A glomerulus is a capillary tuft that performs the first step in filtering blood to form urine.
It is 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).
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 glomerular filtrate, and it enters
lumen of
proximal tubule. Otherwise, it returns through the efferent circulation, discussed below.
[[File:Filtration barrier.svg|thumb|350px|Scheme of filtration barrier (blood-urine) in the kidney. A. The endothelial cells of the glomerulus; 1. pore (fenestra). B. Glomerular basement membrane: 1. lamina rara externa 2. lamina densa 3. lamina rara interna
C. Podocytes: 1. enzymatic and structural protein 2. filtration slit 3. diaphragma]]
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 fenestrations that are 70 to 90 nm in diameter. Hence most proteins cannot pass through except smaller ones like albumin.
Glomerular basement membrane
The glomerular endothelium sits on a very thick (250-350
nm)
glomerular basement membrane. Not only is it uncharacteristically thick compared to most other basement membranes (40-60 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 by contracting and reducing the glomerular surface area, and therefore filtration rate, in response mainly to stretch.
==Selectivity==
The structures of the layers determine their permeability-selectivity permselectivity. The factors that influence permselectivity are the negative charge of the basement membrane and the podocytic epithelium, and the effective pore size of the glomerular wall (8 nm). As a result, large and/or negatively charged molecules will pass through far less frequently than small and/or positively charged ones. 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
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 (i.e., 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
References
External links
Image and article at FGCU
"Kidney (Glomerulus)"
- "Mammal, kidney cortex (LM, Medium)"
UNC Nephropathology
Category:Kidney anatomy
