A glomerulus /ɡlɒˈmɛrələs/ is a capillary tuft that is involved in the first step of
filtering blood to form urine. A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus 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 these arterioles results in
high pressure within 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 capillaries of a glomerulus branches off of
an interlobular artery in the renal cortex. Glomerular capillary pressure, and thus
glomerular filtration rate, can be
influenced by constriction or relaxation of
the afferent arteriole, resulting in
decreases or increases in pressure. As an
example, one study involving rats found that having narrowed afferent arterioles
contributed to the development of increased blood pressure.[1]Sympathetic nervous system action as well as hormones can also impact glomerular filtration rate
by modulating afferent arteriole diameter. Layers If a substance has passed through the
glomerular capillary endothelial cells,
glomerular basement membrane, and podocytes, then it enters the lumen of the tubule and is known as glomerular filtrate.
Otherwise, it exits the glomerulus through
the efferent arteriole and continues
circulation as discussed below. 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 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 See also: Table of permselectivity for different substances 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. [2] 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 (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.
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment