Bowman's capsule

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Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular 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.

Structure

Outside the capsule, there are two "poles":

• The
  afferent arteriole and efferent arteriole
  .
• The
  proximal convoluted tubule
  .

Inside the capsule, the layers are as follows, from outside to inside:^[citation needed]

• 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
  filtration slits.^[1]
• Visceral layer—Lies just above the thickened
  glomerular capillaries.^{[citation needed}
  ]
• 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.^[citation needed]

Function

The process of filtration of the blood in the Bowman's capsule is

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.^{[citation needed]}

Any small molecules such as

Measuring the glomerular filtration rate (GFR) is a diagnostic test of kidney function.^[3]

A decreased GFR may be a sign of kidney failure.^{[citation needed]}

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.^{[citation needed]}

History

Bowman's capsule is named after

• Mesangium
• Glomerulus (kidney)
• Blood–brain barrier
• List of distinct cell types in the adult human body

Additional images

• 
  Glomerulus.

References