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Pressure in Fluids

The amount of force acting on a given area. Pressure = Force/Area

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Breathing Muscles

Breathing Muscles


Credit: United States Navy
Source: http://commons.wikimedia.org/wiki/File:US_Navy_081018-N-0773H-085_Chief_Musician_Tia_F._Wortham_plays_bassoon.jpg
License: CC BY-NC 3.0

When you play a wind instrument like this bassoon, your music teacher may tell you to “breathe with your diaphragm.” What is your diaphragm, and what does it have to do with breathing? Boyle’s law explains it.

Why It Matters

  • We can breathe air in and out of our lungs because of Boyle’s law. According to Boyle’s law, if a given amount of gas has a constant temperature, increasing its volume decreases its pressure, and vice-versa.
  • When you inhale, muscles increase the size of your thoracic (chest) cavity and expand your lungs. This increases their volume, so pressure inside the lungs decreases. As a result, outside air rushes into the lungs. That’s because a gas always flows from an area of higher to lower pressure.
  • When you exhale, muscles decrease the size of your chest cavity and squeeze your lungs. This decreases their volume, so pressure inside the lungs increases. As a result, air rushes out of the lungs, flowing from an area of higher to lower pressure.
  • Watch the following animation of breathing to see how muscles control the volume of the lungs as we inhale and exhale: 


Can You Apply It?

Learn more about breathing and Boyle’s law at the link below. Then answer the questions that follow.

  1. What type of relationship exists between the pressure and volume of a given quantity of gas (assuming the temperature of the gas remains constant)?
  2. What muscles control breathing?
  3. What happens when these muscles contract?
  4. What happens when these muscles relax?
  5. Air enters and leaves the lungs until _____ in fluid pressure is reached.

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