Structure of a nephron

Each kidney contains close to a million nephrons, and every nephron has two main parts: a filtering unit called the renal corpuscle, and a long renal tubule leading away from it. The renal corpuscle consists of a double-walled cup, Bowman's capsule, wrapped around a tuft of capillaries called the glomerulus - together these form the site where blood is first filtered. The capillary walls of the glomerulus are perforated by tiny pores (fenestrae), and the inner (visceral) layer of Bowman's capsule is made of specialised cells called podocytes, whose branching foot processes cling to the capillary basement membrane and leave narrow gaps, the filtration slits, between them; the outer (parietal) layer is ordinary simple squamous epithelium. From Bowman's capsule the renal tubule runs on as the proximal convoluted tubule (PCT), then straightens and doubles back on itself as the U-shaped loop of Henle (a thin descending limb followed by a thick ascending limb), then becomes coiled again as the distal convoluted tubule (DCT), before finally opening into a straight collecting duct that carries urine down through the medulla and, after several collecting ducts fuse into papillary ducts, empties it into the calyces. Nephrons come in two types depending on how deep their loop of Henle reaches into the medulla: cortical nephrons, whose short loops barely enter the medulla, and juxtamedullary nephrons, whose long loops plunge deep into it - it is this second group that makes concentrated urine possible. Two separate capillary beds serve every nephron: the glomerulus itself (fed by a wide afferent arteriole and drained by a narrower efferent arteriole, which is what keeps glomerular pressure high) and the peritubular capi …