Nephron
Nephronum
Also known as: uriniferous tubule, functional unit of the kidney
The structural and functional unit of the kidney (≈1 million per kidney): a renal corpuscle that filters plasma and a tubule — PCT, loop of Henle, DCT — that modifies the filtrate before it drains into a collecting duct.
Overview
A nephron consists of a renal corpuscle (glomerulus within the glomerular capsule) and a renal tubule ≈30–50 mm long. Filtrate passes in sequence through the proximal convoluted tubule (and its straight part), the nephron loop (of Henle) — thin descending limb, thin ascending limb (long loops only) and thick ascending limb — and the distal convoluted tubule, which joins a collecting duct via a short connecting tubule. Collecting ducts are not part of the nephron by strict definition (they have a different embryological origin — the ureteric bud) but function with it; one collecting duct drains several nephrons.
The thick ascending limb returns to its own glomerulus and touches the vascular pole, where its specialised macula densa cells form the juxtaglomerular apparatus with the afferent arteriole — a feedback loop linking each nephron's tubular output to its own filtration.
Nephrons are classified by the position of their glomeruli as cortical (≈85%; short loops) or juxtamedullary (≈15%; long loops that create the medullary gradient). Nephron number is fixed at birth and declines with age (≈10% per decade after 40); reduced nephron endowment (low birth weight) predisposes to hypertension and CKD.
- Location
Renal corpuscles and convoluted tubules in the cortex; loops of Henle in the medulla.
Parts & subdivisions
- Glomerulus + glomerular (Bowman) capsule — ultrafiltration.
- Bulk isosmotic reabsorption (≈65–67% of Na⁺/water) and secretion.
- Descending and ascending limbs creating the medullary gradient.
- NaCl (NCC) and Ca²⁺ reabsorption; fine-tuning.
- Final hormonally regulated adjustment of water, Na⁺, K⁺ and H⁺.
- Macula densa + JG cells + extraglomerular mesangial cells.
Anatomy
- Arterial supply
Afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons).
- Development
Renal corpuscle to DCT from metanephric mesenchyme (renal vesicle → comma-shaped → S-shaped body); collecting duct from the ureteric bud. Nephrogenesis ends at ≈34–36 weeks.
Function
- Ultrafiltration of plasma (≈125 mL/min for both kidneys)
- Selective reabsorption of water, electrolytes, nutrients
- Secretion of H⁺, K⁺, organic ions and drugs
- Formation of dilute or concentrated urine
- Physiology
Excretion = filtration − reabsorption + secretion. About 99% of filtered water and Na⁺ are reabsorbed.
Mnemonics
Order of the tubule
"Please Let Dr Come" — PCT, Loop of Henle, DCT, Collecting duct.
Test yourself
Which nephron segment reabsorbs the largest fraction of filtered sodium?
- Thick ascending limb
- Distal convoluted tubule
- Proximal convoluted tubule
- Cortical collecting duct
Answer: Proximal convoluted tubule
The PCT reabsorbs ≈65–67% of filtered Na⁺ and water isosmotically; the thick ascending limb ≈25%, the DCT ≈5–7% and the collecting duct ≈2–5%.
Which segment of the nephron is called the "diluting segment" because it reabsorbs NaCl actively but is impermeable to water?
- Thin descending limb
- Thick ascending limb
- Inner medullary collecting duct
- Proximal convoluted tubule
Answer: Thick ascending limb
The thick ascending limb lacks aquaporins; NKCC2 removes NaCl so tubular fluid leaving it is hypo-osmotic (≈100 mOsm/kg) whatever the ADH level. This also generates the single effect of the countercurrent multiplier.
Which feature distinguishes juxtamedullary nephrons from cortical nephrons?
- Their glomeruli lie in the outer cortex
- They lack a thick ascending limb
- They have long loops reaching the inner medulla and their efferent arterioles give rise to the vasa recta
- They make up about 85% of all nephrons
Answer: They have long loops reaching the inner medulla and their efferent arterioles give rise to the vasa recta
Juxtamedullary nephrons (≈15%) have glomeruli near the corticomedullary junction, long loops with thin ascending limbs extending to the papilla, and efferent arterioles forming the vasa recta — they create the medullary osmotic gradient.
Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.