Glomerulus
Encyclopedia
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 nephron
s 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).
action as well as hormones can also impact glomerular filtration rate by modulating afferent arteriole diameter.
, and podocyte
s, 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 and as shown on the picture.
) 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 proteins the same size or larger than albumin will be blocked.
.
s 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.
s 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.
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.
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 nephron
s (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.
cells that synthesize renin
. These juxtaglomerular cell
s play a major role in the renin-angiotensin system
, which helps regulate blood volume
and pressure
.
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
tuft that is involved in the first step of filtering blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
to form urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
.
A glomerulus is surrounded by Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
, the beginning component of nephron
Nephron
The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system, which is not part of the nephron....
s in the vertebrate kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
. A glomerulus receives its blood supply from an afferent arteriole of the renal circulation
Renal circulation
The renal circulation receives around 20% of the cardiac output. It branches from the abdominal aorta and returns blood to the ascending vena cava. It is the blood supply to the kidney, and contains many specialized blood vessels.-Circulation:...
. Unlike most other capillary beds, the glomerulus drains into an efferent arteriole
Efferent arteriole
The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus...
rather than a venule
Venule
A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...
. The resistance of these arterioles results in high pressure within the glomerulus, aiding the process of ultrafiltration
Ultrafiltration (renal)
In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent...
, where fluids and soluble materials in the blood are forced out of the capillaries and into Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
.
A glomerulus and its surrounding Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
constitute a renal corpuscle
Renal corpuscle
In the kidney, a renal corpuscle is the initial blood-filtering component of a nephron. It consists of two structures: a glomerulus and a Bowman's capsule. The glomerulus is a small tuft of capillaries containing two cell types. Endothelial cells, which have large fenestrae, are not covered by...
, 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
Renal function
Renal function, in nephrology, is an indication of the state of the kidney and its role in renal physiology. Glomerular filtration rate describes the flow rate of filtered fluid through the kidney...
(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. Sympathetic nervous systemSympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
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 membraneBasement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
, and podocyte
Podocyte
Podocytes are cells in the Bowman's capsule in the kidneys that wrap around the capillaries of the glomerulus...
s, then it enters the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the tubule and is known as glomerular filtrate. Otherwise, it exits the glomerulus through the efferent arteriole and continues circulation as discussed below and as shown on the picture.
Endothelial cells
The endothelial cells of the glomerulus contain numerous pores (fenestraeFenestrae
Fenestræ is a Latin word that means "window".* In histology, fenestræ are small pores in endothelial cells that allow for rapid exchange of molecules between sinusoid blood vessels and surrounding tissue...
) 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 proteins the same size or larger than albumin will be blocked.
Glomerular basement membrane
The glomerular endothelium sits on a very thick (250-350 nm) glomerular basement membraneGlomerular basement membrane
The glomerular basement membrane is the basal laminal portion of the glomerulus. The Glomerular capillary endothelial cells, the GBM and the filtration slits between the podocytes perform the filtration function of the glomerulus, separating the blood in the capillaries from the filtrate that...
.
Podocytes
PodocytePodocyte
Podocytes are cells in the Bowman's capsule in the kidneys that wrap around the capillaries of the glomerulus...
s 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
Podocin
Podocin is a protein component of the slit diaphragm of podocytes. Glomerular capillary endothelial cells, the glomerular basement membrane and the slit diaphragms function as the filtration barrier of the kidney glomerulus....
and nephrin
Nephrin
Nephrin is a protein necessary for the proper functioning of the renal filtration barrier. The renal filtration barrier consists of fenestrated endothelial cells, the glomerular basement membrane, and the podocytes of epithelial cells...
. In addition, foot processes have a negatively-charged coat (glycocalyx
Glycocalyx
Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells. The slime on the outside of a fish is considered a glycocalyx. The term was initially applied to the polysaccharide matrix excreted by epithelial cells forming a...
) that limits the filtration of negatively-charged molecules, such as serum albumin
Serum albumin
Serum albumin, often referred to simply as albumin is a protein that in humans is encoded by the ALB gene.Serum albumin is the most abundant plasma protein in mammals. Albumin is essential for maintaining the osmotic pressure needed for proper distribution of body fluids between intravascular...
.
The podocytes are sometimes considered the "visceral layer of Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
", rather than part of the glomerulus.
Intraglomerular mesangial cell
Intraglomerular mesangial cellIntraglomerular mesangial cell
Intraglomerular mesangial cells are specialized pericytes located among the glomerular capillaries within a renal corpuscle of a kidney.There are three primary functions of intraglomerular mesangial cells: filtration, structural support, and phagocytosis....
s 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
Vascular permeability
Vascular permeability, often in the form of capillary permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules or even whole cells in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells...
-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
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
pass freely, while larger proteins, such as hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
and albumin
Albumin
Albumin refers generally to any protein that is water soluble, which is moderately soluble in concentrated salt solutions, and experiences heat denaturation. They are commonly found in blood plasma, and are unique to other blood proteins in that they are not glycosylated...
have practically no permeability at all.
Efferent circulation
Blood is carried out of the glomerulus by an efferentEfferent arteriole
The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus...
arteriole
Efferent arteriole
The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus...
instead of a venule
Venule
A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...
, 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
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
layer (tunica media
Tunica media
The tunica media is the middle layer of an artery or vein.-Artery:It is made up of smooth muscle cells and elastic tissue...
).
Efferent arterioles of juxtamedullary nephron
Juxtamedullary nephron
A juxtamedullary nephron is a type of nephron, found only in birds and mammals and not in other vertebrates. The "medullary" is referring to the renal medulla, while the "juxta", , refers to the relative position of the renal corpuscle, which in these nephrons is near the medulla...
s (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
Loop of Henle
In the kidney, the loop of Henle is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Named after its discoverer F. G. J...
, these vasa recta
Vasa recta
In the blood supply of the kidney, the vasa recta renis form a series of straight capillaries in the medulla...
play a crucial role in the establishment of the nephron's countercurrent exchange
Countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some component, between two flowing bodies flowing in opposite directions to each other. The flowing bodies can be liquids, gases, or...
system.
The efferent arteriole
Efferent arteriole
The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus...
, into which the glomerulus delivers blood, empties into an interlobular vein.
Juxtaglomerular cells
The walls of the afferent arteriole contain specialized smooth muscleSmooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
cells that synthesize renin
Renin
Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...
. These juxtaglomerular cell
Juxtaglomerular cell
The juxtaglomerular cells are cells in the kidney that synthesize, store, and secrete the enzyme renin. They are specialized smooth muscle cells in the wall of the afferent arteriole that delivers blood to the glomerulus...
s play a major role in the renin-angiotensin system
Renin-angiotensin system
The renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....
, which helps regulate blood volume
Blood volume
Blood volume is the volume of blood in the circulatory system of an individual.-Humans:A typical adult has a blood volume of approximately between 4.7 and 5 liters, with females generally having less blood volume than males....
and pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
.
External links
- Image and article at FGCU "Kidney (Glomerulus)" - "Mammal, kidney cortex (LM, Medium)"
- UNC Nephropathology