Bowman's capsule is a cup-like sac 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 and
further processed along the nephron to form urine. This process is known as
ultrafiltration. The Bowman's capsule is named after Sir William Bowman, who
identified it in 1842. Anatomy
Outside the capsule, there are two "poles":
The vascular pole is the side with the afferent arteriole and efferent
arteriole. the urinary pole is the side with the
proximal convoluted tubule. 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—Between the visceral and parietal layers, into which the filtrate
enters after passing through the filtration slits.
Visceral layer—Lies just above 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. 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.
Physiology 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. Clinical significance
Measuring the glomerular filtration rate is a diagnostic test of kidney function.
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 glomerulonephritis,
mesangioproliferative glomerulonephritis, mesangiocapillary
glomerulonephritis, acute crescentic glomerulonephritis, focal segmental
glomerulonephritis, and diabetic glomerulosclerosis.
Eponym Bowman's capsule is named after Sir
William Bowman, a British surgeon and anatomist.
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. See also
Mesangium Additional images
References External links
Histology image: 16006loa – Histology Learning System at Boston University
Diagram at ircc.edu