Nephron: Structure and Regions

The nephron is the structural and functional unit of the kidney — each kidney has close to a million of them. A single nephron consists of a roughly 4-6 cm long, thin-walled tube called the renal tubule, together with an associated tuft of blood capillaries called the glomerulus. Along its length the renal tubule is divided into a wide, blind, cup-shaped proximal end — the Bowman's capsule — a short neck, the proximal convoluted tubule (PCT), the loop of Henle (LoH), the distal convoluted tubule (DCT), and finally an open, wider collecting tubule.

The glomerulus, sitting inside the cup of the Bowman's capsule, together with the capsule itself, is jointly called the renal corpuscle (or Malpighian body) — each one is only about 200 micrometres across. A small terminal branch of the renal artery, the afferent arteriole, enters the cup and branches extensively into a network of thin-walled, fenestrated capillaries that make up the glomerulus; all these capillaries then reunite into a single efferent arteriole that leaves the cup. Crucially, the afferent arteriole has a larger diameter than the efferent arteriole, and this mismatch is what creates the unusually high hydrostatic pressure inside the glomerulus that drives filtration.

The Bowman's capsule itself is a double-walled, cup-shaped structure, both walls made of squamous epithelium, with a capsular space (urinary space) separating them. The inner, visceral wall is built of specialised squamous cells called podocytes, each bearing foot-like extensions (pedicels) that wrap tightly around the glomerular capillaries, leaving narrow filtration slits between neighbouring podocytes through which filtrate must pass. The outer, parietal wall is a simple, thin sheet of squamous epithelium that continues on into the neck of the nephron, which is lined by ciliated epithelium; the lumen of the neck is called the urinary pole, and the neck leads on into the PCT.

The proximal convoluted tubule is a highly coiled segment lined by cuboidal cells bearing a dense brush border of microvilli, and it is surrounded by peritubular capillaries; because it is so coiled, filtrate moves through it slowly, giving plenty of time for the very efficient reabsorption that makes the PCT the principal site of selective reabsorption.

The loop of Henle is a U-shaped tube with a thin-walled, water-permeable descending limb lined by simple squamous epithelium, and a thick-walled, water-impermeable ascending limb lined by simple cuboidal epithelium. The loop is surrounded by its own set of capillaries, the vasa recta, and its whole job is to run a countercurrent system that helps concentrate urine (osmoregulation). The ascending limb leads on into the DCT.

The distal convoluted tubule is another coiled segment, lined by simple cuboidal epithelium, whose main job is tubular secretion (or 'augmentation') — actively taking up wastes from the surrounding peritubular capillaries and releasing them into the passing filtrate — along with reabsorbing water and helping regulate blood pH. The short, straight collecting tubule that follows continues this water reabsorption and also secretes protons, before opening into the larger collecting duct. …