Glomerulus (kidney)
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The glomerulus is a capillary bed found surrounded by the Bowman's capsule of the nephron in the vertebrate kidney.
Glomerular endothelial cells are perforated by thousands of small 'cracks' known as "fenestrae". These 'cracks' allow water and small solutes to pass through, but not proteins and cells.
Blood enters the glomerulus through the afferent arteriole, and empties into the efferent arteriole. This organisation of the circulation is unusual as blood leaving capillaries passes into an arteriole as opposed to a venule.The resistance of the arterioles results in high pressure in the glomerular capillaries aiding the process of ultrafiltration where fluids and soluble materials in the blood are forced out of the capillaries and into the Bowman's capsule.
Fluids collected in the Bowman's capsule is known as glomerular filtrate, which eventually becomes urine after further processing along the nephron.