release. (b) Micrograph showing the glomerulus and surrounding structures. 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.
Lining 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.
Mesangium 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. The mesangium contains mainly: •
Intraglomerular mesangial cells. They are not part of the filtration barrier but are specialized
pericytes that participate in the regulation of the filtration rate by contracting or expanding: they contain actin and myosin filaments to accomplish this. Some mesangial cells are in physical contact with capillaries, whereas others are in physical contact with podocytes. There is two-way chemical cross talk among the mesangial cells, the capillaries, and the podocytes to fine-tune the glomerular filtration rate. •
Mesangial matrix, an amorphous
basement membrane-like material secreted by the mesangial cells.
Blood supply 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. Blood exits the glomerular capillaries by an
efferent arteriole instead of a
venule, as is seen in the majority of capillary systems (Fig. 4). This provides tighter control over the blood flow through the glomerulus, since arterioles dilate and constrict more readily than venules, owing to their thick circular
smooth muscle layer (
tunica media). The blood exiting the efferent arteriole enters a renal
venule, which in turn enters a renal
interlobular vein and then into the
renal vein. Cortical nephrons near the corticomedullary junction (15% of all nephrons) are called
juxtamedullary nephrons. The blood exiting the efferent arterioles of these nephrons enter the
vasa recta, which are straight capillary branches that deliver blood to the
renal medulla. These vasa recta run adjacent to the descending and ascending
loop of Henle and participate in the maintenance of the medullary
countercurrent exchange system.
Filtrate drainage The filtrate that has passed through the three-layered filtration unit enters Bowman's capsule. From there, it flows into the renal tubule—the nephron—which follows a U-shaped path to the
collecting ducts, finally exiting into a
renal calyx as
urine. ==Function==