Thanks to Simple Contacts for supporting this episode.

Go to simplecontacts.com/scishow and use the promo code “SciShow” to get twenty dollars off your next order of contacts. [ INTRO ]. If you don't have perfect vision, you've probably had your fair share of eye exams.

They sit you down, ask you if things look better with option one… or two… and also blow that puff of air in your eye to test for disease. If you're one of the lucky ones with no vision problems, though, getting your eyes checked on a yearly basis probably isn't your top priority. I mean, you can see just fine, so why go?

Well, visiting an ophthalmologist is about more than just checking your vision — it's also good for your overall health. All kinds of conditions can show up in your eyes, sometimes before they're obvious anywhere else. They include everything from STIs to cancer — but since we can't cover them all, here are four of the more common ones. your eyes are so useful for detecting health conditions because they're packed with different kinds of tissues and cells.

There are blood vessels, nerve cells, muscle cells, and more, which means the eyes are susceptible to diseases that affect any of those things. On top of that, it's really easy to see inside the eye, since the outer covering is transparent — or see-through if you will. It's the only organ where doctors can see your blood vessels without any serious obstructions.

With the right equipment, they can even see red blood cells moving through your capillaries! So the eye is the perfect looking glass into your body's overall health. Some of the things a doctor can learn from your eyeballs are pretty straightforward.

For example, one of the easiest things for them to spot is if you have high blood pressure, otherwise known as hypertension. High blood pressure can develop over many years, but unless you regularly get checked for it, it can be hard to detect. The physical symptoms are often elusive, but can sometimes manifest as really bad headaches, chest pain, trouble breathing, or dizziness.

And if left untreated, hypertension can cause strokes or heart attacks, or can even lead to dementia. A general practitioner can usually catch high blood pressure during a regular checkup. But eye doctors can also notice it, because hypertension leads to some significant changes in blood vessels.

Although researchers aren't sure why, it causes the blood vessels to resist and push against the blood flowing through them, which causes pressure to rise. Over time, that leads to the vessels becoming narrower and stiffer. And while this happens all over the body, it can be easily seen in the retina, which is near the center of your eye.

The narrower the blood vessels get, the harder it is for blood to flow into the retina. That makes the pressure go up in the eyeball and causes it to become swollen, which can lead to blurry vision or seeing white spots. More severe hypertension can also cause hemorrhages and leaking in the eye because the blood vessels get weak and burst, but that can also be easily noticed during routine eye exams.

Another commonly-spotted problem in the eyes is high cholesterol. Cholesterol is a waxy substance that builds up in blood vessels over time, and it makes it harder for blood to get where it needs to go — like the heart or the brain. You know, important places.

If a piece of it breaks off, it can also get stuck somewhere and cause a heart attack or stroke. Like with high blood pressure, a general practitioner can tell you about your cholesterol levels — but again, so can an ophthalmologist. If there's too much cholesterol in the eyes' blood vessels, it can cause something called retinal vein occlusion.

This is where a clot cuts off the blood supply to part of the eye, and it can cause blurry vision, or even vision loss if it's severe enough. It is worth noting that this occlusion can also be caused by high blood pressure or diabetes, but regardless, it's a sign that something isn't quite right in the body. Speaking of diabetes, though, that's another condition that's sometimes first spotted in the eye.

Diabetes affects more than 420 million people worldwide, and it's actually a major cause of blindness. The disease has plenty of signs and symptoms, like being really tired, thirsty, or hungry, but one big one is blurred vision. This can be caused by retinal vein occlusion, but it can also happen because of diabetic retinopathy.

This is damage that happens to blood vessels when too much sugar builds up in the body. In particular, sugar damages the blood vessels that feed the retina. When these vessels are damaged, they can bleed or leak fluid, making your vision fuzzy.

The leaking can also cause the center of the retina to swell, which further contributes to the blurriness. At this point, scientists aren't exactly sure why sugar build-up damages blood vessels, but it definitely can. And if you don't treat diabetic retinopathy, it can eventually lead to blindness.

So while there are other ways to detect diabetes, sometimes, these vision problems are someone's first red flag. Finally, many inflammatory diseases — like Crohn's disease and lupus — can also be detected in the eye. Inflammation is totally normal, and is our body's response to an injury or infection.

When you get hurt or sick, your body dispatches white blood cells, and they work to attack and get rid of the invader. With inflammatory diseases, though, there isn't an injury… but the body responds as if there were one. These conditions are less common than something like hypertension or diabetes, but they still affect more than a million people, and that number is growing.

Inflammatory conditions can be caused by a number of things, from infections to autoimmune disorders, but ultimately, they lead to prolonged inflammation in the body. And that can cause something called uveitis, which is typically when the middle part of the eye, called the uvea, swells. It happens when inflammatory cells flood the eye, and it can cause blurred vision, light sensitivity, or dark floating spots, among other things.

And if you don't treat it, it can lead to permanent eye damage. These examples are just the tip of the iceberg when it comes to conditions that affect your eyes. So while the saying might be “the eyes are the windows to our souls,” it turns out they're more like the windows to our health.

While you definitely need to get your eyes examined, you can now test your vision and renew your contacts without heading to the doctor's office thanks to Simple Contacts. They allow you to renew your contact lens prescription and order your brand of contacts from anywhere, and it only takes a few minutes. They worked with ophthalmologists and doctors to design a self-guided vision test you can do at home to check your vision prescription.

And while it definitely isn't a replacement for a full eye health exam, it can help you confirm that your current prescription is helping you see twenty-twenty. If you want to learn more, you can head over to simplecontacts.com/scishow. And if you use the promo code “SciShow”, you'll get twenty dollars off your next order of contacts. [ OUTRO ].