Mechanics of Breathing

• Inspiration is the process of taking air into the lungs (inhalation)
• The pressure of air in the lungs must be less than the atmospheric pressure outside the lungs
• Decreasing the pressure of the air in the lungs is achieved by increasing volume of the lungs
• To increase volume of the lungs, diaphragm and intercostal muscles contract
• The diaphragm flattens and the rib cage moves upwards and outwards, increasing the volume of the chest cavity
• Expansion of chest cavity increases lung volume, causing decrease in pressure inside the lungs
• Expiration is the process of breathing out (exhalation) - it occurs passively as elastic recoil of the lungs causes them to return to their resting position.
• During expiration, the diaphragm relaxes and returns to its dome shape, while the intercostal muscles also relax
• This decreases the volume of the thoracic cavity, causing an increase in pressure inside the lungs compared to the atmosphere
• Air rushes from high to low pressure, so air enters the lungs during inspiration
• Rib cage over inwards during exhalation