Pulmonary Ventilation: Inhalation

Respiration is a rhythmic process of alternating inhalation and exhalation.

During quiet inhalation, the thoracic cavity expands due to a decrease in the gas pressure within the lungs, prompting air to flow in.

The diaphragm is the most important muscle for inhalation. It contracts and descends about 1 cm and flattens, thereby increasing the height of the thoracic cavity and creating a pressure difference of about 1 to 3 mm Hg.

The contraction of the external intercostal muscles lifts the rib cage and pulls the sternum upwards.

The ribs protrude outward, expanding the diameter of the thorax laterally and anteroposteriorly.

Although the thoracic dimensions change by only a few millimeters, it increases the thoracic volume by about 500 ml.

As the thoracic dimensions enlarge, the intrapulmonary volume increases, leading to a decrease in intrapulmonary pressure by about 1 mm Hg below atmospheric pressure.

Air flows into the lungs until the intrapulmonary pressure equals the atmospheric pressure. Concurrently, the intrapleural pressure falls to approximately -6 mm Hg relative to the atmospheric pressure.