Final answer:
In normal infants, during inhalation, the diaphragm and intercostal muscles contract, expanding the chest wall and lowering intrapleural pressure to draw air in. Exhalation is passive, where the lungs recoil and the diaphragm rises, expelling air. Pulmonary surfactant is essential for reducing alveolar surface tension and proper lung inflation.
Step-by-step explanation:
In normal infants, thoracic wall compliance is crucial for the breathing process. During inhalation, the contraction of the diaphragm and the intercostal muscles cause the chest wall to expand out and away from the lungs. The lungs, being elastic, fill with air and exert pressure back toward the interior of the lungs, contributing to the lung's inflation. The intrapleural pressure drops as the thoracic cavity volume increases, creating a pressure gradient that allows air to be drawn into the airways.
Upon exhalation, the elastic recoil of the lungs helps to push air out as the intercostal muscles relax and the diaphragm moves upward, increasing thoracic pressure relative to the external environment, enabling air to flow out of the lungs. This process, described by Boyle's Law, is a passive event with no muscles actively contracting to expel the air.
During birth, significant changes occur as the fetal chest wall is compressed to expel fluid from the lungs and the first inhalation acts to inflate the lungs. The presence of pulmonary surfactant is critical for lung function, especially in reducing the surface tension within the alveoli for proper gas exchange.