STRUCTURE OF THE KIDNEY

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. Minor calyx • 15. Major calyx • 16. Renal papilla • 17. Renal column The kidney has a bean-shaped structure;
each kidney has a convex and concave
surface. The concave surface, the renal hilum, is the point at which the renal artery enters the organ, and the renal vein and ureter leave. The kidney is surrounded by tough fibrous tissue, the renal capsule, which is itself surrounded by perinephric fat, renal fascia (of Gerota) and paranephric fat. The anterior (front) border of these tissues is the peritoneum, while the posterior (rear) border is the transversalis fascia. The superior border of the right kidney is
adjacent to the liver; and the spleen, for the left kidney. Therefore, both move
down on inhalation. The kidney is approximately 11–14 cm in
length, 6 cm wide and 4 cm thick. The substance, or parenchyma, of the kidney is divided into two major
structures: superficial is the renal cortex and deep is the renal medulla. Grossly, these structures take the shape of 8 to 18
cone-shaped renal lobes, each containing renal cortex surrounding a portion of
medulla called a renal pyramid (of Malpighi).[5] Between the renal pyramids are projections of cortex called renal columns (of Bertin). 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, located in the cortex, which 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, which becomes the ureter. Blood supply 3D-rendered computed tomography, showing renal arteries and veins. The kidneys receive blood from 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.[5] 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 interstitum (or interstitium) is the functional space in the kidney beneath the
individual filters (glomeruli) which are rich
in blood vessels. The interstitum absorbs fluid recovered from urine. Various conditions can lead to scarring and congestion 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. Histology Microscopic photograph of the renal medulla. Microscopic photograph of the renal cortex. Renal histology studies the structure of the kidney as viewed under a microscope. Various distinct cell types occur in the kidney, including: Kidney glomerulus parietal cell Kidney glomerulus podocyte Kidney proximal tubule brush border cell Loop of Henle thin segment cell Thick ascending limb cell Kidney distal tubule cell Kidney collecting duct cell Interstitial kidney cells Innervation The kidney and nervous system communicate via the renal plexus, whose fibers course along the renal arteries to reach the kidney.[7] Input from the sympathetic nervous system triggers vasoconstriction in the kidney, thereby reducing renal blood flow.[7] The kidney is not thought to receive input from the parasympathetic nervous system .[7] Sensory input from the kidney travels to
the T10-11 levels of the spinal cord and is sensed in the corresponding dermatome.[7] Thus, pain in the flank region may be referred from the kidney. [7] Functions Main article: Renal physiology The kidney participates in whole-body homeostasis, regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure. The kidney accomplishes these homeostatic functions both independently
and in concert with other organs,
particularly those of the endocrine system. Various endocrine hormones coordinate
these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others. Many of the kidney's functions are
accomplished by relatively simple
mechanisms of filtration, reabsorption, and
secretion, which take place in the nephron. Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are filtered from the blood
to make an ultrafiltrate that eventually
becomes urine. The kidney generates 180
liters of filtrate a day, while reabsorbing a
large percentage, allowing for the
generation of only approximately 2 liters of urine. Reabsorption is the transport of
molecules from this ultrafiltrate and into
the blood. Secretion is the reverse process,
in which molecules are transported in the
opposite direction, from the blood into the
urine. Excretion of wastes The kidneys excrete a variety of waste
products produced by metabolism. These include the nitrogenous wastes called
"urea", from protein catabolism, as well as uric acid, from nucleic acid metabolism. Formation of urine is also the function of
the kidney. Acid-base homeostasis Main article: Acid-base homeostasis Two organ systems, the kidneys and lungs,
maintain acid-base homeostasis, which is
the maintenance of pH around a relatively stable value. The lungs contribute to acid-
base homeostasis by regulating bicarbonate (HCO3-) concentration. The kidneys have two very important roles in
maintaining the acid-base balance: to
reabsorb bicarbonate from urine, and to
excrete hydrogen ions into urine Osmolality regulation Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone (ADH), 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. ADH binds to principal cells in the collecting
duct that translocate aquaporins to the
membrane, allowing water to leave the
normally impermeable membrane and be
reabsorbed into the body by the vasa recta,
thus increasing the plasma volume of the body. There are two systems that create a
hyperosmotic medulla and thus increase
the body plasma volume: Urea recycling
and the 'single effect.' Urea is usually excreted as a waste product
from the kidneys. However, when plasma
blood volume is low and ADH is released
the aquaporins that are opened are also
permeable to urea. This allows urea to
leave the collecting duct into the medulla creating a hyperosmotic solution that
'attracts' water. Urea can then re-enter the
nephron and be excreted or recycled again
depending on whether ADH is still present
or not. The 'Single effect' describes the fact that
the ascending thick limb of the loop of
Henle is not permeable to water but is
permeable to NaCl. This allows for a countercurrent exchange system whereby the medulla becomes increasingly
concentrated, but at the same time setting
up an osmotic gradient for water to follow
should the aquaporins of the collecting duct
be opened by ADH. Blood pressure regulation Main articles: Blood pressure regulation and Renin-angiotensin system Long-term regulation of blood pressure predominantly depends upon the kidney.
This primarily occurs through maintenance
of the extracellular fluid compartment, the size of which depends on the plasma sodium concentration. Although the kidney cannot directly sense blood pressure,
changes in the delivery of sodium and chloride to the distal part of the nephron alter the kidney's secretion of the enzyme renin. When the extracellular fluid compartment is expanded and blood
pressure is high, the delivery of these ions
is increased and renin secretion is
decreased. Similarly, when the
extracellular fluid compartment is
contracted and blood pressure is low, sodium and chloride delivery is decreased
and renin secretion is increased in
response. Renin is the first in a series of important
chemical messengers that comprise the renin-angiotensin system. Changes in renin ultimately alter the output of this system,
principally the hormones angiotensin II and aldosterone. Each hormone acts via multiple mechanisms, but both increase the
kidney's absorption of sodium chloride,
thereby expanding the extracellular fluid
compartment and raising blood pressure.
When renin levels are elevated, the
concentrations of angiotensin II and aldosterone increase, leading to increased
sodium chloride reabsorption, expansion of
the extracellular fluid compartment, and an
increase in blood pressure. Conversely,
when renin levels are low, angiotensin II
and aldosterone levels decrease, contracting the extracellular fluid
compartment, and decreasing blood
pressure. Hormone secretion The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It
stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Part of the renin-angiotensin-aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels. Development Main article: Kidney development 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.[8] Evolutionary adaptation Kidneys of various animals show evidence
of evolutionary adaptation and have long been studied in ecophysiology and comparative physiology . Kidney morphology, often indexed as the relative
medullary thickness, is associated with
habitat aridity among species of mammals. [9] Etymology 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).[10] For example, surgical removal of the kidney is a nephrectomy, while a reduction in kidney function is called renal dysfunction. Diseases and disorders Main article: Nephropathy Congenital 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.[11][12] Duplicated ureter occurs in approximately one in 100 live births Horseshoe kidney occurs in approximately one in 400 live births 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 is invariably fatal. 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