Breathing is a complex mechanical process involving muscular movement that alters the volume of the thoracic cavity and thereby that of the lung.
• Breathing occurs involuntarily but its rate is controlled by the respiratory centre of brain.
• The space of thoracic cavity increases or decreases by outward and inward movements of the ribs caused by enternal intercostals muscles.
• This action is also assisted by the contraction and expansion of the diaphragm.
• The floor of the thoracic cavity is completely closed by diaphragm. IT is a thin muscular septum separating abdominal and thoracic cavities.
• Inspiration or inhalation is concerned with the taking in of atmospheric air or oxygen into the thoracic cavity.
• Expiration or exhalation is concerned with the expelling of carbon dioxide from lungs. It takes place when the volume of the thoracic cavity decreases and the pressure of the contained air in the thoracic cavity increases.