USMLE Step 1 Respiratory Physiology Review 60 07 Expiration and Muscles of Expiration

USMLE Step 1 Respiratory Physiology Review 60 07 Expiration and Muscles of Expiration

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Play USMLE Audio MP3 60 07 Expiration and Muscles of Expiration Below

Begin 60 07 Expiration and Muscles of Expiration Transcription

Now let’s talk about the muscles of expiration. Exhalation or expiration is caused by a decrease in lung volume. During  eupnoea, expiration is a passive process and the muscles of expiration are not needed.

What is eupnoea?

  • Normal quiet breathing.

During eupnoea, what properties of lung tissues cause a decrease in lung volume? The answer starts with an E.

  •  Elastic properties of lung tissues cause a decrease in lung volume during eupnoea.

We will discuss the elasticity of lung tissue in a later section. After a normal inhalation, the contracting inspiratory muscles relax and the lungs return to their normal or equilibrium volume.

As the lungs return to equilibrium volume, what happens to the pressure of alveolar air? Remember Boyle’s Law.

  • The pressure of alveolar air increases as lung volume decreases.

This difference in pressures between alveolar air and atmospheric air causes air to flow in what direction?

  •       Air flows out of the lungs and into the environment.

Student doctor please pause the tape and summarize what has been discussed about passive expiration.

  •  Exhalation or expiration is caused by a decrease in lung volume. During eupnoea, expiration is a passive process and the muscles of expiration are not needed.  Eupnoea is normal, quiet breathing. Elastic properties of lung tissues cause a decrease in lung volume during eupnoea. After a normal inhalation, the contracting inspiratory muscles relax and the lungs return to their normal or equilibrium volume. In accordance with Boyle’s law, the pressure of alveolar air increases as lung volume decreases. This pressure difference between alveolar air and atmospheric air causes air to flow out of the lungs and into the environment.  The expiratory muscles are needed whenever expiration becomes active rather than passive. Respiratory disease can trigger active expiration.

What is another thing that causes active expiration? I’ll give you a hint. It begins with an E and most of us don’t get enough of it.

  •   Exercise causes active expiration.

Okay what are the muscles of expiration?

  • The main ones are abdominal muscles.

The contraction of what abdominal muscles pulls the lower ribs downward thus decreasing thoracic volume? The three word answer starts with R-A-M.

  • Rectus abdominis muscle.

When abdominal muscles contract, they squeeze the abdomen. This causes the diaphragm to move upward and thoracic volume is decreased.  What is the general term for these abdominal muscles?

  • Abdominal wall muscles.

What are the expiratory muscles in the chest called? The three word answer starts with I-I-M.

  •  The internal intercostal muscles are expiratory muscles located in the chest.

When the internal intercostals contract, the rib cage is compressed.  How does this affect thoracic volume?

  • Thoracic volume is decreased when the internal intercostals contract.

Student doctor please pause the tape and summarize what we have discussed about active expiration and the expiratory muscles.

  • The expiratory muscles are needed whenever expiration becomes active rather than passive. Respiratory disease is one thing that can trigger active expiration. Exercise is another thing that causes active expiration. The main muscles of expiration are abdominal muscles. When the rectus abdominis muscle contracts, it pulls the lower ribs downward and decreases thoracic volume. When the abdominal wall muscles contract, they squeeze the abdomen. This causes the diaphragm to move upward and thoracic volume is decreased. Internal intercostal muscles are expiratory muscles in the chest that compress the rib cage and decrease thoracic volume when they contract.

 

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