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 inspirator…
Expiration begins when the inspiratory muscles relax, causing air to move out of the conducting zone to the external environment.
Relaxation of the diaphragm causes the pleural cavity to contract and exert pressure on the lungs.
As lung volume decreases, it compresses the alveoli and increases alveolar pressure.
When alveolar pressure exceeds atmospheric pressure, air flows out.
The pleural cavity provides a frictionless space for the lung to expand and contract during breathing.
In addition, expiration results from the elastic recoil of the lungs and does not involve any muscular contractions.
During labored exhalation, abdominal muscles push forward the diaphragm, causing the thoracic cavity to contract; simultaneously, the internal intercostal muscles lower the ribcage and decrease the thoracic volume.
Expiration can be voluntary or involuntary.
In voluntary expiration, the air in the lungs is held and released at a fixed rate—for example, controlled exhalation during speech.
Conversely, involuntary expiration is vital for metabolic functions and is not in conscious control, like breathing during sleep.
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Q1: What happens to the diaphragm during expiration?
During expiration, the diaphragm relaxes and moves upward, reducing the volume in the thoracic cavity. This relaxation is the primary driver of the initial expiration phase. As the diaphragm moves upward, the pleural cavity contracts and exerts pressure on the lungs, causing lung volume to decrease and air to flow outward.
Q2: How does pressure change in the alveoli during expiration?
As lung volume decreases during expiration, the alveoli are compressed, which increases alveolar pressure. When alveolar pressure exceeds atmospheric pressure outside the body, air flows out of the lungs. This pressure gradient is essential for moving air from the conducting zone to the external environment.
Q3: What role do the pleural cavity and elastic recoil play in expiration?
The pleural cavity provides a frictionless space between the lungs and chest wall, allowing smooth expansion and contraction during breathing. Expiration results from elastic recoil of the lungs, which does not require muscular contractions during quiet breathing. This passive mechanism makes normal expiration an effortless process.
Q4: Which muscles are involved in forced or labored expiration?
During labored exhalation, the abdominal muscles contract to push the diaphragm further upward, intensifying thoracic volume reduction. The internal intercostal muscles lower the ribcage and decrease thoracic volume. These accessory muscles activate during vigorous breathing, singing, or heavy physical exertion like weightlifting.
Q5: What is the difference between voluntary and involuntary expiration?
Voluntary expiration involves conscious control over breathing to regulate airflow, such as during speech or singing. Involuntary expiration is an automatic process that occurs continuously without conscious effort, even during sleep, and is vital for sustaining metabolic functions and life.
Q6: How do the intercostal muscles contribute to expiration?
The intercostal muscles, situated between the ribs, contract during expiration to pull the ribs downward and inward. This action decreases the size of the chest cavity and reduces thoracic volume. During quiet breathing, this muscular effort combines with elastic recoil to facilitate smooth airflow out of the lungs.
Q7: Why is expiration generally considered a passive process?
Expiration is passive during quiet breathing because it relies on elastic recoil of lung tissue and relaxation of inspiratory muscles rather than active muscular contractions. The natural elasticity of the lungs, combined with decreased pleural pressure, drives air out without requiring conscious effort or significant muscle work.