About Pupil
The human eye is made up of several external structures like the sclera, cornea, iris, pupil, lens, retina, optic nerves, cones, rods. The pupil comes under the external part of the human eye as it opens in the centre of the iris as the iris is the structure that gives colour to the eyes. The main function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight. They are perfectly round in shape, equal in size and of black colour.
Colour of Pupil
The colour of the pupil is mainly black because the light that passes through the pupil is absorbed by the retina and is not reflected back. In case the colour of the pupil is pale then the main reason behind this is the lens of the eye has become opaque due to the formation of a cataract. To obtain the colour of pupils bak, we have to remove the pale colour cataract by operating it. With the help of surgery pale lens is replaced by a clear intraocular lens.
There is one more situation when the colour of pupils get changed, is when someone takes your photo using the camera's flash. When taking a photo, the colour of the pupil may vary depending upon the direction of the gaze. The main reason behind this is intense light coming from the flash being reflected by the red colour of the retina.
Size of Pupil
The size of the pupil keeps on changing depending on the condition i.e. they tend to become bigger in low-intensity light which enables more light to enter into the eyes which make it easier to see anything and in high-intensity light, the pupil size becomes smaller or contracts. When a pupil is fully dilated its size is approx 4 to 8 millimetres and when it is constricted its size varies from 2 to 4 mm range. As per a recent study done by an American ophthalmologist, the normal size of the pupil is approx 2 to 8mm.
Functioning of Pupil
Iris and pupil together control the amount of light entering into the eyes. The pupil is the aperture of the eye and the iris is the diaphragm that controls or maintains the size of the aperture. Muscle within the iris is responsible for controlling the size of the pupil. One muscle constricts the pupil opening and another iris muscle dilates the pupil. This whole process of controlling the size of the pupil occurs within the iris. High-intensity light can cause glare and discomfort and sometimes it may damage the lens and retina.
Injury or Deformities which Cause Damage to Pupil
A. Concussion:
It is a kind of brain injury that occurs from the brain smacking against the hard skull during a fall, a hit to the head, which further shows an impact on the whole body. One of the common symptoms of this is an increase in the size of the pupil than normal.
B. Anisocoria:
In this condition, one pupil is wider than another, which can be because of both natural problems or may be accidental. This problem is found in almost 20% of people.
C. Coloboma:
In this condition, part of the eye doesn’t form the right way before you’re born.
D. Pituitary Gland Tumour:
As pituitary gland controls so many other glands that participate in making different hormones. So tumours in this gland can increase the size of pupils.
How Pupil of the Eye Regulate the Amount of Light?
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Let us see how the size of the pupil is changed to regulate the amount of light.There are muscles and connective tissue between the iris and the pupil. When bright light falls on the pupil, the circular muscles in the iris contract and the radial muscles relax which reduces the size of the pupil. Therefore, the amount of light passing through the pupil for a bright light is reduced and an image is formed on the retina comfortably. This is what happens when we go outside on a brighter sunny day, we will feel uncomfortable at first due to bright light reaching our eyes. Then the size of the pupil decreases and the amount of light passing through the pupil is also reduced. Then we can easily see clearly without any discomfort due to bright light. So, in bright conditions, we have a dilated pupil eye.
On the other hand, when it is dark and not clearly visible, the circular muscles relax and the radial muscles contact. This causes the pupil to dilate and the amount of light passing through the purple increases.This helps to maximise the vision of our eyes. A good example to understand this is when we go inside suddenly to a dark room. We will not be able to see clearly because of the low amount of light reaching our eyes. But as the time progresses, the pupil gets dilated and the amount of light passing through the pupil increases. Thereby increasing our vision at dim conditions.
Our emotions can also affect the size of the pupil.The pupil tends to dilate if we are delighted or surprised and the pupil size becomes small when we are angry. If we are in a fight or flight situation, the pupil dilates. In a situation of fight or flight, it is crucial that the vision of our eyes has to be very clear. To ensure that we are able to see properly, the pupil is dilated along with other sympathetic actions. The pupil of a dead person will always be dilated. To check whether a person is dead or alive, we can check the pupil of the person and if it is not constricting, then we can confirm that the person is dead.
Conclusion
The smaller circular opening of the iris is called pupil of the eye. The function of the pupil of the eye is to control the amount of light passing through the pupil. It is the responsibility of the iris to control the size of the pupil using circular and radial muscles. The contraction and relaxation of these muscles control the size of the pupil. The pupil constricts in bright conditions and the pupil of the eye dilates in dim conditions. The dilation and constriction of the pupil is also triggered by our emotions. The pupil dilates when we are surprised or pleased and the pupil constricts when the person is angry or doubtful. A dead person will have a dilated pupil eye.
FAQs on Pupil - Eye
1. What exactly is the pupil of the eye and where is it located?
The pupil is not a physical structure but an aperture, or opening, located in the absolute centre of the iris (the coloured part of the eye). It appears as a black dot because the light that enters through it is absorbed by the tissues inside the eye, primarily the retina, and is not reflected back.
2. What is the primary function of the pupil in the human eye?
The primary function of the pupil is to control the amount of light that enters the eye and reaches the retina. By adjusting its size, it regulates light intensity to ensure clear vision and protect the sensitive retina from damage caused by excessively bright light.
3. How does the pupil change its size in response to bright and dim light?
The size of the pupil is controlled by the muscles of the iris.
- In bright light, the circular muscles of the iris contract, causing the pupil to constrict (get smaller). This reduces the amount of light entering the eye.
- In dim light, the radial muscles of the iris contract, causing the pupil to dilate (get larger). This allows more light to enter, improving vision in low-light conditions.
4. What is the difference between the iris and the pupil?
The key difference lies in their nature and function. The iris is a pigmented, muscular structure that gives the eye its colour and actively controls the size of the pupil. The pupil, on the other hand, is the passive opening in the centre of the iris through which light passes. Think of the iris as the camera's shutter and the pupil as the aperture it creates.
5. Why does the pupil appear black?
The pupil appears black because the environment inside the eye is very dark. When light enters the pupil, it is almost entirely absorbed by the retina and other deep structures like the choroid. Since very little light is reflected back out, the opening looks black, similar to how an open window of a distant house looks black during the day.
6. What causes the 'red-eye' effect sometimes seen in photographs?
The 'red-eye' effect occurs when a photograph is taken with a flash in low-light conditions. In the dark, the pupil is widely dilated to let in more light. The camera's bright flash is so quick that the pupil doesn't have time to constrict. The intense light travels through the pupil and illuminates the blood-rich retina at the back of the eye. The camera captures the reflection of this red-coloured tissue, making the pupils appear red in the photo.
7. Besides light levels, what other factors can affect the size of the pupil?
Pupil size is not just a reaction to light. It can also be influenced by the autonomic nervous system in response to other stimuli, including:
- Emotions: Fear, excitement, or surprise can cause the pupils to dilate as part of the 'fight or flight' response.
- Focus: Focusing on a very near object can cause the pupils to constrict slightly.
- Medical substances: Certain drugs and medications can cause either constriction (miosis) or dilation (mydriasis).
- Medical conditions: Brain injuries or nerve damage can lead to abnormal pupil sizes or reactions.
8. Why is the pupillary light reflex considered vital for our vision?
The pupillary light reflex is vital for two main reasons:
- Protection: It shields the highly sensitive retina from photic damage (damage from too much light). By constricting rapidly in bright conditions, it prevents overstimulation and potential harm to photoreceptor cells.
- Adaptation: It optimises vision across a wide range of lighting conditions. By dilating in dim light, it maximizes the amount of light captured, improving image clarity and our ability to see in the dark.
9. What does the size and reactivity of a person's pupils indicate about their health?
Observing the pupils is a critical diagnostic tool in medicine. Normally, pupils are equal in size and react briskly to light. Abnormalities can indicate serious health issues. For example, unequally sized pupils (anisocoria) can be a sign of a stroke, head injury, or nerve problem. Pupils that are 'fixed and dilated' (do not constrict in response to bright light) are a key indicator of severe brain trauma or brain death, as this suggests the brainstem pathway controlling the reflex is no longer functioning.
