Vascular Layer of the Eyeball - Online Tutor, Practice Problems & Exam Prep

The vascular layer of the eye, also known as the uvea, consists of the iris, ciliary body, and choroid. The iris controls light entry through the pupil, with melanin determining eye color. The ciliary body suspends the lens and produces aqueous humor, while the choroid absorbs excess light and supplies blood to the retina. The autonomic nervous system regulates pupil size: the pupillary constrictor muscle reduces pupil size in bright light, while the pupillary dilator muscle enlarges it in dim light, enhancing visual clarity.

Vascular Layer

As we continue learning about the different layers of the eye, we're now going to talk about the vascular layer. And remember, the vascular layer is the middle layer of those three layers that make up the eyeball, and we said that it's also sometimes called the uvea. Now, I'm not going to call it the uvea, but you should be familiar with that term because it does come up sometimes. And we said the general role for this vascular layer is that it regulates the light that enters the eye and it also supplies blood to the eye. Now we're going to break the vascular layer up into three parts and talk about them individually, but before we do that let's just orient ourselves to our image here. We have this transverse section of the eyeball right here, this is a top-down view of the eye, and we're looking at the right eye. And we can see that we've colored in these browns and oranges the vascular layer. In the front here, in this brown, we have the iris. As we move back, we have in this sort of reddish color here, the ciliary body. And then all around the back here, in this sort of orangish brown, we have the choroid.

So let's start by talking about the iris. The iris is that colored region around the pupil. So when you ask someone what color eyes do you have, you're asking them what color is your iris. It's going to be made of or comprised of muscles, and those muscles control the size of the iris and therefore control the size of the pupil. So if those muscles squeeze down, the iris is going to get bigger, the pupil will get smaller. If other muscles pull back, that iris is going to get smaller, the pupil will get larger. It's also going to be made of pigmented elastic fibers. So you have these muscles, but then you have these stretchy elastic fibers with pigment, a brown pigment, that gives eyes its color wrapped around those muscles that can stretch back and forth as the muscles change size. And the job of those elastic fibers and the pigment is to block the light. The iris is there really controlling the total amount of light that gets into the eye, so you don't want it transparent. You want to block light from going through it. Now when we said when it changes size, what it's really changing is the size of the pupil. And the pupil, you really just want to think of as a hole. That pupil, that black dot in the middle of your eye, it's really just a hole so that light can get into the eye, and we can change the size of the pupil to control how much light gets into the eye. Now we're going to talk about how all this works in a lot more detail coming up, but right now we just sort of want to look at this. And I want to take a second sort of even closer look to see how this is all put together. And so we're going to look at this super zoomed-in image of an eye here. You can see that iris sort of changing shape a little bit. And I like this view here, sort of at an angle, because you can really see how that pupil is just a hole in the middle of the iris. Right? It's a hole, it's black because it's dark in there. And you can also see those elastic fibers really clearly. You can see how it really looks like they're just these fibers wrapped around those muscles, and as those muscles change size, the elastic fibers with the pigment in them, that color of your eye, is going to change size with it.

We'll now move back and we're going to talk about the ciliary body. Again, that's that section here in this sort of orangish red. And we're going to say the ciliary body is there to suspend the lens of your eye, and it also produces fluid to the eye. We're going to break the ciliary body up into three individual parts though. First, we have the ciliary muscles, and we have that labeled as here and here. It's sort of the larger part of the ciliary body. And the ciliary muscles are there to control the lens's shape and thereby to focus the light. So your lens is flexible and depending on how much those ciliary muscles are pulling on the lens, the lens will be rounder or flatter. Now we'll talk about how that works and how that focuses the light, a lot more coming up. But for now, you just want to know that the ciliary muscles are responsible for changing that shape of the lens. Now in B, we have labeled the ciliary zonules. So you can see here in B, these are the ligaments that connect those ciliary muscles to the lens, and they're also sometimes just called the suspensory ligaments. So we're going to say here they connect the lens to the ciliary muscles. And you can remember that because the word 'zonio' comes from the Latin, which means belt. So these are these sort of belts that are pulling on, tugging on that lens, and helping to change its shape so that it can focus light.

The last part of the ciliary body that we want to talk about are these ciliary processes here, and you can see those in c. And it's just sort of the back part here, you see how it's just a little bit bumpy? Those are those processes, and those processes are what secrete fluid for the anterior or just the front part of the eye. Okay. So the front part of your eye up here is filled with what we said is called aqueous humor. Again, we'll talk about that in more detail coming up, but that aqueous humor, that fluid is actually produced back here and it slowly flows forward through the pupil, and then it's, sort of taken out and it leaves the eye sort of at the corners here. So it flows up through that way. That's where that fluid that fills the front of the eye is produced.

That takes us to our last section. That last section is going to be the choroid, and again the choroid is this orangish brown section that's going all the way around, sort of inside the sclera, between the sclera, and the retina. And this is going to be a pigmented membrane. And so when you think of it being dark in your eye, it's probably because that whole back of the eye is a dark color. You have this choroid, this pigmented membrane, and that is preventing reflection. You want light to come into your eye in this way, get focused by the lens, and then end up on the retina, but not all the light is going to be absorbed by the retina. Some's going to pass through the retina. You don't want that extra light reflecting off and bouncing all over the eye. That could really sort of mess up the image and the image processing that your retina does. So the choroid absorbs that light and sort of just makes the whole thing dark. It's also going to be rich in blood vessels, and that's where that name, the vascular layer, comes from. So it's located between the retina and the sclera there. So those blood vessels are supplying a lot of blood, some to the sclera, but really it's supplying blood to the back half of the retina. The retina uses a lot of energy. It has a high need for oxygen because they're these, you know, nervous cells and they use a lot of oxygen to do their job. So the back half of the retina gets that blood from the choroid. The front half of the retina actually has its own blood vessels that supply it.

So that's the vascular layer. We're going to go into the iris in more detail coming up. Later on we'll talk about how the ciliary body works with a lens to focus light. I'm looking forward to it. I'll see you there.

Vascular Layer of the Eyeball Example 1

Alright, folks. Our example tells us that the table below lists 3 parts of the vascular layer. Then I want to know how does each component help regulate or control the light coming into the eye? And the three structures we're talking about here are the iris, the choroid, and the ciliary body. Now before we answer that, let's look at our diagram here. This is our transverse section or our top-down view of this right eye, and you can see that we have the vascular layer color-coded here.

So let's start with our first structure, the iris. Do you remember what the iris is and how it controls or regulates light coming into the eye? Now the first thing I'm going to do is I'm going to find the iris on my diagram here, and that's that brown structure in the front. The iris is that circular colored part of the eye in the front. And what it's doing, right, the iris is this circular structure that has a hole in the middle, and that hole is the pupil. And the pupil is the only way that light can get into the eye. So what the iris does, I'm going to say, is it controls the size of the pupil, and thereby it controls the total amount of light entering the eye. If that iris opens up, the pupil is bigger, more light gets in, the iris closes down, the pupil is smaller, less light gets in.

Next up, we have the choroid. Do you remember the role of the choroid in terms of regulating or controlling light in the eye? Well in my diagram here the choroid is this part all the way around the back. It's between the sclera and the retina. It is the vascular layer. And so as light comes into the eye, right, light is going to come in through the pupil, it's going to go through the lens, and then what you want is it to be absorbed by the retina so that you can see it. But not all light will be absorbed by the retina, and what you don't want happening is light hitting the back of the eye and then bouncing off and hitting other parts of the retina. That's going to create problems with your vision. What you want to have happen is light enter the eye, get picked up by the retina, but if it isn't picked up by the retina, hit the choroid and stop. So we said that the choroid has pigmented cells in it, and those pigmented cells are going to absorb excess light.

Remember the choroid also has the role of having a lot of blood supply in it, right? So this is the vascular layer and that's really where most of that vascularization is, is in the choroid and it is providing blood to the back half of the retina. So that's another really important job for the choroid. But in terms of controlling light, it's there to absorb excess light in the eye, so light doesn't bounce around and reflect.

Finally, we have the ciliary body. Remember the job of the ciliary body? Well, in our diagram here, the ciliary body's here in this sort of orangish-red color, and importantly, we have the ciliary muscles here, and then we have the ciliary zonules or the suspensory ligaments, sometimes called, and those are suspending the lens. So together, those work to control the shape of the lens. Right? So light is going to come in and you want that light focused by the lens so it creates a really clear image on the retina. The job of changing the shape of the lens so that it can focus is the ciliary body, the ciliary muscles, and the ciliary zonules.

Now we'll talk about how the lens works in more detail coming up. Right now, you just want to understand that the ciliary body has that role. Okay. So the vascular layer, remember vascular means blood supply, so it does provide blood to the eye. But you want to also remember that it has its job of controlling and regulating the light. Alright. With that, there are practice problems to follow. See you there.

The aqueous humor is the fluid that fills the anterior portion of the eye between the lens and the cornea. This fluid is constantly regenerated and drains out through the scleral venous sinus. If the scleral venous sinus is blocked, aqueous humor can accumulate, putting pressure on the structures of the eye. Which structure of the vascular layer is responsible for producing the aqueous humor?

• A. Iris.
• B. Choroid.
• C. Ciliary Body.
• D. Ciliary Process.

Which structure(s) of the middle layer contains muscles?

• A. Choroid & iris.
• B. Choroid & ciliary body.
• C. Iris only.
• D. Iris & ciliary body.

Control of Light Entering the Eye

As we continue talking about the vascular layer of the eye, we now want to talk about control of light entering the eye in a lot more detail. And remember, light enters through the pupil, but the pupil is just a hole that's in the center of the iris. So to understand how we control the light entering the eye, we need to understand how the iris works in more detail. Well, the iris is the color of your eye, so we're going to start off by saying that that color of the iris just comes from the pigment melanin. And you'll remember from the integumentary system that melanin is a brown pigment, and it's the same thing that gives your skin its color. People with very dark skin colors have a lot of melanin. People with very fair or pale skin have very little melanin in their skin. The same thing happens in the eyes. We're going to say that more pigment equals brown eyes. And as we look up here, we have this sort of scale of eye color here. We have 5 eyes laid out from blue to green to darker and darker browns. And as we go to the right on this scale, we see that it says, "High melanin," we're putting more and more melanin in the eyes. We can see that the eyes go from sort of a light brown to darker and darker browns. And some people have very dark brown eyes because they just have a lot of melanin in their eyes. But you'll notice as we go to the left on the scale, it's not like skin color. It doesn't just get paler or a lighter brown. Eventually, it sort of looks kinda greenish or blue. So we're going to say here that less pigment is going to equal a green or blue color of the eyes. But, importantly, we're still just talking about melanin. There's nothing blue or green in the irises of people who have blue or green eyes. So I have fairly blue eyes. If you were to gouge out my eye, rip out my iris, and lay it on the table, you would see it would look probably just kind of like this unpigmented, pale, beige tissue. The reason it looks blue in my eye is the same reason that the sky is blue. Blue is a shorter wavelength light. So like all light, when the light passes through materials of different densities, in this case the elastic tissue of the eye, it's going to refract or bend. But shorter wavelengths refract or bend more than longer wavelengths. So that blue light hits the iris, starts bending, and more of the blue light bends and comes back out of the eye for people to see, the eye looks blue. If I had more melanin in my eye, that blue light would just get absorbed and my eyes would look brown.

What we really want to talk about here, though, is how light gets past the iris, though, and that's going through the pupil. So we're going to say here that changing the size of the pupil changes the amount of light coming into the eye, and of course, we change the size of the pupil by changing the size of the iris. Now before we get into the details here, I just want to watch a video of this, a super close-up vision, video of the eye, so that we can sort of think about how all this is working. So we're going to see here someone whose eye or whose pupil is dilated. So the pupil is going to be really big starting out, and then that iris is going to squeeze in and make the pupil smaller. Here we go. We have this big pupil and you can see that iris squeezing in the elastic tissue with the melanin in it, squeezing in with it, getting pulled along with it. Now, I'm going to play this again for you one more time. And as we do, I want you to think about how the muscles in this iris must be working. How must those iris be those muscles in the iris be arranged to squeeze in in this way? And then if we wanted to open up the pupil, how must those muscles be arranged and how would they work as well?

So now that we're thinking about that, let's look back at our page. So there are 2 basic actions that the iris can do. We can have constriction of the pupil—the iris can squeeze inwards. Or we can have dilation of the pupil. The iris can pull outwards and make that pupil bigger. We are going to talk about 2 muscles here, and they're pretty easy to remember the names because they're just named after the action. So to constrict the pupil, we have the pupillary constrictor muscle, and to dilate the pupil, we have the pupillary dilator. So let's start with this pupillary constrictor muscle. We can see here we have well, we have two images of the iris here where we removed that elastic tissue off of the iris. And so now we're just looking at the muscles, and we can see this pupillary constrictor in the middle here. This is this sort of circular-shaped muscle. And this circular muscle well, like all circular muscles, when it squeezes in, it constricts in and makes the hole in the middle smaller. Alright? So that is going to make that iris bigger, the pupil smaller. Now in contrast, on the outside of the iris here, we have these muscles, this pupillary dilator muscle that's almost arranged like spokes on a wheel, and we're going to call this a radial shape, sort of pulling outwards. And so to open up this iris, to dilate the pupil, these muscles are going to pull outwards.

Okay? So now why would they do that? In what cases would each different muscle contract? Well, the pupillary constrictor is going to contract for bright lights. That makes sense. If it's really bright out, you want to shut down that pupil so you don't get too much light in the eye and damage the retina. It's also going to do it though for close vision. And why that is, we'll talk about in more detail when we talk about focusing light on the retina using a lens. For now, though, just remember that for close vision, that's another thing that's going to cause this iris to close down, this pupillary constrictor muscle to contract. Alright. Well, in contrast, the dilation of the pupil, the pupillary dilator, that's going to happen for dim lights. And that makes sense if it's not very bright. Need to open up the pupil, let more light into the eye, and also for distant vision. Again, why for distant vision? We'll talk about when we talk about focusing light on the retina using the lens. For now, just know that it happens when you're looking at something far away. That pupil is going to be bigger.

Control of the pupil is by the autonomic nervous system. This is something that you have no conscious control of. So for innervation, we want to think about which division of the autonomic nervous system is controlling it. For the constriction, that's going to be the parasympathetic division of the autonomic nervous system. And remember, the parasympathetic division of the autonomic nervous system, we sometimes think of as our rest and digest nervous system. So when I think about it that way, I sort of think, well, if I want to rest and digest, I kind of want to just shut things down. I want to reduce the total amount of sensory information coming into my body. So I'm just gonna kinda turn those headlights down and just, you know, block stuff out. Now in contrast, the pupillary dilators are going to be controlled by the sympathetic division of the automatic nervous system. Remember, the sympathetic division, that is like your fight or flight response. So again, I think of it this way. If I, you know, am in my fight or flight, I'm scared, I need to fight something, I want as much sensory information coming into my body as possible. So I open up that pupil, I let a lot of the light in so I can see everything super clearly. All right. Associated with that, we can think of what emotional states are going to lead to constriction or dilation. So, that pupillary constrictor muscle is going to contract in states of boredom. Right? Shut things down, rest and digest, but also things like an unpleasant sight. And I think of that again as if I don't want to see something I can kind of shut things down a little bit and reduce the total amount of information coming in. In contrast, these dilators, you want to open up the pupil, well, for things like fear, desire, problem-solving. All these states where you really want a lot of sensory information coming in so that you can really process everything that's going on. Alright. So with that, we're done talking about the vascular. But like always, we have an example and practice problems to follow. Give them a try.

Vascular Layer of the Eyeball Example 2

Alright, folks. Our example asks us to imagine that you are waking up from a nap in a dark room with heavy curtains drawn. Sounds lovely. You walk to the window and open the curtains, letting direct sunlight flood the room. Describe how the muscles of the iris would respond to the two light environments described.

Alright. So, our two light environments here, we have the darkened room and the room with sunlight. And for each one, we want to know which muscle is contracted, which one is relaxed, and then the size of the pupil. Alright. I'm going to start with the size of the pupil because figuring out the size of the pupil is going to tell me everything else, and I think that part should be pretty easy to figure out. So, you wake up in a dark room, you open your eyes, it's dark. You need to let a lot of light into the eye to be able to see things. Well, that pupil is going to be nice and big, it's going to open up. So, I'm going to say that the pupil here will be large, letting a lot of light into the eye.

Now, you walk over, you open the curtains, bright light hits your face, you're looking at the sun. You want to turn down those headlights, you want to shut that pupil down. So here, the pupil is going to be small in a room with a lot of sunlight.

Alright. So, what is the muscle that contracts to get your pupil to be large? Remember we had two muscles that we're talking about here, the pupillary dilator and the pupillary constrictor. And those names just kinda tell you what they do. Right? So first off, we know it's a pupillary muscle. So, pupillary. Well, to get a large pupil, you want to dilate the eye. That means to make something bigger, to make that hole in the iris bigger. So, pupillary dilator. Remember, those were those muscles arranged in sort of this radial fashion, and they're going to pull out on the eye or on the iris, making that pupil larger. So therefore, our relaxed muscle will have to be our pupillary constrictor. Right? And that's that circular muscle that squeezes down like a sphincter to close the eye or to close the iris. And so, if you want a large pupil, that one needs to relax so it can be pulled back.

In the room with sunlight, well, it's just going to be the opposite. Right? So the pupillary constrictor, that round circular muscle, is going to squeeze down to make the pupil smaller, and the muscle that needs to relax is going to be the pupillary dilator. And when that relaxes, the pupillary constrictor can squeeze down. You're going to get a small pupil blocking light entering the eye. Okay. We got more practice problems on this coming up. See you there.

Which of the following would increase the total amount of light coming into the eye?

• A. Activation of the parasympathetic nervous system.
• B. Looking at something close to the face.
• C. Resting.
• D. Feelings of rage.

True or False: if false, choose the answer that corrects the statement.

Pupils enlarge when the pupillary dilators contract after receiving stimuli from the parasympathetic nervous system.

• A. True.
• B. False; pupils enlarge when the pupillary dilators contract after receiving stimuli from the sympathetic nervous system.
• C. False; pupils enlarge when the pupillary constrictors contract after receiving stimuli from the parasympathetic nervous system.
• D. False; pupils enlarge when the pupillary constrictors contract after receiving stimuli from the sympathetic nervous system.

Here’s what students ask on this topic:

What is the vascular layer of the eye and what are its main components?

The vascular layer of the eye, also known as the uvea, is the middle layer of the eyeball. It consists of three main components: the iris, the ciliary body, and the choroid. The iris controls the amount of light entering the eye by adjusting the size of the pupil. The ciliary body suspends the lens and produces aqueous humor, which nourishes the eye. The choroid is a pigmented membrane that absorbs excess light to prevent reflection and supplies blood to the retina, ensuring it receives the necessary oxygen and nutrients.

How does the iris control the amount of light entering the eye?

The iris controls the amount of light entering the eye by adjusting the size of the pupil, which is the hole in the center of the iris. It contains two types of muscles: the pupillary constrictor muscle and the pupillary dilator muscle. The pupillary constrictor muscle, arranged in a circular pattern, contracts to make the pupil smaller in bright light. The pupillary dilator muscle, arranged radially, contracts to enlarge the pupil in dim light. This regulation is controlled by the autonomic nervous system, with the parasympathetic division causing constriction and the sympathetic division causing dilation.

What role does melanin play in determining eye color?

Melanin is the pigment responsible for determining eye color. It is the same pigment that gives skin its color. The amount of melanin in the iris determines the eye color: more melanin results in brown eyes, while less melanin results in lighter eye colors such as blue or green. The blue or green appearance is due to the scattering of light, similar to why the sky appears blue. The light refracts through the elastic tissue of the iris, with shorter wavelengths (blue light) bending more and being more visible.

What is the function of the ciliary body in the eye?

The ciliary body has two main functions: suspending the lens and producing aqueous humor. It consists of ciliary muscles, ciliary zonules, and ciliary processes. The ciliary muscles control the shape of the lens, allowing it to focus light on the retina. The ciliary zonules, or suspensory ligaments, connect the ciliary muscles to the lens. The ciliary processes produce aqueous humor, the fluid that fills the front part of the eye, providing nourishment and maintaining intraocular pressure.

How does the choroid contribute to vision?

The choroid is a pigmented membrane located between the retina and the sclera. Its primary function is to absorb excess light to prevent reflection within the eye, which could interfere with image processing. The choroid is also rich in blood vessels, supplying oxygen and nutrients to the retina, particularly the back half, which has a high demand for energy. This blood supply is crucial for the retina's function in converting light into neural signals for vision.