The glomerulus is the beginning section of the nephron, in the kidney, made of a capilliary which is wound in a tight knot structure.  The glomerulus is surrounded by a cup-like structure known as the Bowman's Capsule and it is the first stage of filtration of the blood as it is entering the kidney. Blood from the afferent arteriole enters the glomerulus and is put under high amounts of pressure to force molecules out of the blood stream and into the Bowman's Capsule as the first stage of ultrafiltration. The efferent arteriole has a very narrow lumen, in order to maintain the high pressure created, enough so that it will force molecules out of the plasma into the Bowman's space. Together, the glomerulus and the Bowman's Capsule form what is known as the renal corpuscle. 
 - Grant's Atlas of Anatomy, Thirteenth Edition, 2001. A.M.R.Agur, A.F.Dalley.
 - Wheater's Functional Histology A Text and Colour Atlas, Fifth Edition 2006. B.Young, J.S.Lowe, A.Stevens, J.W.Heath.
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