The Bowman's capsule (other names: capsula glomeruli, glomerular capsule) is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney. A glomerulus is enclosed in the sac. Fluids from blood in the glomerulus are collected in the Bowman's capsule (i.e., glomerular filtrate) and further processed along the nephron to form urine. This process is known as ultrafiltration.
Inside the capsule, the layers are as follows, from outside to inside:
|parietal layer||A single layer of simple squamous epithelium. Does not function in filtration.|
|Bowman's space (or "urinary space", or "capsular space")||Between the visceral and parietal layers, into which the filtrate enters after passing through the podocytes' filtration slits.|
|visceral layer||Lies just beneath the thickened glomerular basement membrane and is made of podocytes. Beneath the visceral layer lie the glomerular capillaries.|
|FIltration Barrier||The filtration barrier is composed of the fenestrated endothelium of the glomerular capillaries, the fused basal lamina of the endothelial cells and podocytes, and the filtration slits of the podocytes. The barrier permits the passage of water, ions, and small molecules from the bloodstream into the Bowman's space. The barrier prevents the passage of large and/or negatively charged proteins (such as albumin). The basal lamina of the filtration barrier is composed of three layers. The first layer is the lamina rara externa, adjacent to the podocyte processes. The second layer is the lamina rara interna, adjacent to the endothelial cells. The final layer is the lamina densa which is a darker central zone of the basal lamina. It consists of the meshwork of type IV collagen and laminin which act as a selective macromolecular filter.|
Any proteins under roughly 30 kilodaltons can pass freely through the membrane, although there is some extra hindrance for negatively charged molecules due to the negative charge of the basement membrane and the podocytes.
A decreased GFR may be a sign of renal failure.
A number of diseases can result in various problems within the glomerulus. Examples include acute proliferative (endocapillary) glomerulonephritis, mesangioproliferative glomerulonephritis, mesangiocapillary (membranoproliferative) glomerulonephritis, acute crescentic glomerulonephritis, focal segmental glomerulonephritis, and diabetic glomerulosclerosis.
Together with the glomerulus it is known as a renal corpuscle, or a Malpighian corpuscle, named after Marcello Malpighi (1628-1694), an Italian physician and biologist. This name is not used widely anymore, probably to avoid confusion with a Malpighian corpuscle in the spleen.