glomeruli

Glomeruli are tiny, tufted networks of capillaries that serve as the primary site of blood filtration within the kidney. Each nephron contains a single glomerulus, which lies within Bowman's capsule to form the renal corpuscle. Blood reaches the glomerulus through the afferent arteriole, flows through the capillary tuft, and exits via the efferent arteriole.

The glomerular filtration barrier consists of three layers: fenestrated endothelial cells lining the capillaries, a shared basement membrane, and podocyte foot processes with filtration slits. This barrier selectively allows water and small solutes to pass into Bowman's space while restricting the passage of large proteins and blood cells. The resulting filtrate, which is essentially plasma without most proteins, is processed along the nephron to become urine. Mesangial cells within the tuft provide structural support, regulate surface area, and remove trapped debris.

Glomerular filtration rate (GFR) measures the kidney’s filtering capacity and is tightly regulated. Autoregulatory mechanisms, including adjustments of the afferent and efferent arterioles and tubuloglomerular feedback involving the macula densa, help maintain stable GFR across a range of blood pressures. Hormonal influences, notably the renin-angiotensin-aldosterone system, can modulate arteriolar tone to preserve filtration under stress.

Clinical relevance includes diseases that damage the glomerular filtration barrier, such as glomerulonephritis and diabetic nephropathy, which can cause proteinuria and reduced GFR. Evaluation often involves estimating GFR from serum creatinine and assessing urinary protein excretion. In humans, the kidneys contain about one million glomeruli per kidney, collectively filtering a large volume of plasma daily.