Everything about Glomeruli totally explained
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, and thus the measure of the overall renal function, is the
glomerular filtration rate (GFR).
The first place where urine is formed in the kidney, filters fluid from the blood.
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's 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 (250-350
nm)
glomerular basement membrane. It isn't only uncharacteristically thick compared to most other
basement membranes (40-50 nm), but it's 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.
Selectivity
The structures of the layers determine their
permeability-selectivity
permselectivity. For instance, small ions such as
sodium and
potassium pass freely, while larger proteins, such as
hemoglobin and
albumin have practically no permeability at all.
Efferent circulation
Blood is carried out of the glomerulus by an
efferent arteriole 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
Image:Histology-kidney.jpg |1 Glomerulus, 2 proximal tubule, 3 distal tubule
Image:Glomerulus pas.JPG|Glomerulus.
Image:Gray1132.png|Section of cortex of human kidney.
Image:Glomerulum of mouse kidney in Scanning Electron Microscope, magnification 1,000x.GIF|Mouse glomerulus in the Scanning Electron Microscope ("SEM"), magnification 1,000x
Image:Glomerulum of mouse kidney in Scanning Electron Microscope, magnification 5,000x.GIF|Mouse glomerulus in the SEM, magnification 5,000x
Image:Glomerulum of mouse kidney with broken capillary in Scanning Electron Microscope, magnification 10,000x.GIF|Mouse glomerulus in the SEM with one capillary broken, magnification 10,000x
Image:Inner view of fenestrae in capillary of glomerulus in Scanning Electron Microscope, magnification 100,000x.GIF|View on the inside of broken capillary with fenestrae visible, magnification 100,000x
Further Information
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