Skip to main content
[ "article:topic", "license:ccbysa", "authorname:burrananddesrochers" ]
Biology LibreTexts

Part 2: Breathing Measurements

  • Page ID
  • During inhalation, volume increases as a result of contraction of the diaphragm, and pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere. Because of this drop in pressure, air rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. The chest wall expands out and away from the lungs. The lungs are elastic; therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original positi  on. The diaphragm also relaxes and moves higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement of air out of the lungs is a passive event. No muscles are contracting to expel the air.

    Breathing Measurements.PNG

    Figure \(\PageIndex{1}\) The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration.


    1. Put one hand on your chest and take three deep inspirations followed by three forced expirations. Describe your observation during each inspiration and expiration.
    2. Repeat Step 1 with your hands on your abdomen. Now try to breathe in and out without any movement of your chest. Describe your observation during each inspiration and expiration.