Constriction: Definition & Meaning

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  • 0:00 What Is Constriction?
  • 0:52 Constriction vs. Dilation
  • 1:27 Example: Pupils of the Eye
  • 2:20 Example: Blood Vessels
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: John Williams
Constriction is a key regulatory process within the body that is important for proper functioning in many of our physiological systems. Learn more about the process of constriction and when it is utilized within the human body.

What Is Constriction?

The word constriction, in terms of human physiology, means a narrowing or closing of a passageway. This usually refers to structures where materials are allowed to pass. Constriction is an autonomic process, which means you cannot consciously control it. It is used as a method of regulating, or controlling, the passage of materials, fluids, and solids within the body.

So how does it work? Two words: smooth muscle. Smooth muscle is one of three muscle types in the body and is often found in internal organs such as the stomach, bronchi, and bladder. Smooth muscle is also found in several other structures, such as in blood vessels and muscle sphincters. When smooth muscle contracts, it allows for constriction to occur in these organs and structures. This usually happens when a decrease of bodily materials is needed.

Constriction vs. Dilation

Constriction and dilation are opposite, but related, processes in the body. Whereas dilation is an opening or widening of a structure or passageway to increase material flow, constriction leads to a decrease in material flow by closing the passage. Both of these processes are controlled by smooth muscle, so dilation and constriction can usually be seen in the same places within the body, such as in blood vessels, the pupils of the eyes, and other structures. It's important to know that constriction and dilation work together to ensure proper flow of materials within the body.

Example of Constriction: Pupils of the Eye

Imagine that you are at the eye doctor's office. As she is performing her routine checks, she pulls out a pen light and shines it in your eyes. She tells you that your pupils constricted, and therefore, they are functioning properly. What does this mean?

The pupils of the eye are responsible for allowing light to pass through to the receptors of the eye so that you're able to see. However, the amount of light necessary to see a particular image may change depending on the light source. If the source is too bright, it could damage the cells of the eye and cause impaired vision.

To prevent this from happening, the smooth muscles of the eye will constrict when light increases in order to reduce the amount of light that is allowed to enter the eye. By constricting, the pupil allows you the ability to focus on the images in front of you without damage. So, when your doctor shines the light in your eye, she is checking to make sure that your pupils are constricting properly.

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