Microscopic Anatomy of the Kidney

Core Concepts

The renal corpuscle: a pressurized filter

The renal corpuscle consists of the glomerulus — a tuft of fenestrated capillaries — wrapped in Bowman's capsule. Blood enters via the afferent arteriole and leaves via the smaller efferent arteriole; this difference helps maintain high pressure inside the tuft (commonly taught). The capsule's visceral layer is made of podocytes, cells with finger-like foot processes; gaps between them form filtration slits bridged by slit diaphragms. Between the capillaries sit mesangial cells, which contract, clear debris, and regulate flow. The three-layer filtration membrane — fenestrated endothelium, basement membrane, filtration slits — blocks blood cells and almost all plasma proteins while letting water, ions, and small solutes through.

The renal tubule: segment by segment

Proximal convoluted tubule (PCT). Simple cuboidal cells with a brush border of microvilli and abundant mitochondria (fuel for active transport); it reabsorbs most filtered water and solutes and secretes some wastes — the nephron's workhorse.

Loop of Henle. The thin descending limb is permeable to water but not solutes, so water leaves and the filtrate concentrates; the ascending limb is impermeable to water, and its thick portion transports Na⁺, K⁺, and Cl⁻ out via the Na-K-2Cl cotransporter (NKCC2), diluting the fluid and building the medullary salt gradient.

Distal convoluted tubule (DCT). Cuboidal cells with few microvilli; reabsorbs Na⁺ and Cl⁻ (Na-Cl cotransporter, NCC) and Ca²⁺ under parathyroid hormone (PTH).

Collecting duct. Runs from cortex to medulla, receiving fluid from many nephrons. Principal cells reabsorb Na⁺ (aldosterone) and water (ADH, via aquaporin-2) and secrete K⁺; intercalated cells secrete H⁺ or reabsorb HCO₃⁻ for acid-base balance.

The juxtaglomerular apparatus

Where the DCT's wall touches the afferent arteriole of its own nephron, three cell groups form the juxtaglomerular (JG) apparatus. JG cells are modified smooth-muscle cells in the afferent arteriole that synthesize, store, and release renin. The macula densa is a patch of tall, closely packed DCT cells that sense the NaCl concentration of the fluid flowing past. Extraglomerular mesangial cells sit between the two. The apparatus does two jobs: tubuloglomerular feedback (macula densa adjusts arteriole tone based on NaCl delivery) and renin release when blood pressure or NaCl delivery is low, starting the renin-angiotensin cascade.

Cortical versus juxtamedullary nephrons

About 85% are cortical nephrons: corpuscles in the outer cortex, short loops of Henle, wrapped in peritubular capillaries. About 15% are juxtamedullary nephrons: corpuscles near the corticomedullary junction, loops plunging deep into the medulla, with vasa recta — straight vessels — running alongside. The long loops plus vasa recta are essential for building and preserving the medullary concentration gradient that allows the kidney to produce concentrated urine.

How It Works / Step-by-Step Process

Follow a drop of filtrate. 1) The three-layer membrane filters plasma into Bowman's capsule. 2) The PCT reabsorbs most water, glucose, amino acids, and ions; wastes and some drugs are secreted in. 3) The descending limb lets water leave, concentrating the fluid. 4) The ascending limb moves salt out but no water, diluting the fluid while the medulla gains salt. 5) The DCT reabsorbs a little more salt and (with PTH) calcium. 6) Aldosterone and ADH set the final Na⁺, K⁺, and water amounts in the collecting duct; the result drains toward the papilla as urine.

Blood-pressure signal. Low NaCl at the macula densa → JG cells release renin → renin-angiotensin cascade raises blood pressure (commonly taught pathway).

Common Confusions