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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.

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Ventilation: Inspiration and Expiration

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 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 pushes the air out of the lungs.

Boyle’s Law

The close relationship between volume and pressure
Boyles Law Diagram

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: 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

The diaphragm
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 on the left side of the image above, 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 on the right side of the image above, 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

Overall, ventilation requires muscular contraction. Let’s break down ventilation and see exactly where muscular contraction is required. Quiet inspiration is inspiration under resting conditions. Quiet inspiration requires muscular contraction and is, therefore, an active process. Similarly, quiet expiration is expiration under resting conditions.

However, quiet expiration is a passive process, as it requires no muscular contraction.

Contraction of abdominal muscles causes forced expiration
Forced Ventilation Body Diagram

Ventilation becomes forced under stressful conditions such as with exercise. Forced inspiration and expiration require muscular contraction, and therefore, they’re both active processes. Additional muscles are recruited to help with forced ventilation. For example, our abdominal muscles contract during forced expiration (that helps to push the air out of our lungs). Contraction of the abdominal muscles compresses the abdominal contents and pushes up on the diaphragm, and that further decreases the thoracic volume, increasing thoracic pressure and pushing the air out.

Lesson Summary

In summary, ventilation includes both inspiration, in which air moves in, and expiration, in which air moves out of the lungs. Ventilation is an active example of Boyle’s Law, which states that the pressure of a container of gas decreases as the volume of that container increases. Inspiration occurs when intrapulmonary pressure falls below atmospheric pressure, and air moves into the lungs. Intrapulmonary pressure falls below atmospheric pressure when the diaphragm contracts and increases the thoracic volume. The diaphragm is the major inspiratory muscle.Inspiration continues until the intrapulmonary pressure equilibrates with atmospheric pressure. Expiration occurs when intrapulmonary pressure is increased above atmospheric pressure.

After the diaphragm contracts, it relaxes, thus decreasing thoracic volume and increasing intrapulmonary pressure. Under resting conditions, expiration is passive, as no muscular contraction is needed. Expiration becomes active with stress, as muscular contraction is required to force out additional air.

Learning Outcome

At the end of this lesson, you will be able to:

  • Explain ventilation in terms of Boyle’s Law
  • Describe what occurs during inspiration and expiration
  • Differentiate between passive and active expiration and list the muscles involved

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