Structure of the Nephron

Each human kidney contains roughly one million tiny, densely packed tubular structures called nephrons, and it is within these nephrons, not in the kidney's gross anatomy as such, that urine is actually formed — which is why the nephron, rather than the kidney as a whole, is described as the structural and functional unit of the excretory system. Every nephron is made up of two principal parts joined end to end: the Malpighian corpuscle at its start, where blood filtration begins, and a long renal tubule that follows it, along which the filtrate is progressively modified.

The Malpighian corpuscle (renal corpuscle) consists of two closely fitted structures. The glomerulus is a compact tuft of specialised capillaries formed when a small branch of the renal artery, the afferent arteriole, enters the corpuscle and immediately divides into a knot of thin-walled, highly permeable capillaries; blood leaves the glomerulus again by a second, narrower vessel, the efferent arteriole. Surrounding the glomerulus on all sides, and continuous at its outer edge with the renal tubule proper, is Bowman's capsule, a double-walled, cup-shaped structure whose inner wall (made of specialised cells called podocytes, which wrap branching foot-processes tightly around the glomerular capillaries) is intimately applied to the glomerulus, while its outer wall bounds a narrow, fluid-filled space — Bowman's space — into which the filtrate first collects before entering the tubule.

From Bowman's capsule, the renal tubule continues as the proximal convoluted tubule (PCT), a much-coiled segment lying entirely within the cortex, whose epithelial cells bear dense microvilli (a brush border) that greatly increase the surface area available for reabsorbing useful substances back out of the filtrate. The PCT then straightens and descends from the cortex into the medulla as the descending limb of the loop of Henle; the loop makes a sharp hairpin turn at varying depths within the medulla and returns to the cortex as the ascending limb of the loop of Henle, part of which is notably thicker-walled than the rest. Re-entering the cortex, the tubule coils once again as the distal convoluted tubule (DCT), before finally opening, together with the DCTs of several neighbouring nephrons, into a shared collecting duct. Several collecting ducts in turn merge as they descend through a renal pyramid, ultimately emptying urine at the papilla into a minor calyx.

Every nephron receives its blood supply from a small branch of the renal artery entering as the afferent arteriole and leaving the glomerulus as the narrower efferent arteriole, described in the next section; after leaving the glomerulus, the efferent arteriole immediately branches again into a network of peritubular capillaries that wrap closely around the proximal and distal convoluted tubules, well placed to reabsorb substances the tubule reclaims from the filtrate. In nephrons whose loop of Henle dips deep into the medulla, the peritubular capillary network extends further as a long, straight, hairpin-shaped vessel called the vasa recta, running parallel to the loop and playing an essential part in the counter-current mechanism described later in this chapter. …