The pupil is one of the first things you likely notice when you look someone in the eye, even before you realize what color their eyes are. But what does the pupil do, and what exactly is it?

What is the Eye’s Pupil and What Does It Do

The pupil is that black circle at the center of your eyes, but it’s not “really” black. It’s also not really a “thing” – it’s an open space in your eye. Through the clear lens covering your eye, light comes through and is absorbed by the tissues at the back of the eye – and it is this absorption of light that makes the pupil appear black.

Around your pupil, the colored part that gives you brown, hazel, blue, or green eyes, is the iris. The iris serves a purpose, as well, besides giving your eyes their color. This part of the eye dilates and contracts, which is what changes the size of your pupil. Both the pupil and the iris are responsible for how much light gets into your eyes.

It is this pupillary reflex that makes your eyes smaller when the lights are bright and bigger when the lights are turned down low. The pupil controls what amount of light flows through the clear lens of the eye back to your retina (which is at the back of the eye).

Oddities of the Eye’s Pupils

There are some interesting facts about pupils. Not all pupils are created equal.

  • Pupils are most often larger in younger people than they are in older people.
  • Around 20% of the world’s population has pupils that vary in size – meaning one is larger/smaller than the other.
  • While humans all have round pupils, not all mammals and animals do. Some animals have slit-shaped pupils.
  • Pupils that appear cloudy or pale in color are often this way because of cataracts (you can notice this in animals and people). Cataract surgery can reverse this, making the pupils appear black again.
  • When someone takes your photo, and your eyes look red in it, that is the fault of your pupils. The intense light of the flash and where you’re looking cause the light to reflect off the retina in the color of red.

What Makes Your Pupils Do What They Do?

What causes pupillary reflex? Your eyes constrict and dilate, but it’s not magic happening – there is a process in your eyes that makes them do what they do.

The thing is, your eye is equipped with sphincter muscles that work like a drawstring to constrict your pupils. Then, there are dilator muscles in the center of the iris working to dilate the pupil.

Within your eye, there is a long nerve meant to help with the changing conditions of sight, like more or less light coming in. That helps the eye react to changing conditions. It’s a nerve that starts in the brain and travels down the spinal cord, making its way down to the lungs before traveling back up through the neck, back into the brain, and then reaching your pupil. It’s because of this route that people suffering from certain brain or lung issues can have eye problems, or the illness can affect how the pupils work.

Why Does the Pupil Adjust to Light?

Knowing how the pupil functions isn’t always enough to help you understand why it does what it does. There is a method to the pupil restricting light at times – and understanding why it does this can be better imagined if you think of how a camera works.

Your pupil is much like the aperture, and your iris is the diaphragm, which controls the size of the pupil (or aperture). So, much like you want light controlled in a camera, the iris and pupil work to control light coming into your eye – with one muscle constricting to make the pupil smaller and another dilating for a larger pupil.

Your eye is protecting itself in this way – as too much light into the eye in brightly lit situations can cause glare, as well as damage to the lens and even the retina. By dilating when it’s dark, your pupil is helping you see better (so you don’t stub your toe).

Pupils and Your Eyesight

The light your eyes let in helps you see clearly – but are the pupils your main object of sight? They aren’t. All of the parts of your eyes work together to help you see.

When you open your eyes, light passes first through your cornea, then into your pupils – your pupils will adjust depending on the light source. The light then goes to your retina where images are flipped upside-down. The images travel through the optic nerves into your brain where the images are processed (and flipped back the right way).

Everybody’s Pupils are Different

Pupils don’t come in a one-size-fits-all fashion – one person may have larger (or smaller) pupils than another. Pupil size also changes with age – with larger pupils when you’re young and smaller ones when you reach your senior years.

Average pupil size ranges between 2 and 4 millimeters when the lights are bright, and ranging from 4 to 8 millimeters when you’re in the dark. Pupils also constrict when you’re reading something close to you.

Checking Your Pupils at the Doctor’s Office

When you go in for your normal eye exam, your pupils will get tested – it’s a normal thing since they are an important part of your visual experience. When testing your pupil function, you’ll be in a room with dim lighting where your doctor will be looking at your eyes using a small light.

The test exams how your pupils react both directly and indirectly to light. There are other tests that check your pupils, and sometimes you’ll get your pupils dilated (usually for older people or people with specific eye issues).

Problems with Pupil Health

If you have problems with your pupils, it could be for any number of reasons. Here are a few common eye conditions that can affect the pupils.

1. Marcus Gunn Pupil

Marcus Gunn pupil is also sometimes referred to as relative afferent pupillary defect or afferent pupillary defect. It is an abnormal result that shows up in a test called swinging-flashlight. The patient’s pupils don’t constrict as much as they should between eyes when the light is swung.

This abnormality is often the result of damage to the posterior region of the optic nerve. It can also be caused by severe retinal disease.

2. Argyll Robertson Pupil

Argyll Robertson pupil is often recognized by the smaller size of the pupils, usually both of them. They don’t react to light at all.

This is a rare condition, and there is no known cause of it. However, some tests have associated it with neuropathy caused by diabetes and syphilis.

3. Adie’s Tonic Pupil

When someone has Adie’s Tonic pupil, their pupil has almost no reaction to light. There can be a delayed reaction when including accommodation. This eye abnormality is sometimes referred to as Adie’s syndrome.

Typically, this eye condition only affects one eye. The pupil with the problem will appear larger than the unaffected one. The cause of Adie’s syndrome is unknown, but it has happened due to infections, trauma, and post-surgery.

Problems with the pupils can also be caused by trauma to the eye. Trauma that affects the shape of a pupil, eye surgeries, and more can cause abnormal pupillary response to light.

Caring for Your Pupils

Caring for your pupils is one more step in caring for the overall health of your eyes. Neglect of your pupillary health can cause problems with your retinas and more.

To care for your eyes, there are some simple things you can do:

  • Visit your eye doctor for routine eye exams every 1 to 2 years.
  • Always wear sunglasses when you’re outdoors in the sun, even in the winter.
  • If you notice any changes, mild or major, in your vision, don’t ignore them – make an eye appointment and get it checked out.

Final Thoughts on the Function of the Pupil

Pupil care is important – since the vision you have is all you’ve got. Improper pupil care can lead to retinal damage, which can lead to blindness. Pupils do a lot to help you see clearly, so take good care of them.

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