Information about Glomeruli

Glomerulus.
Distribution of bloodvessels in cortex of kidney.
Latin	glomerulus renis
	subject #253 1221
Precursor	Metanephric blastema
MeSH	Kidney+Glomerulus
Dorlands/Elsevier	g_07/12394744

A glomerulus is a capillary tuft surrounded by Bowman's capsule in nephrons of the vertebrate kidney. It receives its blood supply from an afferent arteriole of the renal circulation. Unlike most other capillary beds, the glomerulus drains into an efferent arteriole rather than a venule. The resistance of the arterioles results in high pressure in the glomerulus aiding the process of ultrafiltration where fluids and soluble materials in the blood are forced out of the capillaries and into Bowman's capsule.

A glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle, the basic filtration unit of the kidney. The rate at which blood is filtered through all of the glomeruli is the glomerular filtration rate (GFR), measurements of which are often used to determine renal function.

Afferent circulation

The afferent arteriole that supplies the glomerulus is a branch off of an interlobular artery in the cortex.

Layers

If a substance can pass through the endothelial cells, glomerular basement membrane, and podocytes, then it is known as ultrafiltrate, and it enters proximal tubule. Otherwise, it returns through the efferent circulation, discussed below.

Endothelial cells

The endothelial cells of the glomerulus contain numerous pores (fenestrae) that, unlike those of other fenestrated capillaries, are not spanned by diaphragms. The cells have openings which are so large that nearly anything smaller than a red blood cell passes through that layer.

Because of this, the endothelial cells lining the glomerulus are not usually considered part of the renal filtration barrier.

Glomerular basement membrane

The glomerular endothelium sits on a very thick (100-200 nm) glomerular basement membrane. It is not only uncharacteristically thick compared to most other basement membranes (40-50 nm), but it is also rich in negatively charged glycosaminoglycans such as heparan sulfate.

The negatively-charged basement membrane repels negatively-charged proteins from the blood, helping to prevent their passage into Bowman's space.

Podocytes

Podocytes line the other side of the glomerular basement membrane and form part of the lining of Bowman's space. Podocytes form a tight interdigitating network of foot processes (pedicels) that control the filtration of proteins from the capillary lumen into Bowman's space.

The space between adjacent podocyte foot processes is spanned by a slit diaphragm formed by several proteins including podocin and nephrin. In addition, foot processes have a negatively-charged coat (glycocalyx) that limits the filtration of negatively-charged molecules, such as serum albumin.

The podocytes are sometimes considered the "visceral layer of Bowman's capsule", rather than part of the glomerulus.

Intraglomerular mesangial cell

Intraglomerular mesangial cells are found in the interstitium between endothelial cells of the glomerulus. They are not part of the filtration barrier but are specialized pericytes that participate indirectly in filtration.

Efferent circulation

Blood is carried out of the glomerulus by an efferentarteriole instead of a venule, as is observed in most other capillary systems. This provides tighter control over the bloodflow through the glomerulus, since arterioles can be dilated and constricted more readily than venules, owing to arterioles' larger smooth muscle layer (tunica media).

Efferent arterioles of juxtamedullary nephrons (ie, the 15% of nephrons closest to the medulla) send straight capillary branches that deliver isotonic blood to the renal medulla. Along with the loop of Henle, these vasa recta play a crucial role in the establishment of the nephron's countercurrent exchange system.

The efferent arteriole, into which the glomerulus delivers blood, empties into an interlobular vein.

Juxtaglomerular cells

The walls of the afferent arteriole contain specialized smooth muscle cells that synthesize renin. These juxtaglomerular cells play a major role in the renin-angiotensin system, which helps regulate blood volume and pressure.

Additional images

Malphigian corpuscle	1 Glomerulus, 2 proximal tubule, 3 distal tubule	Glomerulus.	Glomerulus.
Section of cortex of human kidney.

External links

• Image and article at FGCU
• Histology at KUMC epithel-epith02 "Kidney (Glomerulus)"
• Organology at UC Davis Urinary/mammal/cortex1/cortex1 - "Mammal, kidney cortex (LM, Medium)"
• Histology at BU 16010loa
• {http://www.uncnephropathology.org UNC Nephropathology}

•  • [ e] Urinary system - Kidney
Layers	Renal fascia • Renal capsule • Renal cortex  (Renal column) • Renal medulla (Renal sinus, Renal pyramids) • Renal lobe • Cortical lobule • Medullary ray • Nephron
Afferent circulation	Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Cortical radial arteries → Afferent arterioles → Renal corpuscle (Glomerulus, Bowman's capsule)
Renal tubule	Proximal tubule → Loop of Henle (Descending, Thin ascending, Thick ascending) → Distal convoluted tubule → Connecting tubule → Collecting ducts → Duct of Bellini → Renal papilla → Minor calyx → Major calyx → Renal pelvis → Ureter
Efferent circulation	Glomerulus → Efferent arterioles → Peritubular capillaries/Vasa recta → Arcuate vein → Interlobar veins → Renal vein
Juxtaglomerular apparatus	Macula densa • Juxtaglomerular cells • Extraglomerular mesangial cells
Filtration	Glomerular basement membrane • Podocyte • Filtration slits • Intraglomerular mesangial cells