Glomerular Filtration Dynamics
Overview
Glomerular Filtration Rate (GFR) estimates approximately 125 mL of plasma filtered through the glomeruli per minute in healthy adults. It is governed by the Starling forces across the glomerular membrane and is the gold standard metric for assessing renal function in the context of contrast media administration.
Starling Forces at the Glomerulus
Net Filtration Pressure (NFP) = (Glomerular capillary hydrostatic pressure - Bowman's capsule pressure) - Glomerular oncotic pressure.
- Glomerular Hydrostatic Pressure (~60 mmHg): Key driving force. Regulated by afferent and efferent arteriolar resistance.
- Bowman's Capsule Pressure (~15 mmHg): Opposes filtration. Greatly increased in ureteral obstruction.
- Glomerular Oncotic Pressure (~25-35 mmHg): Opposes filtration. Increases progressively along the capillary loop as protein-free filtrate is removed.
- Bowman's Space Oncotic Pressure: Normally negligible (healthy glomerulus filters no protein).
Autoregulation Mechanisms
- Myogenic Mechanism: Afferent arterioles reflexively constrict in response to raised wall tension (arterial hypertension), protecting the glomerulus from barotrauma.
- Tubuloglomerular Feedback (TGF): Macula densa senses increased NaCl delivery and releases adenosine, constricting the afferent arteriole.
WarningNSAID + ACE-Inhibitor Combination = Acute Renal Failure
NSAIDs block prostaglandin-mediated afferent arteriolar dilation, dropping glomerular hydrostatic pressure. ACE inhibitors block Angiotensin II-mediated efferent arteriolar constriction, also dropping glomerular pressure. Combined, they can precipitate acute tubular necrosis, especially in high-risk patients (CKD, heart failure, hypovolaemia).
High Yield Facts
LightbulbFRCR / MD Prep Pearl
Inulin clearance is the gold-standard measurement of GFR (freely filtered, not secreted or reabsorbed). Creatinine clearance slightly overestimates GFR (~10-20%) due to tubular secretion. For contrast media decisions, use eGFR (CKD-EPI equation). eGFR <30 requires pre-hydration and avoidance of high-osmolality contrast.