In terms of human respiration, the act of expelling air, known as exhalation (or expiration), operates on the principle of pressure gradients. During expiration, the pressure within the lungs exceeds that of the surrounding atmosphere. Under normal conditions, quiet breathing involves passive exhalation and is free of muscular contractions. This is because the exhalation process is driven by the natural elastic recoil of the lungs and chest wall, both of which have an inherent tendency to return to their original positions after expansion.

As the muscles involved in inspiration cease their activity, the diaphragm and external intercostals relax. This causes the diaphragm to ascend and the ribs to descend. These movements reduce the thoracic cavity's dimensions, reducing lung volume and increasing alveolar pressure to approximately 762 mm Hg. This pressure difference forces airflow from the lungs (higher pressure) to the atmosphere (lower pressure). Exhalation only requires active effort during heightened physical exertion, such as exercise or when playing a wind instrument. At such times, the muscles involved in exhalation—the abdominal and internal intercostals—engage, causing pressure within the abdominal area and thorax. The contraction of abdominal muscles compresses abdominal organs, pushing the diaphragm upward, while the internal intercostals draw the ribs downward through their inferior and posterior extension between adjacent ribs. Although the pressure within the pleura always remains lower than that within the alveoli, it can momentarily exceed atmospheric pressure during a forceful exhalation, such as a cough.

Some Clinical Conditions Related to Pulmonary Ventilation

1. Neonatal Respiratory Distress Syndrome

Neonatal RDS, or hyaline membrane disease, primarily affects premature infants. This condition arises due to the insufficient production of surfactant in the lungs, a substance that helps keep the alveoli open. Without adequate surfactant, the alveoli collapse during exhalation, making breathing harder for the infant.

Symptoms of RDS include rapid and shallow breathing, flaring of the nostrils, grunting sounds, and a bluish tint to the skin (cyanosis). Treatment often involves providing supplemental oxygen and administering surfactant replacement therapy to improve lung function. Most infants recover from RDS within a few days to weeks with proper medical intervention.

2. Atelectasis

Atelectasis is the partial collapse of a lung portion. This condition can occur for various reasons, such as blocked airways, stab or gunshot wounds to the chest, surgical opening of the chest, , or weakened lung tissue. Common causes include mucus plugs, tumors, foreign objects, and post-surgical complications.

Symptoms of atelectasis depend on the extent of lung collapse and may include chest pain, shortness of breath, increased heart rate, and coughing. Treatment options range from deep breathing exercises and coughing techniques to bronchodilators or surgery in severe cases.

3. Bronchitis

Bronchitis is an inflammation of the bronchial tubes. The condition can be acute or chronic, with viral infections being the most common cause of acute bronchitis. Chronic bronchitis, on the other hand, is often associated with long-term exposure to irritants such as tobacco smoke and air pollution.

Symptoms of bronchitis may include a persistent cough, mucus production, chest discomfort, and wheezing. Treatment typically involves rest, staying hydrated, oxygen, bronchodilators,, and addressing the underlying cause. If a bacterial infection is present, antibiotics may be prescribed.