Physiology · Renal, fluids and acid–base

Functional anatomy of the nephron !

Concept
Exam priority: medium

Each kidney has around a million nephrons: a filter (the glomerulus) and a long tube that recovers what is useful and lets waste through.

Updated 2026-11-01

Also called: Nephron, Functional unit of the kidney

See it in 3D: Kidneys · Renal pelvis · Vessels inside the kidney · Left renal artery · Right renal artery · Left renal vein · Ureters

Key points

≈ 1 million nephrons per kidney (wide range between people); they do not regenerate, and after age 40 they are slowly lost.
Cortical nephrons (the majority) have short loops; juxtamedullary nephrons (≈ 15–30%) have long loops that build the medullary gradient.
Order: glomerulus → proximal tubule → descending limb → thin ascending limb → thick ascending limb → macula densa → distal tubule → collecting duct.
Two capillaries in series: glomerular (high pressure, filters) and peritubular (low pressure, reabsorbs); juxtamedullary nephrons give the vasa recta.
The kidneys are ≈ 0.4% of body weight but receive ≈ 20–25% of cardiac output.
The juxtaglomerular apparatus (macula densa + granular cells + extraglomerular mesangium) links the tubule with its own arteriole.

What it is

The nephron is the kidney's functional unit: a glomerulus that filters and a tubule that modifies the filtrate (proximal tubule, loop of Henle, distal tubule), draining into the collecting duct. Its blood supply is unusual: two capillary beds in series, glomerular and peritubular. The shape and position of each segment explain what it does.

Key facts

Arterial path: renal artery → segmental → interlobar → arcuate → cortical radiate → afferent → glomerulus → efferent.
The proximal tubule has a brush border and many mitochondria: it reabsorbs most of the filtrate.
The thick limb and distal tubule have many mitochondria but no brush border: active transport without water.
The collecting duct has principal cells (Na⁺, K⁺, water) and intercalated cells (acid–base).
The cortex receives ≈ 90% of renal blood flow; the medulla gets little, which makes it sensitive to hypoxia.

The nephron: segments, transporters and diuretics

GFR ≈ 125 mL/minFiltrate ≈ 180 L/dayUrine ≈ 1–2 L/day

Tap a nephron segment: what it reabsorbs, its transporters, the hormones, the diuretics that act there and the classic disorders.

Glomerulus and arterioles
Proximal tubule
Thin descending limb
Thin ascending limb
Thick ascending limb
Distal convoluted tubule
Collecting tubule and duct
Juxtaglomerular apparatus (macula densa)
Cortex
Medulla
Interstitial osmolality (≈ 300 → 1200 mOsm/kg)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

Segments and their function

Segment	Epithelial feature	Main function
Glomerulus	Fenestrated endothelium, podocytes	Filters ≈ 180 L/day
Proximal tubule	Brush border, mitochondria	Reabsorbs ≈ 65–70% of Na⁺ and water, all glucose
Loop of Henle (thin)	Flat epithelium	Passive water and salt exchange
Thick ascending limb	Cuboidal, water-tight	Removes NaCl: dilutes and builds the gradient
Distal tubule	Cuboidal	Na⁺ (NCC) and Ca²⁺ adjustment
Collecting duct	Principal and intercalated cells	Fine tuning: water (ADH), Na⁺/K⁺ (aldosterone), acid

Clinical notes

Nephron loss in chronic kidney disease overloads the survivors (hyperfiltration), which are damaged in turn over time.
The medulla, with low flow and heavy work, suffers first in ischemia and with some drugs (papillary necrosis from NSAIDs).
Polycystic kidney disease: cysts arise from the tubule and replace functional tissue.

Test yourself

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

Why does the kidney have two capillary beds in series?

The glomerular bed needs high pressure to filter; after the efferent, low hydrostatic and high oncotic pressure in the peritubular capillary favour reabsorption of the filtrate.

Which nephrons are essential for concentrating urine, and why?

Juxtamedullary nephrons: their long loops reach the inner medulla and, with the vasa recta, build and preserve the osmotic gradient.

Why is the renal medulla especially sensitive to ischemia?

It receives a small share of renal flow through slow vasa recta, while the thick limb spends a lot of O₂ on active transport.

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See it in 3D

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

In Biochemistry

Creatinine and estimated glomerular filtration rate (eGFR)

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 25.3 Gross Anatomy of the Kidney (OpenStax (Rice University), 2022); 25.4 Microscopic Anatomy of the Kidney (OpenStax (Rice University), 2022); Anatomy, Abdomen and Pelvis: Kidneys (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 26). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.