Bowman's capsule is a cup-like sack 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. Fluids from blood in the glomerulus are collected in the Bowman's capsule.
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 chargedproteins. 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.
Function
The process of filtration of the blood in the Bowman's capsule is ultrafiltration, and the normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume. 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. Any small molecules such as water, glucose, salt, amino acids, and urea pass freely into Bowman's space, but cells, platelets and large proteins do not. As a result, the filtrate leaving the Bowman's capsule is very similar to blood plasma in composition as it passes into the proximal convoluted tubule.
Bowman's capsule is named afterSir William Bowman, a British surgeon and anatomist. However, thorough microscopical anatomy of kidney including the nephronic capsule was first described by Ukrainian surgeon and anatomist from the Russian Empire, Prof. :ru:Шумлянский, Александр Михайлович|Alexander Schumlansky, in his 1782 doctoral thesis "De structura renum" ; thus, much prior to Bowman. Together with the glomerulus it is known as a renal corpuscle, or a Malpighian corpuscle, named after Marcello Malpighi, an Italian physician and biologist. This name is not used widely anymore, probably to avoid confusion with Malpighian bodies of the spleen.
