Glomerulusfiltration

Glomerulus filtration, or glomerular filtration, is the process by which plasma from the glomerular capillaries is filtered into Bowman's capsule, forming the initial filtrate that will be modified along the nephron to become urine. It is the first step in urine formation and depends on the structure of the filtration barrier and the pressures within the glomerulus.

The filtration barrier consists of three layers: the fenestrated glomerular capillary endothelium, the glomerular basement membrane, and the podocyte foot processes with filtration slits. This barrier restricts passage primarily by size and charge, allowing water, electrolytes, and small solutes to pass while restricting cells and large proteins. Filtrate is approximately isosmotic with plasma.

Filtration rate is governed by Starling forces: glomerular capillary hydrostatic pressure tends to push fluid into Bowman's capsule, while capillary oncotic pressure and capsular hydrostatic pressure oppose filtration. In healthy adults, the glomerular filtration rate (GFR) is about 90–120 mL/min, corresponding to roughly 180 L of filtrate produced each day. The filtration fraction—the proportion of plasma entering the glomerulus that becomes filtrate—is about 20%.

Regulation involves autoregulatory mechanisms that maintain GFR across a range of arterial pressures, including adjustments of afferent and efferent arteriolar resistance and tubuloglomerular feedback. Hormonal influences, such as the renin-angiotensin-aldosterone system and atrial natriuretic peptide, also modulate glomerular perfusion and barrier properties.

Clinical relevance: impairment of the filtration barrier can cause proteinuria, while a reduced GFR is a key indicator of kidney function and disease. GFR can be measured directly with inulin clearance or estimated from serum creatinine–based equations, with considerations for age, sex, and body size.