Kidney: cortex, medulla and the nephron !

Organ

The nephron is the unit that filters blood and makes urine; each segment has its own epithelium, which explains its job.

Updated 2026-11-01

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What it is

The kidney has a granular cortex, holding the renal corpuscles and convoluted tubules, and a medulla of pyramids with loops of Henle and collecting ducts. Each kidney holds about a million nephrons. Knowing the epithelium of each segment tells you where diuretics act and which segment is hurt in acute kidney injury.

Structure

Cortex: round renal corpuscles, many pink proximal tubule profiles with a fuzzy lumen, and fewer, paler distal tubules with a wide lumen.
Medullary rays: bundles of straight tubules and collecting ducts running up from the medulla into the cortex.
Medulla: pyramids with thin limbs of squamous epithelium, thick limbs and collecting ducts of cuboidal to columnar cells with sharp cell borders.
Papilla: collecting ducts merge into papillary ducts that empty into the minor calyces.

Function

Filters plasma in the glomerulus and reabsorbs most of the filtrate along the tubule.
Regulates volume, sodium, potassium and pH, and concentrates or dilutes urine under ADH.
Also works as an endocrine gland: renin, erythropoietin and active vitamin D.

Nephron segments: epithelium and main job

Segment	Epithelium	Main job
Renal corpuscle	Simple squamous parietal layer of Bowman's capsule; visceral layer of podocytes	Filtration of plasma
Proximal convoluted tubule	Simple cuboidal with brush border, eosinophilic cytoplasm and many mitochondria	Reabsorbs most sodium and water and nearly all glucose and amino acids
Loop of Henle, thin limbs	Simple squamous	The descending limb lets water out into the hypertonic medulla
Thick ascending limb	Simple cuboidal without brush border	Reabsorbs Na, K and Cl without water; dilutes the filtrate; makes uromodulin
Distal convoluted tubule	Simple cuboidal, few microvilli, lower and paler cells	Reabsorbs sodium and calcium; the macula densa senses arriving chloride
Collecting tubule and duct	Cuboidal to columnar with sharp borders: principal and intercalated cells	Final adjustment of sodium, potassium, acid and water under aldosterone and ADH

Clinical notes

Acute tubular necrosis: the proximal tubule loses its brush border and its cells slough; muddy brown granular casts appear in the urine.
Chronic kidney disease: the microscope shows sclerosed glomeruli, tubular atrophy and interstitial fibrosis.
Nephrogenic diabetes insipidus: the collecting duct does not respond to ADH and urine stays dilute.

Test yourself

Think of the answer, then tap the question to see it.

Which nephron segment has a brush border and is the most damaged in acute tubular necrosis?

The proximal convoluted tubule: cuboidal epithelium with long microvilli and many mitochondria, which make it very sensitive to ischemia and toxins.

Why does the thick ascending limb dilute the filtrate?

It reabsorbs sodium, potassium and chloride through the Na-K-2Cl cotransporter but is impermeable to water, so solute leaves without water.

Which collecting duct cells insert aquaporin 2 when ADH acts?

The principal cells.

Which type of nephron builds the medullary gradient that concentrates urine?

The juxtamedullary nephron, whose long loop of Henle reaches deep into the medulla.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

Kidneys

In Pharmacology

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: 25.4 Microscopic Anatomy of the Kidney (OpenStax, Rice University, 2022); 25.3 Gross Anatomy of the Kidney (OpenStax, Rice University, 2022); Kidneys (National Cancer Institute, SEER Training Modules, 2026).

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Representative model, for educational purposes only.