Glomerular filtration begins with ultrafiltration of plasma across the three-layer glomerular filtration barrier (fenestrated endothelium, basement membrane, and podocyte slit diaphragms), driven by the Starling pressure gradient and determined by glomerular filtration rate (GFR, ~120 mL/min). Regulation of afferent and efferent arteriolar resistance adjusts GFR to maintain body fluid composition.
From your study of renal physiology and capillary filtration, you know that the kidneys filter blood to regulate fluid balance and eliminate waste, and that fluid movement across capillary walls is governed by hydrostatic and oncotic pressure gradients (the Starling forces). Glomerular filtration takes these familiar principles and applies them in a specialized structure optimized for high-volume plasma filtration.
Each kidney contains about one million nephrons, and each nephron begins with a glomerulus — a tuft of capillaries enclosed within Bowman's capsule. Blood enters the glomerulus through the afferent arteriole and exits through the efferent arteriole (notably, this is a capillary bed sandwiched between two arterioles, not between an arteriole and a venule like most capillary beds). The glomerular capillary pressure is unusually high — about 55 mmHg, roughly twice the pressure in most systemic capillaries — because the efferent arteriole's resistance maintains back-pressure. This high hydrostatic pressure is the engine driving filtration. Opposing it are Bowman's capsule hydrostatic pressure (~15 mmHg, from fluid already filtered) and the glomerular capillary oncotic pressure (~30 mmHg, from plasma proteins that cannot cross the filter). The net filtration pressure of about 10 mmHg drives roughly 180 liters of plasma ultrafiltrate per day — an extraordinary volume that the tubules then selectively reabsorb and modify.
The glomerular filtration barrier itself is a three-layer structure exquisitely designed for selective permeability. The innermost layer is the fenestrated endothelium of the capillary, with pores that freely pass water and small solutes but block blood cells. The middle layer is the glomerular basement membrane (GBM), a dense meshwork of collagen and negatively charged proteoglycans that restricts passage of large and negatively charged molecules — this charge barrier is a key reason why albumin (a large, negatively charged plasma protein) is almost entirely excluded from the filtrate. The outer layer consists of podocytes, specialized epithelial cells whose foot processes interdigitate to form slit diaphragms — the final size-selective barrier. Together, these three layers ensure that the filtrate is essentially protein-free plasma: water, electrolytes, glucose, amino acids, urea, and other small molecules pass freely, while proteins and blood cells are retained.
The body regulates glomerular filtration rate (GFR) primarily by adjusting the resistance of the afferent and efferent arterioles. Constricting the afferent arteriole reduces blood flow into the glomerulus, lowering capillary pressure and decreasing GFR — this is what happens during sympathetic activation in severe hemorrhage, diverting blood away from the kidneys. Constricting the efferent arteriole has a more nuanced effect: moderate constriction actually increases glomerular capillary pressure (by impeding outflow) and raises GFR, while severe constriction reduces blood flow so much that GFR falls. Angiotensin II preferentially constricts the efferent arteriole, helping maintain GFR even when systemic blood pressure drops. The kidney also employs tubuloglomerular feedback: specialized cells in the distal tubule (the macula densa) sense the filtrate's sodium chloride concentration and signal the adjacent afferent arteriole to constrict or dilate, forming a local feedback loop that stabilizes GFR. These regulatory mechanisms ensure that despite wide fluctuations in blood pressure, the kidneys maintain remarkably constant filtration — a process called autoregulation — keeping GFR near 120 mL/min across a mean arterial pressure range of roughly 80–180 mmHg.