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How Ventilation Muscles Cause Inspiration and Expiration

  • 0:05 Ventilation:…
  • 0:53 Boyle's Law
  • 1:20 Boyle's Law and Ventilation
  • 2:26 Ventilation Muscles
  • 4:09 Quiet and Forced Ventilation
  • 5:24 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

What is ventilation? It includes both inspiration and expiration, the movement of air into and out of our lungs. In this lesson, learn about how the diaphragm contracts and relaxes and its impact on lung volume.

Ventilation: Inspiration and Expiration

Boyles Law explains the close relationship between volume and pressure
Boyles Law Diagram

Have you ever wondered why we breathe? We breathe for the purpose of getting oxygen into our body and carbon dioxide out of our body. Our cells need oxygen to make the energy-containing molecule ATP. Carbon dioxide is a product of ATP synthesis, and it can be toxic so we got to get rid of it! Ventilation refers to the process of air movement into and out of our lungs, and it includes both inspiration and expiration. Inspiration occurs when lung pressure is decreased below atmospheric pressure, and that causes the air to move into the lungs. Expiration, on the other hand, occurs when lung pressure is increased above atmospheric pressure, and that pushed the air out of the lungs.

Boyle's Law

Gases, including the air we breathe, follow scientific laws. Boyle's Law states that the pressure of a container of gas decreases as the volume of the container increases. In other words, there is an inverse relationship between the volume and the pressure of a gas. As you can see in the animation on the screen, the pressure decreases when the container's volume increases.

Boyle's Law and Ventilation

Now, our lungs can be thought of as containers of gas, and as such, they observe Boyle's Law. Inspiration occurs when our lungs expand. That is, they get bigger, and that increases the volume of the lungs. According to Boyle's Law, lung pressure decreases as a result of the increased volume. The lung pressure (or more specifically, intrapulmonary pressure), becomes less than atmospheric pressure, and that creates a pressure gradient, or a difference between the two pressures. With the mouth and/or the nose open, air moves into the lungs until the pressures equilibrate. Expiration, on the other hand, occurs when the lungs return to a smaller volume. That decreased lung volume increases the intrapulmonary pressure, and that pushes air out of the lungs. Air moves out until equilibrium is achieved between intrapulmonary (that's the pressure inside the lungs) and atmospheric pressure. Then, the ventilation cycle repeats.

Ventilation Muscles

A flattened diaphragm increases volume in the thoracic cavity
Diaphragm Flattens

As stated previously, inspiration occurs as a result of increased lung volume and then decreased intrapulmonary pressure. Okay, then what causes lung volume to increase? The answer is muscular contraction. It's important to note that the lungs are not muscles. Inspiratory muscles are muscles that increase the volume of the thoracic cavity. The major inspiratory muscle is the diaphragm. The diaphragm flattens when it contracts. As seen in the image to the left, a flattened diaphragm increases the volume of the thoracic cavity (that's above the diaphragm). In addition to the diaphragm, external intercostal muscles are muscles located on the outside of our ribs, and they expand the chest. You see, as they contract, they elevate the rib cage, and that expands thoracic volume. According to Boyle's Law, the increased volume decreases pressure, and that results in air movement into the lungs. It's important to note that the lungs are stretched during inspiration. So, think of the lungs as being like a balloon that is stretched when inflated.

Expiration occurs when the diaphragm relaxes. As seen to the right of the screen, the diaphragm domes up, thus decreasing thoracic volume and increasing intrapulmonary pressure. Additionally, the lungs are elastic and will snap back to a smaller volume breath after breath. The elastic recoil of the lungs will help to push the air out.

Quiet and Forced Ventilation

Contraction of abdominal muscles cause forced expiration
Forced Ventilation Body Diagram

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