The renal medulla is the innermost part of the

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...

. 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, 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 which supply the renal lobes. The interlobar arteries branch from the lobar arteries which branch from the segmental arteries, from the renal artery. They give rise to arcuate arteries ...

. The interlobar arteries each in turn branch into arcuate arteries, which in turn branch to form interlobular arteries, and these finally reach the

glomeruli ''Glomerulus'' () is a common term used in anatomy to describe globular structures of entwined vessels, fibers, or neurons. ''Glomerulus'' is the diminutive of the Latin ''glomus'', meaning "ball of yarn". ''Glomerulus'' may refer to: * the filter ...

. 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 Loop of Henle, 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 surface (lumen ...

and finally leaves the kidney by means of the collecting duct, leading to the renal pelvis, the dilated portion of the ureter. The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') contains the structures of the nephrons 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 th ...

(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 loop of Henle in the medulla. It functions in renal water reabsorption by building up a high

hypertonicity In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane- ...

, which draws water out of the

thin descending limb of the loop of Henle Within the nephron of the kidney, the descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle. Physiology The permeability is as follows: Also, the medullary interstitium is highly c ...

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 system is the last part of nephron and participates in elect ...

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

urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important r ...

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 system is the last part of nephron and participates in elect ...

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Pyramids

Renal pyramids (or malpighian pyramids or Malpighi's pyramids named after Marcello Malpighi, 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, and its apex, or

papilla Papilla (Latin, 'nipple') or papillae may refer to: In animals * Papilla (fish anatomy), in the mouth of fish * Basilar papilla, a sensory organ of lizards, amphibians and fish * Dental papilla, in a developing tooth * Dermal papillae, part of ...

, points internally towards the pelvis. The pyramids appear striped because they are formed by straight parallel segments of nephrons' 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 are chambers of the kidney through which urine passes. The minor calyces surround the apex of the renal pyramids. Urine formed in the kidney passes through a renal papilla at the apex into the minor calyx; two or three minor cal ...

, made of parallel bundles of urine collecting tubules.

Papilla

The renal papilla is the location where the

renal pyramids The renal medulla 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, which then splits up to form the segmental arteries whi ...

in the medulla empty urine into the

in the

. Histologically it is marked by medullary

collecting ducts 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 system is the last part of nephron and participates in elect ...

converging to form a

papillary 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 system is the last part of nephron and participates in electr ...

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 substances, both toxic chemicals and medications, on kidney function. There are various forms, and some drugs may affect kidney function in more than one way. Nephrotoxins ...

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 Renal papillary necrosis is a form of nephropathy involving the necrosis of the renal papilla. Lesions that characterize renal papillary necrosis come from an impairment of the blood supply and from subsequent ischemic necrosis that is diffuse. Sig ...

. 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 for treating pain, fever, and inflammation. This includes painful menstrual periods, migraines, and rheumatoid arthritis. It may also be used to close a patent ductus arte ...

, acetylsalicylic acid, and phenylbutazone, in combination with

dehydration In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...

. 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 plaque.

Additional Images

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:Gray1127.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