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[Physiology](https://durgical.com/en/physiology/) · [Neurophysiology](https://durgical.com/en/physiology/#s-neuro)

# Aqueous humor and intraocular pressure

- Regulation
- Exam priority: medium

The eye makes and drains a clear fluid all the time; if drainage fails, pressure rises and can harm the optic nerve.

Updated 2026-11-01

Also called: IOP, Trabecular outflow

**See it in 3D:** [Ciliary body](https://durgical.com/en/anatomy/ciliary-body/) · [Drainage angle](https://durgical.com/en/anatomy/iridocorneal-angle/) · [Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/) · [Iris](https://durgical.com/en/anatomy/iris/) · [Optic disc](https://durgical.com/en/anatomy/optic-disc/)

## Key points

- Normal IOP is about 10–21 mmHg; it depends on production versus outflow resistance.
- Production: active secretion by ciliary epithelium using carbonic anhydrase and Na⁺/K⁺-ATPase, driven partly by β-adrenergic signalling.
- Outflow: trabecular meshwork → Schlemm canal → episcleral veins (main route); uveoscleral route is smaller.
- Open-angle glaucoma: slow, painless, peripheral field loss with optic disc cupping. Angle-closure: iris blocks the angle, sudden painful red eye.
- Pupil dilation can trigger angle closure in narrow-angle eyes, so anticholinergic and sympathomimetic drugs are a classic trap.

## What it is

Aqueous humor is secreted by the ciliary processes into the posterior chamber, flows through the pupil into the anterior chamber and leaves through the trabecular meshwork and Schlemm's canal, with a smaller uveoscleral route. Intraocular pressure is set by the balance between production and outflow resistance. Glaucoma is an optic neuropathy, often but not always linked to high pressure.

## Reference values

| Parameter | Value | Note |
| --- | --- | --- |
| Intraocular pressure | ≈10–21 mmHg | mean ≈15 mmHg |
| Aqueous production | ≈2–3 µL/min | |
| Trabecular share of outflow | most of it (≈70–90%) | the rest uveoscleral; varies with age |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Regulation

- *+* **β-adrenergic stimulation of ciliary epithelium**: Increases aqueous production; β-blockers lower IOP this way.
- *+* **Carbonic anhydrase**: Supplies bicarbonate for secretion; its inhibitors reduce production.
- *−* **Ciliary muscle contraction (muscarinic)**: Pulls on the scleral spur and opens the trabecular meshwork, increasing outflow.
- *−* **Prostaglandin F receptor signalling**: Remodels the ciliary muscle matrix and increases uveoscleral outflow.
- *−* **α2-adrenergic stimulation**: Reduces production and adds some uveoscleral outflow.

## Clinical notes

- Glaucoma is a leading cause of irreversible blindness; in Colombia it is often found late because early field loss is silent.
- Acute angle-closure: sudden eye pain, headache, nausea, halos, a hard eye and a mid-dilated, unreactive pupil; an emergency.
- Glucocorticoids, topical or systemic, can raise IOP in susceptible people.

## Memory aid

A sink with the tap always on: the ciliary body is the tap, the trabecular meshwork is the drain. Glaucoma drugs either turn down the tap or clear the drain.

## Test yourself

Think of the answer, then tap the question to see it.

## How do topical β-blockers lower intraocular pressure?

They block β-adrenergic signalling in the ciliary epithelium and reduce aqueous production; outflow is not the main target.

## Why can dilating the pupil precipitate acute angle-closure glaucoma?

In a narrow angle, the thickened, relaxed peripheral iris folds into the angle and blocks the trabecular meshwork, so outflow stops while production continues.

## How do muscarinic agonists improve aqueous outflow?

Contracting the ciliary muscle pulls on the scleral spur and opens the trabecular meshwork; constriction of the pupil also pulls the iris away from the angle.

## Why is open-angle glaucoma often found late?

Optic nerve damage starts in the peripheral field, which the other eye and the brain compensate for, and the raised pressure causes no pain.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/aqueous-humor-and-intraocular-pressure/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Ciliary body](https://durgical.com/en/anatomy/ciliary-body/)
- [Drainage angle](https://durgical.com/en/anatomy/iridocorneal-angle/)
- [Anterior chamber](https://durgical.com/en/anatomy/anterior-chamber-of-eyeball/)
- [Iris](https://durgical.com/en/anatomy/iris/)
- [Optic disc](https://durgical.com/en/anatomy/optic-disc/)

## Related topics

- [Optics of the eye, accommodation and refractive errors](https://durgical.com/en/physiology/optics-of-the-eye-accommodation-and-refractive-errors/)
- [Sympathetic and parasympathetic effects on each organ](https://durgical.com/en/physiology/sympathetic-and-parasympathetic-effects-on-each-organ/)
- [Adrenergic and cholinergic receptors](https://durgical.com/en/physiology/adrenergic-and-cholinergic-receptors/)

## Related conditions

- [Open-angle glaucoma](https://durgical.com/en/conditions/open-angle-glaucoma/)
- [Acute angle-closure glaucoma](https://durgical.com/en/conditions/acute-angle-closure-glaucoma/)

## In Pharmacology

- [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Physiology, Aqueous Humor Circulation](https://www.ncbi.nlm.nih.gov/books/NBK553209/) (StatPearls (NCBI Bookshelf), 2024); [Open Angle Glaucoma](https://www.ncbi.nlm.nih.gov/books/NBK441887/) (StatPearls (NCBI Bookshelf), 2024); [Acute Angle-Closure Glaucoma](https://www.ncbi.nlm.nih.gov/books/NBK430857/) (StatPearls (NCBI Bookshelf), 2024); [14.1 Sensory Perception](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-1-sensory-perception/) (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 50). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/aqueous-humor-and-intraocular-pressure/