It involves inspiration and expiration:
It is a process by which fresh air enters the lungs. The diaphragm, intercostal muscles and abdominal muscles play an important role.
(i) Diaphragm-. The diaphragm becomes flat and gets lowered by the contraction of its muscle fibres thereby increasing the volume of the thoracic cavity in length.
(ii) External Intercostal Muscles: They occur between the ribs. These muscles contract and pull the ribs and sternum upward and outward thus increasing the volume of the throacic cavity.
(iii) Abdominal Muscles: These muscles relax and allow compression of abdominal organs by the diaphragm. Abdominal muscles play a passive role in inspiration.
The muscles of the diaphragm and external intercostal muscles are principal muscles of inspiration.
Movement of Fresh Air into the Lungs: Thus overall volume of the thoracic cavity increases and as a result there is a decrease of the air pressure in the lungs. The greater pressure outside the body now causes air to flow rapidly into external nares (nostrils) and through nasal chambers into internal nares. Thereafter the sequence air flow is like this: External nares Nasal chambers internal nares Pharynx Glottis Larynx trachea Bronchi bronchioles alveolar ducts alveoli.
It is a process by which the foul air (carbon dioxide) is expelled out from the lungs. Expiration is a passive process which occurs as follows:
(i) Diaphragm: The muscle fibres of the diaphragm relax making it convex, decreasing volume of the thoracic cavity.
(ii) Internal Intercostal Muscles'. These muscles contract so that they pull the ribs down ward and inward the lower ribs and decreasing the size of the thoracic cavity.
(iii) Abdominal Muscles: Contraction of the abdominal muscles such as external and internal oblique muscles compresses the abdomen and pushes its contents (viscera) towards the diaphragm.
The internal intercostal and abdominal muscles are muscles of expiration.
Movement of Foul Air out of the lungs: Thus overall volume of the thoracic cavity decreases and foul air goes outside from the cavities of the alveoli in the following manner: Alveoli -» alveolar ducts -> bronchioles -> trachea -> larnyx -> glottis pharynx -» internal nares nasal chambers -» external nares -» outside.