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; blo ...
. 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 arterie ...
. 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 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
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 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 el ...
. 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 ...
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 elec ...
.
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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
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; blo ...
. 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. It ...
, and its
apex, or
papilla, points internally towards the pelvis. The pyramids appear striped because they are formed by straight parallel segments of
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' 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, made of parallel bundles of urine collecting tubules.
Papilla
The renal papilla is the location where the
renal pyramids in the medulla empty urine into the
minor calyx in 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; blo ...
. 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 electr ...
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
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 ar ...
,
acetylsalicylic acid
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat ...
, 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
See also
*
Medullipin Medullipin is a hormone created by the interstitial cells of renal papilla, which is converted to medullipin II in the liver. This, in turn, results in vasodilation and decreased blood pressure.
There are two kinds of medullipin, known as medulli ...
*
Kokko and Rector Model, a theory to explain how a gradient is generated in the inner medulla
*
Renal sinus
*
Medullary interstitium
*
Renal capsule
References
External links
*
* - "Posterior Abdominal Wall: Internal Structure of a Kidney"
* - "Urinary System: neonatal kidney"
* ()
{{DEFAULTSORT:Renal Medulla
Kidney anatomy