Features of renal disorder

Understanding glomerular dynamics is essential for medical students preparing for the USMLE Step 1. Changes in factors such as the glomerular filtration rate (GFR), renal plasma flow (RPF), and filtration fraction (FF) can occur due to various physiological and pathological changes. This guide presents how different factors affect GFR, RPF, and FF, organized in a format for easy memorization and understanding.

Key Terms

GFR (Glomerular Filtration Rate): The rate at which blood is filtered in the glomeruli of the kidney.
RPF (Renal Plasma Flow): The plasma flow through the renal arteries, impacts overall kidney perfusion.
FF (Filtration Fraction): The fraction of plasma that is filtered through the glomerulus, calculated as GFR/RPF.

Effects on GFR, RPF, and FF

Factor	Effect on GFR	Effect on RPF	Effect on FF (GFR/RPF)
Afferent Arteriole Constriction	Decreases	Decreases	No change
Efferent Arteriole Constriction	Increases (initially)	Decreases	Increases
↑ Plasma Protein Concentration	Decreases	No change	Decreases
↓ Plasma Protein Concentration	Increases	No change	Increases
Constriction of Ureter	Decreases	No change	Decreases
Dehydration	Decreases	Decreases	Increases

Effects on GFR, RPF, and FF

Detailed Effects of Various Conditions

1. Afferent Arteriole Constriction

Impact on GFR: Decreases due to reduced blood flow to the glomerulus.
Impact on RPF: Decreases because of reduced plasma entering the glomerulus.
Impact on FF: Remains unchanged as both GFR and RPF decrease proportionally.

2. Efferent Arteriole Constriction

Impact on GFR: Initially increases due to increased glomerular capillary pressure but may decrease if constriction is severe.
Impact on RPF: Decreases due to the outflow resistance.
Impact on FF: Increases because GFR increases or remains stable while RPF decreases.

3. Increased Plasma Protein Concentration

Impact on GFR: Decreases due to increased oncotic pressure in glomerular capillaries, reducing filtration.
Impact on RPF: No effect.
Impact on FF: Decreases due to reduced GFR while RPF remains constant.

4. Decreased Plasma Protein Concentration

Impact on GFR: Increases due to decreased oncotic pressure, promoting filtration.
Impact on RPF: No effect.
Impact on FF: Increases as GFR increases while RPF remains constant.

5. Constriction of Ureter

Impact on GFR: Decreases due to increased hydrostatic pressure in Bowman’s space.
Impact on RPF: No effect.
Impact on FF: Decreases due to a reduction in GFR while RPF remains constant.

6. Dehydration

Impact on GFR: Decreases as plasma volume is reduced.
Impact on RPF: Decreases due to reduced overall blood volume.
Impact on FF: Increases due to proportionally greater reduction in RPF compared to GFR.

Points to Remember

Constriction of the afferent arteriole generally reduces both GFR and RPF without affecting FF.
Constriction of the efferent arteriole increases FF by reducing RPF more than it affects GFR.
Changes in plasma protein concentration alter oncotic pressure, impacting GFR and FF without affecting RPF.
Dehydration decreases both GFR and RPF, with a relatively greater decrease in RPF, thereby increasing FF.

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USMLE Step 1 Review Series

Curriculum

Features of renal disorder

Key Terms

Effects on GFR, RPF, and FF

Detailed Effects of Various Conditions

1. Afferent Arteriole Constriction

2. Efferent Arteriole Constriction

3. Increased Plasma Protein Concentration

4. Decreased Plasma Protein Concentration

5. Constriction of Ureter

6. Dehydration

Points to Remember

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