Filtration Fraction
The filtration fraction is the proportion of plasma flowing through the kidney that is filtered at the glomerulus — the ratio of the glomerular filtration rate to the renal plasma flow. It expresses how much of the plasma delivered to the glomeruli becomes filtrate rather than passing on to the efferent circulation.
Definition
The filtration fraction is the dimensionless ratio of the glomerular filtration rate to the renal plasma flow (FF = GFR / RPF), representing the fraction of plasma entering the glomeruli that is filtered into Bowman's space.
Scope
This entry defines the filtration fraction, gives its relationship to GFR and renal plasma flow, and explains how it changes when afferent and efferent arteriolar resistances shift the balance of glomerular pressure and flow. It treats the concept as integrative renal physiology and does not address clinical interpretation in disease.
Core questions
- How is the filtration fraction defined in terms of GFR and renal plasma flow?
- What happens to the filtration fraction when glomerular flow or pressure changes?
- How do afferent and efferent arteriolar tone alter the filtration fraction?
- How does the filtration fraction relate to peritubular oncotic pressure?
Key concepts
- Filtration fraction (GFR / RPF)
- Renal plasma flow
- Afferent and efferent arteriolar resistance
- Glomerular capillary hydrostatic pressure
- Effect on peritubular oncotic pressure
Mechanisms
Because the filtration fraction is GFR divided by renal plasma flow, anything that changes filtration relative to flow changes it. Constriction of the efferent arteriole, for example, raises glomerular capillary hydrostatic pressure while reducing plasma flow, tending to increase the filtration fraction, whereas afferent constriction lowers both pressure and flow. Micropuncture studies that measured glomerular pressures and flows directly characterized how these hemodynamic adjustments alter single-nephron filtration and the proportion of plasma filtered (brenner-1971; robertson-1972). Filtering a larger fraction of plasma also concentrates the protein left behind, raising the oncotic pressure of blood entering the peritubular capillaries, which is one link between glomerular filtration and downstream tubular reabsorption. The ratio is interpreted within the clearance framework of renal physiology systematized by Smith (smith-1951).
Clinical relevance
The filtration fraction is a conceptual tool for reasoning about how changes in renal blood flow and arteriolar tone affect filtration and the forces governing tubular reabsorption. This entry presents the concept for reference; it does not provide clinical thresholds or management guidance.
Evidence & guidelines
The hemodynamic behaviour described here is grounded in direct micropuncture measurements of glomerular pressures and flows (brenner-1971; robertson-1972), interpreted within the clearance framework of classical renal physiology (smith-1951).
History
The ratio of GFR to renal plasma flow was defined within the clearance methods of mid-twentieth-century renal physiology (smith-1951). Its hemodynamic determinants were clarified when micropuncture studies of the rat glomerulus measured the pressures and flows that set single-nephron filtration and showed how arteriolar resistance and autoregulation shape the proportion of plasma filtered (brenner-1971; robertson-1972).
Key figures
- Homer W. Smith
- Barry M. Brenner
- Channing R. Robertson
Related topics
Seminal works
- smith-1951
- brenner-1971
- robertson-1972
Frequently asked questions
- How is the filtration fraction calculated?
- It is the glomerular filtration rate divided by the renal plasma flow (FF = GFR / RPF), expressed as a fraction or percentage of plasma that is filtered.
- Why does efferent arteriolar constriction tend to raise the filtration fraction?
- Constricting the efferent arteriole raises pressure in the glomerular capillaries while reducing plasma flow through them, so a larger fraction of the plasma that does flow through is filtered.