Glomerulus (kidney)

The glomerulus (pl.: glomeruli) is a network of small blood vessels (capillaries) known as a tuft, located at the beginning of a nephron in the kidney. Each of the two kidneys contains about one million nephrons. The tuft is structurally supported by the mesangium (the space between the blood vessels), composed of intraglomerular mesangial cells. The blood is filtered across the capillary walls of this tuft through the glomerular filtration barrier, which yields its filtrate of water and soluble substances to a cup-like sac known as Bowman's capsule. The filtrate then enters the renal tubule of the nephron.

The glomerulus receives its blood supply from an afferent arteriole of the renal arterial circulation. Unlike most capillary beds, the glomerular capillaries exit into efferent arterioles rather than venules. The resistance of the efferent arterioles causes sufficient hydrostatic pressure within the glomerulus to provide the force for ultrafiltration.

The 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 kidney function, is the glomerular filtration rate.

The glomerulus is a tuft of capillaries located within Bowman's capsule within the kidney. Glomerular mesangial cells structurally support the tufts. Blood enters the capillaries of the glomerulus by a single arteriole called an afferent arteriole and leaves by an efferent arteriole. The capillaries consist of a tube lined by endothelial cells with a central lumen. The gaps between these endothelial cells are called fenestrae. The walls have a unique structure: there are pores between the cells that allow water and soluble substances to exit and after passing through the glomerular basement membrane and between digitating podocyte foot processes, enter the capsule as ultrafiltrate.

Capillaries of the glomerulus are lined by endothelial cells. These contain numerous pores—also called fenestrae—, 50–100 nm in diameter. Unlike those of other capillaries with fenestrations, these fenestrations are not spanned by diaphragms. They allow for the filtration of fluid, blood plasma solutes and protein, while at the same time preventing the filtration of red blood cells, white blood cells, and platelets.

The glomerulus has a glomerular basement membrane sandwiched between the glomerular capillaries and the podocytes. It consists mainly of laminins, type IV collagen, agrin, and nidogen, which are synthesized and secreted by both endothelial cells and podocytes. The glomerular basement membrane is 250–400 nm in thickness, which is thicker than basement membranes of other tissue. It is a barrier to blood proteins such as albumin and globulin.

The part of the podocyte in contact with the glomerular basement membrane is called a podocyte foot process or pedicle (Fig. 3): there are gaps between the foot processes through which the filtrate flows into Bowman's capsule. The space between adjacent podocyte foot processes is spanned by slit diaphragms consisting of a mat of proteins, including podocin and nephrin. In addition, foot processes have a negatively charged coat (glycocalyx) that repels negatively charged molecules such as serum albumin.

The mesangium is a space which is continuous with the smooth muscles of the arterioles. It is outside the capillary lumen but surrounded by capillaries. It is in the middle (meso) between the capillaries (angis). It is contained by the basement membrane, which surrounds both the capillaries and the mesangium.