Glomerulus

A glomerulus 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. 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.



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
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 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.