The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

. The renal medulla is split up into a number of sections, known as the

renal pyramids The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, whic ...

. Blood enters into the kidney via the renal artery, which then splits up to form the segmental arteries which then branch to form

interlobar arteries The interlobar arteries are vessels of the renal circulation The renal circulation supplies the blood to the kidneys via the renal artery, renal arteries, left and right, which branch directly from the abdominal aorta. Despite their relatively sm ...

. The interlobar arteries each in turn branch into

arcuate arteries The arcuate arteries of the kidney, also known as arciform arteries, are vessels of the renal circulation. They are located at the border of the renal cortex and renal medulla. They are named after the fact that they are shaped in arcs due to the ...

, which in turn branch to form

interlobular arteries Cortical radial arteries, formerly known as interlobular arteries, are renal blood vessels given off at right angles from the side of the arcuate arteries looking toward the cortical substance. The interlobular arteries pass directly outward betw ...

, and these finally reach the glomeruli. At the glomerulus the blood reaches a highly disfavourable pressure gradient and a large exchange surface area, which forces the serum portion of the blood out of the vessel and into the renal tubules. Flow continues through the renal tubules, including the

proximal tubule The proximal tubule is the segment of the nephron in kidneys which begins from the renal (tubular) pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s ...

, the

loop of Henle In the kidney, the loop of Henle () (or Henle's loop, Henle loop, nephron loop or its Latin counterpart ''ansa nephroni'') is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after it ...

, through the

distal tubule The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule. Physiology It is partly responsible for the regulation of potassium, sodium, calcium, and pH. On its Apical membrane, apica ...

and finally leaves the kidney by means of the

collecting duct The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through rea ...

, leading to the renal pelvis, the dilated portion of the

ureter The ureters are tubes composed of smooth muscle that transport urine from the kidneys to the urinary bladder. In an adult human, the ureters typically measure 20 to 30 centimeters in length and about 3 to 4 millimeters in diameter. They are lin ...

. The renal medulla contains the structures of the

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structu ...

s responsible for maintaining the salt and water balance of the blood. These structures include the

vasa rectae The vasa recta of the kidney, (vasa recta renis) are the straight arterioles, and the straight venules of the kidney, – a series of blood vessels in the blood supply of the kidney that enter the medulla as the straight arterioles, and leave ...

(both spuria and vera), the venulae rectae, the medullary capillary plexus, the loop of Henle, and the collecting tubule. The renal medulla is hypertonic to the filtrate in the nephron and aids in the reabsorption of water. Blood is filtered in the glomerulus by solute size. Ions such as sodium, chloride, potassium, and calcium are easily filtered, as is glucose. Proteins are not passed through the glomerular filter because of their large size, and do not appear in the filtrate or urine unless a disease process has affected the glomerular capsule or the proximal and distal convoluted tubules of the nephron. Though the renal medulla only receives a small percentage of the renal blood flow, the oxygen extraction is very high, causing a low oxygen tension and more importantly, a critical sensitivity to hypotension, hypoxia, and blood flow. The renal medulla extracts oxygen at a ratio of ~80% making it exquisitely sensitive to small changes in renal blood flow. The mechanisms of many perioperative renal insults are based on the disruption of adequate blood flow (and therefore oxygen delivery) to the renal medulla.

Interstitium

The medullary interstitium is the tissue surrounding the

in the medulla. It functions in renal water reabsorption by building up a high hypertonicity, which draws water out of the

thin descending limb of the loop of Henle Thin may refer to: * ''Thin'' (film), a 2006 documentary about eating disorders * Thin, a web server based on Mongrel * Thin (name), including a list of people with the name * Mal language, also known as Thin See also * * * Body shape * ...

and the

collecting duct system The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through rea ...

. Hypertonicity, in turn, is created by an efflux of

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

from the

inner medullary collecting duct The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through rea ...

. Page 837

Pyramids

Renal pyramids (or malpighian pyramids or Malpighi's pyramids named after

Marcello Malpighi Marcello Malpighi (10 March 1628 – 30 November 1694) was an Italians, Italian biologist and physician, who is referred to as the "founder of microscopical anatomy, histology and father of physiology and embryology". Malpighi's name is borne by ...

, a seventeenth-century anatomist) are cone-shaped tissues of the

. In humans, the renal medulla is made up of 10 to 18 of these conical subdivisions. The broad ''base'' of each pyramid faces the

renal cortex The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections ( cortical columns) that extend down between the pyramids. I ...

, and its

apex The apex is the highest point of something. The word may also refer to: Arts and media Fictional entities * Apex (comics) A-Bomb Abomination Absorbing Man Abraxas Abyss Abyss is the name of two characters appearing in Ameri ...

, or papilla, points internally towards the pelvis. The pyramids appear striped because they are formed by straight parallel segments of

s' Loops of Henle and collecting ducts. The base of each pyramid originates at the corticomedullary border and the apex terminates in a papilla, which lies within a

minor calyx The renal calyces ( calyx) are conduits in the kidney through which urine passes. The minor calyces form a cup-shaped drain around the apex of the renal pyramids. Urine formed in the kidney passes through a renal papilla at the apex into the m ...

, made of parallel bundles of urine collecting tubules.

Papilla

The renal papilla is the location where the

in the medulla empty urine into the

in the

. Histologically it is marked by medullary collecting ducts converging to form a papillary duct to channel the fluid. Transitional epithelium begins to be seen.

Clinical significance

Some chemicals toxic to the kidney, called

nephrotoxin Nephrotoxicity is toxicity in the kidneys. It is a poisonous effect of some chemical substance, substances, both toxic chemicals and pharmaceutical drug, medications, on kidney function. There are various forms, and some drugs may affect kidney fun ...

s, damage the renal papillae. Damage to the renal papillae may result in death to cells in this region of the kidney, called renal papillary necrosis. The most common toxic causes of renal papillary necrosis are NSAIDs, such as

ibuprofen Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain, fever, and inflammation. This includes dysmenorrhea, painful menstrual periods, migraines, and rheumatoid arthritis. It can be taken oral administration, ...

acetylsalicylic acid Aspirin () is the Generic trademark, genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Specific inflammatory conditions ...

, and

phenylbutazone Phenylbutazone, often referred to as "bute", is a nonsteroidal anti-inflammatory drug (NSAID) for the short-term treatment of pain and fever in animals. In the United States and United Kingdom, it is no longer approved for human use (except in t ...

, in combination with

dehydration In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild deh ...

. Perturbed renal papillary development has also been shown to be associated with onset of functional obstruction and renal fibrosis. Renal papillary damage has also been associated with nephrolithiasis and can be quantified according to the papillary grading score, which accounts for contour, pitting, plugging and Randall's plaque.

Image gallery

File:Slide4iii.JPG, Renal medulla File:Slide21iii.JPG, Renal medulla File:Slide5iii.JPG, Renal papilla File:Illu kidney2.jpg, Frontal section through the kidney File:Kidney section vertical.png, Vertical section of kidney. (Label "medullary sub." visible near top.) File:Blausen 0592 KidneyAnatomy 01.png, Kidney anatomy, with pyramids labeled at right

References

External links

* * - "Posterior Abdominal Wall: Internal Structure of a Kidney" * - "Urinary System: neonatal kidney" * () {{DEFAULTSORT:Renal Medulla Kidney anatomy