Factors Affecting Glomerular Filtration Rate (GFR)

Glomerular Filtration Rate (GFR) is volume of fluid filtered from renal glomerular capillaries into Bowman’s capsule per unit of time. It is primary indicator of kidney function and is determined by balance of hydrostatic and oncotic pressures (Starling forces) and permeability of filtration barrier.

The mathematical relationship is expressed as:

Where:

• Kf: Capillary filtration coefficient

• PG: Glomerular capillary hydrostatic pressure

• PB: Bowman’s space hydrostatic pressure

• PiG: Glomerular capillary oncotic pressure

• PiB: Bowman’s space oncotic pressure (usually considered zero)

1. Pressure Gradients

The most dynamic factors affecting GFR are pressures within the capillary of the nephron.

• Glomerular Hydrostatic Pressure (PG): This is main force pushing fluid into capsule. It is regulated by resistance ofafferent and efferent arterioles.

  • Afferent Constriction: Decreases PG and GFR.

  • Efferent Constriction: Increases PG and GFR (up to a point).

• Plasma Oncotic Pressure (PiG): This force opposes filtration by holding water in blood. Conditions like severe dehydration increase protein concentration, raising PiG and lowering GFR.

• Bowman’s Capsule Hydrostatic Pressure (PB): This opposes filtration. If urinary tract is obstructed (e.g., kidney stones or prostate issues), pressure backs up into capsule, increasing PB and decreasing GFR.

2. Capillary Filtration Coefficient (Kf)

Kf is a product of surface area available for filtration and permeability of glomerular membrane.

• Surface Area: Mesangial cells can contract or relax to change surface area of capillaries.

• Permeability: Chronic diseases like diabetes or hypertension can thicken basement membrane or damage podocytes, reducing Kf and leading to a long-term decline in GFR.

3. Renal Blood Flow (RBF)

GFR is highly dependent on total volume of blood entering kidneys.

• Autoregulation: Kidneys use myogenic mechanism and tubuloglomerular feedback to keep GFR stable even when systemic blood pressure fluctuates between roughly 80 and 180 mmHg.

• Systemic Blood Pressure: If mean arterial pressure drops below autoregulatory range (e.g., in hemorrhage or shock), GFR will fall.

4. Neuro-Hormonal Influences

• • Sympathetic Nervous System: During fight or flight, sympathetic nerves constrict afferent arterioles to divert blood to heart and muscles, lowering GFR.

  • Angiotensin II: Constricts efferent arteriole, which helps maintain GFR even when renal blood flow is low.

  • Atrial Natriuretic Peptide (ANP): Dilates afferent arteriole and constricts efferent, significantly increasing GFR to help body excrete excess sodium and water.