Pulmonary Cycle: Exhalation

During quiet exhalation, the lungs deflate primarily through elastic recoil of stretched fibers and surface tension from intrapleural fluid between the visceral and parietal pleurae. As inhalation muscles relax, the diaphragm ascends and ribs descend, reducing thoracic cavity dimensions and lung volume. This passive process requires no muscular effort in healthy individuals.

During exhalation, intrapulmonary pressure increases to 1 mm Hg above atmospheric pressure. When lung pressure exceeds atmospheric pressure, gas flows outward. This pressure gradient is fundamental to respiration, as air naturally moves from higher to lower pressure regions, expelling air from the lungs to the atmosphere.

Forceful exhalation involves contraction of abdominal and internal intercostal muscles. The abdominal muscles compress abdominal organs, pushing the diaphragm upward, while internal intercostals draw ribs downward. This active muscular effort expels air forcefully during exercise, coughing, or playing wind instruments.

Neonatal RDS, or hyaline membrane disease, occurs when premature infants lack sufficient surfactant to keep alveoli open. Without adequate surfactant, alveoli collapse during exhalation, making breathing difficult. Symptoms include rapid shallow breathing, nostril flaring, grunting, and cyanosis. Treatment involves supplemental oxygen and surfactant replacement therapy.

Atelectasis is partial lung collapse caused by blocked airways, chest trauma, surgery, or weakened tissue. This condition prevents normal exhalation mechanics, causing chest pain, shortness of breath, increased heart rate, and coughing. Treatment ranges from deep breathing exercises and bronchodilators to surgery in severe cases.

Bronchitis is inflammation of bronchial tubes, commonly caused by viral infections or long-term irritant exposure like tobacco smoke. The condition produces persistent cough, mucus production, chest discomfort, and wheezing, disrupting normal exhalation. Treatment includes rest, hydration, oxygen, bronchodilators, and antibiotics if bacterial infection is present.

Quiet exhalation is passive because it relies on elastic recoil and surface tension rather than active muscle contraction. When inhalation muscles relax, the diaphragm and external intercostals naturally return to resting positions, reducing thoracic cavity volume. This automatic recoil mechanism requires no energy expenditure in healthy individuals during normal breathing.