Pulmonary Ventilation: Inhalation

The diaphragm is the most important muscle for inhalation. It contracts and descends approximately 1 cm while flattening, which increases the height of the thoracic cavity. This action creates a pressure difference of about 1 to 3 mm Hg, enabling air to flow into the lungs during quiet breathing.

The external intercostal muscles contract during inhalation, lifting the rib cage and pulling the sternum upward. This action causes the ribs to protrude outward, expanding the thorax laterally and anteroposteriorly. Although thoracic dimensions change by only a few millimeters, this expansion increases thoracic volume by approximately 500 ml.

During inhalation, intrapulmonary pressure decreases by about 1 mm Hg below atmospheric pressure. This pressure drop creates a gradient that allows air to flow into the lungs until intrapulmonary pressure equals atmospheric pressure. Understanding pressure relationships in thoracic cavity is essential for comprehending how ventilation occurs.

Tidal volume (TV) refers to the volume of air inspired or expired during normal breathing without conscious effort. During quiet inhalation, the coordinated contraction of the diaphragm and external intercostal muscles increases thoracic volume by approximately 500 ml, which represents the typical tidal volume during resting respiration.

Intrapleural pressure is the pressure within the pleural cavity, the space between the parietal and visceral pleurae. During inhalation, intrapleural pressure falls to approximately -6 mm Hg relative to atmospheric pressure. This negative pressure is crucial because it keeps the lungs expanded against the chest wall, enabling proper ventilation.

As the diaphragm contracts and external intercostal muscles engage, the thoracic cavity expands in multiple directions, increasing intrapulmonary volume. This expansion lowers the pressure inside the lungs below atmospheric pressure, creating a pressure differential. Air then flows into the lungs, filling the alveoli for gas exchange until pressures equalize.

During forced inhalation, accessory muscles supplement the diaphragm and external intercostal muscles. The sternocleidomastoid muscle in the neck elevates the sternum, the scalene muscles lift the upper ribs, and the pectoralis minor muscle raises the ribs. These muscles work together to maximize chest cavity expansion during periods of increased oxygen demand.