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The filtration barrier has three layers: fenestrated endothelium, a negatively charged basement membrane, and podocyte slit diaphragms. Together, they permit water and small solutes to pass while restricting cells and many larger or negatively charged molecules. Filtration is driven by Starling forces, including glomerular capillary pressure, Bowman’s capsule pressure, and glomerular oncotic pressure. Net filtration pressure combines these influences, and GFR depends on both that pressure and the filtration coefficient (Kf), which reflects surface area and permeability. Changes in afferent or efferent arteriole tone can alter GFR. Plasma protein concentration and urinary obstruction also affect filtration by changing relevant pressures.