CorpusHuman Anatomy
UrinaryPhysiological process

Glomerular filtration

Also known as: GFR, glomerular filtration rate, ultrafiltration, Starling forces

The bulk flow of protein-free plasma from the glomerular capillaries into Bowman space, driven by a net filtration pressure of ≈10 mmHg across a very permeable barrier; normal GFR ≈125 mL/min (≈180 L/day).

Show on body map Nephron — Juxtamedullary

Overview

GFR≈125 mL/min; ≈180 L/day
Net filtration pressure≈10 mmHg
Filtration fraction≈20%
P_GC / P_BS / π_GC≈55–60 / ≈15 / ≈30 mmHg

GFR = Kf × NFP, where Kf is the filtration coefficient (hydraulic conductivity × surface area — very high in glomeruli) and NFP is the net filtration pressure given by the Starling forces:

NFP = (P_GC − P_BS) − (π_GC − π_BS) ≈ (55 − 15) − (30 − 0) ≈ 10 mmHg.

Glomerular hydrostatic pressure (P_GC ≈55–60 mmHg) is the main force favouring filtration; Bowman space pressure (≈15 mmHg) and plasma oncotic pressure (≈30 mmHg at the afferent end, rising toward ≈35 mmHg along the capillary) oppose it. About 20% of the renal plasma flow is filtered (filtration fraction ≈0.2).

Determinants: GFR falls with afferent arteriolar constriction (sympathetic, NSAIDs), efferent dilatation (ACE inhibitors), reduced renal blood flow, raised Bowman space pressure (obstruction), raised plasma protein (dehydration, myeloma) or reduced Kf (glomerular disease, mesangial contraction). GFR rises with afferent dilatation, moderate efferent constriction (angiotensin II), increased renal plasma flow, or low plasma protein. GFR is held relatively constant by autoregulation.

Location

Renal corpuscles (glomerular capillaries → Bowman space).

Function

  • Produce ≈180 L/day of ultrafiltrate, the starting fluid for urine
  • Allow rapid excretion of wastes and turnover of the plasma volume (≈60 times/day)
Physiology

Normal GFR ≈125 mL/min (men) and ≈110 mL/min (women), indexed to 1.73 m² body surface area; falls ≈1 mL/min/year after age ≈40. Clinically estimated (eGFR) from serum creatinine (and cystatin C) using the CKD-EPI equation; measured with inulin, iohexol or ⁵¹Cr-EDTA. Creatinine rises appreciably only after ≈50% of GFR is lost (hyperbolic relationship).

Clinical significance

Serum creatinine pitfalls

Creatinine depends on muscle mass (falsely reassuring in the elderly, amputees, cachexia), and trimethoprim and cimetidine raise it by blocking tubular secretion without reducing GFR. Cystatin C is less affected by muscle mass.

Test yourself

The clearance of which substance gives the best measure of glomerular filtration rate?
  1. Para-aminohippurate (PAH)
  2. Glucose
  3. Urea
  4. Inulin

Answer: Inulin

Inulin is freely filtered and neither reabsorbed, secreted nor metabolised, so its clearance equals GFR. Creatinine is used clinically but slightly overestimates GFR because some is secreted. PAH clearance estimates renal plasma flow.

More urinary system questions

Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.