Intraocular pressure is set by a fluid balance between how fast the ciliary body makes aqueous humor and how fast it leaves the eye. At steady state this is the Goldmann equation:
IOP = (F − F_u) / C + P_v
F = aqueous humor production rate (µL/min)
F_u = pressure-independent uveoscleral outflow (µL/min)
C = trabecular outflow facility (µL/min per mmHg)
P_v = episcleral venous pressure (mmHg)
Unlike a snapshot of that formula, this model integrates the underlying differential equation that produces it: the eye behaves as a pressure vessel with finite ocular compliance Ce (µL of volume change per mmHg), so pressure relaxes toward equilibrium as
dIOP/dt = (F − F_u − C·(IOP − P_v)) / C_e
which integrates in closed form to an exponential approach to the Goldmann steady state with time constant τ = C
e/C — a stiffer eye (low C
e) swings pressure faster per unit fluid imbalance; a floppy, compliant eye buffers it. The top panel is a hydraulic circuit view of this same equation: a tank fills from the ciliary inflow pipe and drains through a pressure-driven trabecular valve (resistance 1/C) into the episcleral vein reservoir at P
v, plus a constant-rate uveoscleral bleed valve. The bottom panel is an anatomical cross-section: particles secreted at the ciliary body drift through the anterior chamber and exit through the trabecular meshwork at the iridocorneal angle (teal) or slip through the uveoscleral gap (amber) — drag to pan, scroll/pinch to zoom.
- Inflow rate F — ciliary body secretion. Beta-blockers and carbonic-anhydrase inhibitors reduce F.
- Outflow facility C — trabecular meshwork conductance. In primary open-angle glaucoma the meshwork stiffens and clogs with debris, C falls, and IOP rises even though F is unchanged.
- Episcleral venous pressure Pv — the downstream back-pressure the trabecular pathway drains against; it sets the floor IOP can never go below regardless of C.
- Uveoscleral outflow Fu — prostaglandin analogs (latanoprost) increase this pressure-independent term.
- Ocular compliance Ce — corneoscleral shell "give"; a real physiological quantity (inverse of the Friedenwald coefficient of ocular rigidity) that sets how fast, not just where, IOP settles.