eGFR

The estimated glomerular filtration rate (eGFR) is the best overall index of kidney function, representing the volume of plasma filtered through the glomeruli per unit time (normal: 90-120 mL/min/1.73 m²). The glomerular filtration barrier consists of three layers: the fenestrated capillary endothelium, the glomerular basement membrane (GBM), and the podocyte foot processes with their filtration slit diaphragms. GFR is determined by the balance of Starling forces across these layers: the net filtration pressure equals glomerular capillary hydrostatic pressure (driving filtration, ~55 mmHg) minus Bowman capsule hydrostatic pressure (opposing, ~15 mmHg) minus glomerular capillary oncotic pressure (opposing, ~30 mmHg), yielding a net filtration pressure of approximately 10 mmHg. GFR is autoregulated through the tubuloglomerular feedback mechanism: the macula densa senses distal tubular NaCl concentration and signals afferent arteriolar tone adjustment, maintaining stable GFR across a mean arterial pressure range of 80-180 mmHg. Serum creatinine is the most common endogenous marker used to estimate GFR, but it is an imperfect surrogate: creatinine is produced at a rate proportional to muscle mass, is freely filtered at the glomerulus, and undergoes tubular secretion (10-15%...