The Glomerular Filtration Rate (GFR) is a indicator of renal function, representing volume of fluid filtered from glomerular capillaries into Bowman’s capsule per unit of time.
In a healthy adult, average GFR is approximately 125 mL/min or 180 L/day.
Physiologically, GFR is governed by balance of hydrostatic and oncotic pressures (Starling forces) and the permeability of the filtration barrier.
1. Pressure Gradients
Net filtration pressure (NFP) determine direction and rate of fluid movement. It is calculated using formula:
Glomerular Capillary Hydrostatic Pressure (Pgc):
- The primary force favoring filtration.
- It is usually around 60 mmHg.
- High systemic blood pressure or dilation of the afferent arteriole increases this pressure, thereby increasing GFR.
Bowman’s Space Hydrostatic Pressure (Pbc):
- This force opposes filtration (approx. 18 mmHg).
- Obstructions like kidney stones or prostate enlargement can increase this pressure, causing GFR to drop.
Glomerular Capillary Oncotic Pressure (πgc):
- Driven by plasma proteins (like albumin) that remain in the blood.
- This force (approx. 32 mmHg) pulls fluid back into the capillaries.
- Dehydration increases protein concentration, raising oncotic pressure and lowering GFR.
2. Renal Blood Flow and Autoregulation
Kidneys is able to maintain a constant GFR despite fluctuations in systemic blood pressure (between 80 and 180 mmHg) through autoregulation.
Myogenic Mechanism
- When blood pressure rises, smooth muscle inafferent arteriole stretches.
- In response, it contracts (vasoconstriction), which limits blood flow into glomerulus and prevent a spike in the GFR.
Tubuloglomerular Feedback (TGF)
It involves macula densa cells in distal tubule.
- If GFR increases, more NaCl reaches the macula densa.
- These cells sense increase and release mediators (like adenosine).
- This causes afferent arteriole to constrict, bringing GFR back to normal.
3. Arteriolar Resistance
The relative tone of afferent and efferent arterioles dictates pressure inside glomerulus.
| Action | Effect on Pgc (Glomerular Capillary Hydrostatic Pressure) | Effect on GFR |
| Afferent Constriction (e.g., Sympathetic Nerves) | Decreases | Decreases |
| Afferent Dilation (e.g., Prostaglandins) | Increases | Increases |
| Efferent Constriction (e.g., Angiotensin II) | Increases | Increases |
| Efferent Dilation (e.g., ACE Inhibitors) | Decreases | Decreases |
4. Capillary Filtration Coefficient (Kf)
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Surface Area: Mesangial cells can contract or relax to change the surface area of the capillaries. Contraction (stimulated by Angiotensin II) reduces surface area and lowers GFR.
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Permeability: Diseases like diabetes or hypertension can thicken glomerular basement membrane or destroy the podocytes (slit diaphragms), significantly altering Kf.
5. Hormonal and Neural Factors
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Sympathetic Nervous System: During stress or hemorrhage, strong sympathetic activation constricts afferent arterioles to divert blood to vital organs, drastically reducing GFR.
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Atrial Natriuretic Peptide (ANP): Secreted when the heart is stretched (volume overload), ANP dilates the afferent arteriole and constricts efferent arteriole to increase GFR and promote fluid excretion.