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; bloo ...

. 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 each in turn branch into arcuate arteries, which in turn branch to form interlobular arteries, 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 Serum may refer to: *Serum (blood), plasma from which the clotting proteins have been removed **Antiserum, blood serum with specific antibodies for passive immunity * Serous fluid, any clear bodily fluid * Truth serum, a drug that is likely to mak ...

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 and finally leaves the kidney by means of the collecting duct, leading to the renal pelvis, the dilated portion of the

ureter The ureters are tubes made of smooth muscle that propel urine from the kidneys to the urinary bladder. In a human adult, the ureters are usually long and around in diameter. The ureter is lined by urothelial cells, a type of transitional e ...

. The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') 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 structure ...

s responsible for maintaining the salt and water balance of the blood. These structures include the vasa rectae (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, which draws water out of the thin descending limb of the loop of Henle and the collecting duct system. Hypertonicity, in turn, is created by an efflux of urea from the inner medullary collecting duct. 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 Italian biologist and physician, who is referred to as the "Founder of microscopical anatomy, histology & Father of physiology and embryology". Malpighi's name is borne by several ph ...

, 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, 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 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 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 nephrotoxins, 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 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 ...

, 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. Mi ...

. 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