The pupil of the human eye is a hole in the iris. The pupil’s job is to control the light entering the retina.   

The size of the pupil changes depending on how the stream of light irritates it, whether the eyes cross, whether they are tense or relaxed. In other words, the size of the pupils can tell a lot about the condition. Therefore, it is necessary to pay attention to the size of the pupils when diagnosing any vision problems, as well as when examining for any diseases of the internal systems and organs.

Pupil size is changed by the activity of the muscles of the iris.

By the way, the size of the pupil is never in a constant state, of course, except for the time when a person sleeps. The size of the pupils can change in width if a person is afraid or worried if he is sharply frightened. In other words, the size of the pupils is responsible for obtaining the most complete information about the surrounding reality, in particular about the source of irritation. 

The size of the pupil changes even before the person dies. A decrease in the size of the pupil occurs in a calm state, with depression and depression, in a state of fatigue. Thus, the pupil, as it were, prevents the penetration of unnecessary information interfering with relaxation.

As a person grows up and ages, the size of the pupil undergoes changes in the direction of reducing the circumference, since all the processes occurring in the body become more slow with age. This kind of transformation of pupil size is explained by the degree of brain activity at the bioenergetic level. An unnatural change in the pupil towards constriction indicates that the potential of the brain is reduced.

The correct transformation of the size of the pupil when it is irritated by light is constriction. If the light intensity is reduced, then the pupil begins to dilate. When the axes of view are crossed, the size of the pupils becomes smaller; when diluted, they, on the contrary, expand. 

The adaptation of vision also explains certain transformations in the size of the pupil: when looking at close objects, the pupil narrows, when looking at objects in the distance, it expands. 

Refraction of the eyes also affects the change in the pupil, that is, with myopia, wider pupils are noted than with farsightedness.

In the process of drawing air into the lungs, the pupils dilate, and on exhalation they become narrow again. Even brain activity affects the size of the pupil. For example, if a person mentally draws a night in his imagination, then a signal is sent to the pupils to expand, and vice versa. 

Visual acuity also has an effect on the ability of the pupils to change – as vision deteriorates, the pupils become wider. Blind people have pupils motionless and to the limit.

The size of the pupils of a person’s eyes may indicate that a person is addicted to drugs. And experienced specialists can even reliably establish by the size of the pupils on which particular drug a person is addicted.

Smokers and people who abuse alcohol have small pupils.

How disease affects pupil size

With an increase in the activity of the thyroid gland, the pupils become wider, with an insufficiency of the thyroid gland , the pupils narrow. 

Inflammatory processes of the meninges and high pressure readings inside the skull cause a sharp narrowing of the pupil. At the advanced stage of the disease, the pupils, on the contrary, dilate. If the change in the pupil is noted only in one eye, then the inflammation covers only the corresponding half of the brain.