Glomerular filtration
Also known as: renal filtration
Glomerular filtration is the first step of urine formation, in which pressure in the glomerular capillaries drives water and small solutes out of the blood and into Bowman's capsule. The rate of this process, the GFR, is the standard measure of kidney function.
Blood entering the glomerulus is filtered across three layers: the fenestrated capillary endothelium, the glomerular basement membrane, and the podocyte foot processes with their filtration slits. Together these form a barrier selective for both size and charge. Water, electrolytes, glucose, amino acids, urea, and creatinine pass freely into Bowman's space, while cells and most plasma proteins are retained. The negatively charged basement membrane repels albumin in particular, which is why proteinuria signals barrier damage.
Filtration is driven by Starling forces. Glomerular capillary hydrostatic pressure pushes fluid out; the hydrostatic pressure in Bowman's space and the oncotic pressure of plasma proteins remaining in the capillary oppose it. The net result is the glomerular filtration rate, roughly 90 to 125 mL/min in a healthy adult. Because the glomerulus lies between two arterioles, the kidney can adjust GFR precisely: constricting the efferent arteriole raises glomerular pressure and GFR, while constricting the afferent arteriole lowers it. Angiotensin II preferentially constricts the efferent arteriole, and prostaglandins dilate the afferent one, which explains why NSAIDs and ACE inhibitors can each reduce GFR in vulnerable patients.
GFR is estimated clinically from serum creatinine, since creatinine is freely filtered and only minimally secreted. Inulin clearance is the theoretical gold standard because inulin is filtered and neither reabsorbed nor secreted. A falling GFR is the defining feature of chronic kidney disease staging, and autoregulation — including the tubuloglomerular feedback loop at the macula densa — keeps GFR stable across a wide range of systemic blood pressures.
USMLE Step 1 tests filtration barrier anatomy, Starling force manipulations, clearance calculations, and arteriolar effects on GFR and filtration fraction. The MCAT covers glomerular filtration within the excretory system as part of the biological and biochemical foundations section.
Key takeaways
- Glomerular filtration moves water and small solutes from blood into Bowman's capsule through a size- and charge-selective barrier.
- Starling forces determine the filtration rate: capillary hydrostatic pressure drives it, while Bowman's space pressure and plasma oncotic pressure oppose it.
- Efferent arteriolar constriction raises GFR; afferent arteriolar constriction lowers it.
- GFR is estimated from creatinine clearance, and inulin clearance is the gold-standard measure.
- Loss of the barrier's negative charge allows albumin to escape, producing proteinuria.