammalian mesonephros is similar to the kidneys of
Aquatic Amphibian s and
Fish es.
During the fifth week of gestation, the mesonephric duct develops an outpouching, the
Ureteric Bud , near its attachment to the cloaca. This bud, also called the metanephrogenic diverticulum, grows posteriorly and towards the head of the embryo. The elongated stalk of the ureteric bud, the
Metanephric Duct , later forms the
Ureter . As the cranial end of the bud extends into the intermediate mesoderm, it undergoes a series of branchings to form the
Collecting Duct System of the kidney. It also forms the major and minor
Calyces and the
Renal Pelvis .
The portion of undifferentiated intermediate mesoderm in contact with the tips of the branching ureteric bud is known as the metanephrogenic blastema. Signals released from the ureteric bud induce the differentiation of the metanephrogenic blastema into the
Renal Tubule s. As the renal tubules grow, they come into contact and join with
Connecting Tubule s of the collecting duct system, forming a continuous passage for flow from the renal tubule to the collecting duct. Simultaneously, precursors of vascular endothelial cells begin to take their position at the tips of the renal tubules. These cells differentiate into the cells of the definitive
Glomerulus .
.]]
.]]
- : The membranous covering of the kidney.
- ''' tubes and is supported by a fibrous matrix.
- : The opening in the middle of the concave medial border for nerves and blood vessels to pass into the renal sinus.
- : The structures which support the cortex. They consist of lines of blood vessels and urinary tubes and a fibrous material.
- : The cavity which houses the renal pyramids.
- : The recesses in the internal medulla which hold the pyramids. They are used to subdivide the sections of the kidney. (singular - ''calyx'')
- '''. They have openings through which urine passes into the calyces. (singular - Papilla )
- '''s''.
- ''', 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.
- '''.
- ''', the renal pelvis accepts the urine and channels it out of the hilus into the Ureter .
- '''. The ureter carries urine from the kidneys to the Bladder by means of Peristalsis .
- : Each pyramid together with the associated overlying cortex forms a renal lobe
Generally, humans can live normally with just one kidney, as one has more functioning renal tissue than is needed to survive, possibly due to the nature of the prehistoric human diet. Only when the amount of functioning kidney tissue is greatly diminished will
Chronic Renal Failure develop. If the
Glomerular Filtration Rate (a measure of renal function) has fallen very low (
End-stage Renal Failure ), or if the renal dysfunction leads to severe symptoms, then
Renal Replacement Therapy is indicated, either
Dialysis or
Renal Transplantation .
- Medical terms related to the kidneys involve the prefixes ''renal-'' and ''nephro-''.
- Surgical removal of the kidney is a '' Nephrectomy '', while a ''radical nephrectomy'' is removal of the kidney, its surrounding tissue, lymph nodes, and potentially the adrenal gland. A radical nephrectomy is performed for the removal of the Cancer s.
The kidneys of animals can be
Cooked and eaten by humans (along with other
Offal ). If prepared properly, they can be nutritious and pleasant tasting (if somewhat bland). Veal kidneys and lamb kidneys are particularly prized for their tenderness and flavour. Kidneys can be grilled or sautéed, though they become tough and unpleasant if overcooked.
Chinese cuisine includes sauteed pork kidneys, which are first soaked in water and then cleaned with scissors to remove nephrons and excess urine.
Pork kidneys, along with pork tongue and beef tongue, are some of the most cholesterol intense sources. A serving of pork kidney or beef tongue can yield more than 200% of the allotted daily intake of cholesterol.
World Kidney Day is observed on the second Thursday of March every year.
http://www.ifkf.net/worldkidneyday.php
It was held for the first time in 2006, to increase awareness of kidney disease and educate persons at risk regarding the importance of prevention and early detection.
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5608a1.htm It is a joint initiative by the International Society of Nephrology (ISF) and International Federation of Kidney Foundations (IFKF).
The next World Kidney Day will be held on 13 March 2008. In 2007, it was held on 8th March.
