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Glomerular filtration rate (GFR) is driven by net filtration pressure—the balance of hydrostatic pressure in the glomerulus and oncotic pressure in both the glomerulus and Bowman's space. Autoregulation maintains constant GFR despite blood pressure changes via myogenic and tubuloglomerular feedback mechanisms. Changes in GFR must be matched by reabsorption or excretion to maintain fluid balance.
From your study of Starling forces in the microcirculation, you know that fluid movement across capillary walls is governed by the balance between hydrostatic pressure (pushing fluid out) and oncotic pressure (pulling fluid back in via plasma proteins). The glomerulus applies this same principle, but with a crucial anatomical twist: it is designed to maximize filtration rather than balance it. Glomerular hydrostatic pressure is unusually high — about 55 mmHg, compared to roughly 35 mmHg in most systemic capillaries — because the glomerulus sits between two arterioles (afferent and efferent) rather than between an arteriole and a venule. The efferent arteriole's resistance keeps pressure elevated throughout the entire length of the glomerular capillary.
The net filtration pressure (NFP) at any point along the glomerulus equals glomerular hydrostatic pressure minus both the oncotic pressure of glomerular blood and the hydrostatic pressure in Bowman's capsule. At the afferent end, this works out to roughly 55 − 30 − 15 = 10 mmHg favoring filtration. As blood flows through the glomerulus and fluid is filtered out, the protein concentration in the remaining blood rises, increasing oncotic pressure. By the efferent end, oncotic pressure may reach 35 mmHg or more, narrowing the NFP and eventually approaching filtration equilibrium — the point where net driving pressure approaches zero. Despite this declining pressure gradient, the enormous surface area and high permeability of the glomerular capillaries produce a GFR of approximately 125 mL/min, or about 180 liters per day.
The kidney cannot afford to let GFR fluctuate with every change in systemic blood pressure — losing even 10% more filtrate than usual would rapidly deplete blood volume. Two autoregulatory mechanisms stabilize GFR across a wide range of arterial pressures (roughly 80–180 mmHg). The myogenic response is intrinsic to the afferent arteriole: when blood pressure rises and stretches the vessel wall, smooth muscle cells contract reflexively, increasing resistance and preventing the pressure increase from reaching the glomerulus. The tubuloglomerular feedback mechanism operates through the macula densa, a cluster of specialized cells in the distal tubule that senses the flow rate and NaCl concentration of the filtrate. If GFR is too high, more NaCl reaches the macula densa, which signals the adjacent afferent arteriole to constrict, reducing glomerular pressure and restoring GFR toward normal.
Understanding GFR is clinically essential because it is the single best measure of overall kidney function. When GFR declines — as in chronic kidney disease — waste products like creatinine accumulate in the blood, and the kidney loses its ability to regulate fluid volume, electrolyte balance, and acid-base status. Clinicians estimate GFR from serum creatinine levels precisely because creatinine is freely filtered at the glomerulus and minimally secreted, making its plasma concentration inversely proportional to filtration rate. A falling GFR is often the first quantitative signal that kidney function is deteriorating.