Internal & External Stimuli: Definition & Examples

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  • 0:02 Homeostasis & Stimuli
  • 1:24 Internal vs External
  • 3:12 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

What are internal and external stimuli? Learn how the body uses messages from outside and inside of you to maintain perfect balance and keep you healthy and safe.

Homeostasis & Stimuli

People often talk about the balance of nature and how everything has to stay in harmony. But your body is also part of nature, and it has its own balance. We're not talking about staying upright or balancing on a ledge. No, this is a different kind of balance. This kind of balance is called homeostasis.

Homeostasis is the property of biological systems where things are controlled to keep internal conditions stable and unchanging. For example, your body must maintain a temperature of 98.6°F, your blood's oxygen saturation needs to stay between 95% and 100%, and your blood has to sustain a pH between 7.35 and 7.45. If these things change even slightly, they can cause major problems inside the body. Even an increase in temperature of 1°F is considered elevated, and it only takes a temperature of 100.4°F to be considered a fever. Your body also needs to stay safe and protect itself to maintain homeostasis.

The body maintains homeostasis by responding to stimuli. These complex responses rapidly return the body to its usual state. If they didn't, we would be constantly getting fevers or otherwise feeling sick. Let's take a look at how the body responds to stimuli that might affect its homeostasis.

Internal vs External Stimuli

There are two types of stimuli that affect the body: external and internal. External stimuli are changes to conditions outside of the body, or in general, information from outside the body that our senses detect. For example, our bodies respond to changes in light and temperature and to sources of danger. Light affects our bodies' circadian rhythms, which cause us to feel sleepy or awake at the correct times. Temperature causes our bodies to rapidly find ways to heat up or cool down. It does this by sweating, shivering, increasing or decreasing the size of the blood vessels near the surface of the skin, and creating goosebumps to reposition hair. And danger causes a release of hormones, which create either a fight or flight response: to stay and fight the danger or to flee to safety.

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