13.5 : Mechanism of Breathing II: Expiration

The Physiology of Expiration: A Seamless Respiratory Process

Expiration, or exhaling, is a complex physiological process that begins as the inspiratory muscles begin to relax. This relaxation triggers a series of events that epitomize the efficiency of the respiratory system.

Mechanism of Expiration:

1. Relaxation of the Diaphragm: As the diaphragm muscle, a key player in this process, relaxes, it moves upwards, reducing volume in the thoracic cavity. This critical muscle's relaxation is the primary driver in the initial phase of expiration.
2. Increase in Pleural Pressure: The pleural cavity contracts after the diaphragm relaxes. This contraction exerts pressure on the lungs, causing a decrease in lung volume. The intercostal muscles, situated between the ribs, also contract during expiration. This action pulls the ribs downward and inward, further decreasing the size of the chest cavity.
3. Compression of the Alveoli: As the lung volume decreases, the alveoli are compressed, which increases alveolar pressure. This pressure becomes higher than the atmospheric pressure outside the body.
4. Airflow Out of the Lungs: The higher internal pressure drives air out of the lungs, facilitating a smooth and effortless outflow.

The pleural cavity, a space between the lungs and the chest wall, acts like a finely tuned stage, accommodating the rhythmic expansion and contraction of the lungs during the respiratory cycle. Muscular effort and the natural elastic recoil of the lung tissue also facilitate exhalation, aiding in expelling air from the lungs.

Active Expiration: During vigorous breathing or activities requiring forced exhalation, such as singing, playing wind instruments, or heavy physical exertion like weightlifting, additional muscles come into play:

• Abdominal Muscles: These muscles contract to push the diaphragm further upwards, intensifying the reduction in thoracic volume.
• Internal Intercostal Muscles: These muscles help lower the rib cage, further decreasing the thoracic volume and aiding in more forceful expiration.

Voluntary vs. Involuntary Expiration: Understanding this distinction is crucial to grasp the full complexity of the respiratory process.

• Voluntary Expiration: This type of expiration, such as during speech or singing, involves conscious control over breathing to regulate airflow precisely.
• Involuntary Expiration: This automatic process is crucial for sustaining life, functioning continuously without conscious effort, even during sleep.