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Concept
Bowman's capsule
Summary
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.
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 (or "urinary space", or "capsular 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 (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.
The process of filtration of the blood in the Bowman's capsule is ultrafiltration (or glomerular filtration), and the normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume.
Official source
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