Notes on Glomerular Filtration Rate (GFR) – Renal Physiology, MCQ with answer and Lecture
What is GFR?
- Definition: Glomerular Filtration Rate (GFR) is the volume of fluid filtered from the glomerular capillaries into the Bowman’s capsule per unit of time. It measures kidney function and filtration efficiency.
- Normal Value: Approximately 125 mL/min in healthy adults (180 L/day), though it varies with age, sex, and body size.
- Purpose: GFR reflects the kidneys’ ability to filter blood and remove waste while retaining essential substances.
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Renal Physiology Related to GFR
- Glomerulus: A network of capillaries in the nephron where filtration occurs.
- Bowman’s Capsule: Collects the filtrate from the glomerulus.
- Filtration Membrane: Composed of three layers:
- Fenestrated Endothelium: Allows passage of water, ions, and small molecules; restricts large proteins and cells.
- Basement Membrane: Acts as a selective barrier, preventing filtration of proteins like albumin.
- Podocytes: Specialized cells with filtration slits that fine-tune filtration.
- Filtration Process: Driven by Starling forces (hydrostatic and oncotic pressures).
Starling Forces Affecting GFR
- Net Filtration Pressure (NFP) = GFR driving force:
- Glomerular Hydrostatic Pressure (P_GC): ~60 mmHg (pushes fluid out of capillaries).
- Bowman’s Capsule Hydrostatic Pressure (P_BC): ~15 mmHg (opposes filtration).
- Glomerular Oncotic Pressure (π_GC): ~25 mmHg (opposes filtration due to plasma proteins).
- Formula: NFP = P_GC – (P_BC + π_GC) ≈ 10 mmHg.
Factors Affecting GFR
- Blood Pressure:
- Increased systemic BP increases P_GC → ↑ GFR.
- Decreased BP (e.g., hemorrhage) → ↓ GFR.
- Afferent Arteriolar Resistance:
- Constriction (e.g., sympathetic activation) → ↓ GFR.
- Dilation → ↑ GFR.
- Efferent Arteriolar Resistance:
- Constriction (e.g., angiotensin II) → ↑ P_GC → ↑ GFR (up to a point).
- Dilation → ↓ GFR.
- Plasma Protein Concentration:
- Hypoproteinemia (↓ π_GC) → ↑ GFR.
- Hyperproteinemia → ↓ GFR.
- Bowman’s Capsule Pressure:
- Obstruction (e.g., kidney stones) → ↑ P_BC → ↓ GFR.
- Renal Blood Flow (RBF):
- Directly proportional to GFR; reduced RBF (e.g., dehydration) → ↓ GFR.
- Autoregulation:
- Maintains stable GFR despite BP changes (via myogenic mechanism and tubuloglomerular feedback).
- Effective between 80-180 mmHg mean arterial pressure.
- Hormonal Regulation:
- Angiotensin II: Constricts efferent arteriole → ↑ GFR.
- ANP (Atrial Natriuretic Peptide): Dilates afferent arteriole → ↑ GFR.
- Sympathetic NS: Constricts afferent arteriole → ↓ GFR.
Clinical Relevance
- GFR Estimation: Measured via creatinine clearance or estimated using equations (e.g., Cockcroft-Gault, MDRD).
- Decreased GFR: Indicates kidney damage (e.g., chronic kidney disease).
- Increased GFR: Seen in early diabetes (hyperfiltration).
Chart: Factors Affecting GFR
| Factor | Effect on GFR | Mechanism | Example Condition |
|---|---|---|---|
| ↑ Glomerular Hydrostatic Pressure | Increases GFR | More fluid pushed into Bowman’s capsule | Hypertension |
| ↓ Afferent Arteriolar Resistance | Increases GFR | More blood enters glomerulus | ANP release |
| ↑ Efferent Arteriolar Resistance | Increases GFR (moderately) | Higher pressure in glomerulus | Angiotensin II action |
| ↓ Plasma Protein Concentration | Increases GFR | Reduced oncotic pressure opposing filtration | Liver disease (hypoalbuminemia) |
| ↑ Bowman’s Capsule Pressure | Decreases GFR | Opposes filtration | Kidney stone obstruction |
| ↓ Renal Blood Flow | Decreases GFR | Less blood available for filtration | Dehydration |
| Sympathetic Activation | Decreases GFR | Constricts afferent arteriole | Stress or shock |
Multiple-Choice Questions (MCQs)
- What is the primary driving force for glomerular filtration?
- A) Bowman’s capsule hydrostatic pressure
- B) Glomerular hydrostatic pressure
- C) Plasma oncotic pressure
- D) Tubular reabsorption
- Answer: B) Glomerular hydrostatic pressure
- Which of the following would most likely decrease GFR?
- A) Dilation of the afferent arteriole
- B) Constriction of the efferent arteriole
- C) Increased Bowman’s capsule pressure
- D) Increased plasma protein concentration
- Answer: C) Increased Bowman’s capsule pressure
- A patient with severe dehydration would likely exhibit:
- A) Increased GFR due to higher renal blood flow
- B) Decreased GFR due to reduced renal blood flow
- C) No change in GFR due to autoregulation
- D) Increased GFR due to higher oncotic pressure
- Answer: B) Decreased GFR due to reduced renal blood flow
- Angiotensin II primarily affects GFR by:
- A) Dilating the afferent arteriole
- B) Constricting the efferent arteriole
- C) Increasing plasma protein concentration
- D) Reducing glomerular hydrostatic pressure
- Answer: B) Constricting the efferent arteriole
- The normal GFR in a healthy adult is closest to:
- A) 50 mL/min
- B) 125 mL/min
- C) 200 mL/min
- D) 300 mL/min
- Answer: B) 125 mL/min