Respiration can be divided into three parts:
Inspiration
Expiration
Respiratory pause
During the Respiratory Pause, the accumulation of carbon dioxide (CO2) in the arterial blood triggers off the respiratory centre in the medulla, which in turn sends a message to the muscles of respiration, i.e. the intercostal muscles and the diaphragm.
The amount of CO2 in the arterial blood also controls the depth and rate of respiration.
Inspiration: The cavity of the thorax is enlarged by the upward and outward movements of the ribs, caused by the contraction of the intercostal muscles. The lungs in turn stretch because of their elasticity. Air then rushes in to fill the vacuum created. (Atmospheric pressure is greater than the pressure in the lungs.)
Expiration: the intercostal muscles and the diaphragm relax which exerts pressure on the lungs and the air is expelled.
Normal rate of respiration = 16 – 18 breaths per minute.
