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Physiology · Renal, fluids and acid–base

Measuring GFR and plasma flow: inulin, creatinine, PAH and filtration fraction !

  • Measurement
  • Exam priority: medium

How we measure how much the kidneys filter and how much blood they get, using substances the kidney handles in a known way.

Updated 2026-11-01

Also called: Filtration fraction, PAH clearance, Effective renal plasma flow

See it in 3D: Kidneys · Vessels inside the kidney

What it is

GFR is measured with a substance that is freely filtered and neither reabsorbed nor secreted by the tubule: its clearance equals GFR. Renal plasma flow is measured with a substance the kidney strips almost completely from the blood in a single pass (PAH). The ratio of the two, the filtration fraction, tells you which arteriole is changing calibre.

The nephron: segments, transporters and diuretics

The nephron: segments, transporters and diuretics CortexOuter medullaInner medulla3006009001200mOsm/kgUrineGlomerulusProximal tubuleDistal tubuleThin descendingThin ascendingThick ascendingCollecting ductJuxtaglomerularapparatusNHE3 · SGLT2Filters ≈ 180 L/dayAQP1Passive NaClNKCC2 · ROMKParacellular Ca²⁺, Mg²⁺NCC · Ca²⁺ (PTH)ENaC · ROMK/BKAQP2 (ADH)α: H⁺-ATPase · β: pendrin

Glomerulus and Bowman capsule Filters ≈ 180 L/day · GFR ≈ 125 mL/min

Glomerulus and Bowman capsule

  • What it does: Filters plasma: ≈ 180 L/day (GFR ≈ 125 mL/min), filtration fraction ≈ 20%. The barrier (fenestrated endothelium, negatively charged basement membrane and podocytes with slit diaphragms) lets water and small solutes through, not cells or most proteins.
  • Transporters: No transporters: Starling forces drive it (capillary hydrostatic pressure for; capsule pressure and plasma oncotic pressure against).
  • Hormones: Ang II constricts the efferent arteriole more (holds GFR up); prostaglandins dilate the afferent; autoregulation (myogenic and tubuloglomerular feedback) keeps GFR steady.
  • Drugs: NSAIDs (afferent constriction) and ACE inhibitors or ARBs (efferent dilation) can lower GFR.
  • Disorders: Nephritic syndrome (hematuria, inflammation) and nephrotic syndrome (proteinuria > 3.5 g/day, podocyte injury).
  • 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.

Equations

ClearanceCx = (Ux × V) / Px
Ux
urine concentration of x
V
urine flow (mL/min)
Px
plasma concentration of x

With inulin, C = GFR; with PAH, C ≈ effective RPF.

Filtration fractionFF = GFR / RPF
GFR
glomerular filtration rate
RPF
renal plasma flow

Filtered fluid leaves the blood: a high FF concentrates peritubular capillary proteins and favours proximal reabsorption.

Renal blood flowRBF = RPF / (1 − Hct)
Hct
haematocrit as a fraction

Reference values (young adult, approx.)

ParameterValueNote
Glomerular filtration rate≈125 mL/min (≈180 L/day)falls with age
Renal plasma flow≈600–650 mL/min
Renal blood flow≈1.1–1.2 L/min≈20–22 % of cardiac output
Filtration fraction≈0.16–0.20
Renal PAH extraction≈0.9hence PAH measures “effective” flow

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

What happens to RPF, GFR and FF, arteriole by arteriole

ChangeRPFGFRFF
Afferent constriction (NSAIDs, intense sympathetic tone)↓↓≈ unchanged
Efferent constriction (angiotensin II)↓↑ or ≈↑
Efferent dilation (ACE inhibitors, ARBs)↑ or ≈↓↓
Raised plasma proteins≈↓↓
Urinary obstruction (↑ Bowman’s capsule pressure)≈↓↓

Clinical notes

  • In practice GFR is estimated from creatinine-based equations (with age and sex); creatinine depends on muscle mass, so it misleads in very thin, elderly or amputee patients.
  • With bilateral renal artery stenosis, GFR depends on angiotensin II efferent constriction: an ACE inhibitor removes it and creatinine jumps.
  • Some drugs that block tubular creatinine secretion raise serum creatinine without changing true GFR.

Test yourself

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

Why does creatinine clearance overestimate GFR, and why does the error grow in advanced kidney disease?

Creatinine is secreted as well as filtered. As GFR falls, the secreted part makes up a bigger share, so the calculated clearance sits further above the true GFR.

A drug constricts only the efferent arteriole. What happens to RPF, GFR and filtration fraction?

RPF falls (more resistance), glomerular pressure rises and GFR is maintained or rises: the filtration fraction goes up.

With an RPF of 600 mL/min and a haematocrit of 0.40, what is renal blood flow?

RBF = 600 / (1 − 0.40) = 1000 mL/min. Plasma is only 60 % of the blood.

Why is PAH clearance called “effective” rather than total renal plasma flow?

PAH extraction is not 100 %: some blood perfuses tissue that does not secrete PAH (capsule, medulla). With extraction ≈0.9 the clearance slightly underestimates total flow.

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

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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: 25.5 Physiology of Urine Formation (OpenStax (Rice University), 2022); 25.7 Regulation of Renal Blood Flow (OpenStax (Rice University), 2022); Acute Kidney Injury (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (chs. 27, 28). 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.