- Breathing is a physical process of taking or inhaling the fresh air in and expelling CO2 out.
- It is accomplished through changes in the volume and air pressure of the thoracic cavity.
- It is accomplished by the movement of ribs, internal and external intercostal muscles between each pair of ribs, diaphragm and abdominal muscles.
- Breathing includes two processes:
A) Inspiration or inhalation
It takes place when the volume of the thoracic cavity is increased and the air pressure is decreased. Enlargement of the thoracic cavity involves the following movements which occur simultaneously.
- The external intercostal muscles contract and internal intercostal muscles relax.
- Due to the contraction of external intercostal muscles, the ribs are pulled upward and outward and thereby increase the size of the thoracic cavity.
- The muscles of the dome shaped diaphragm contract. This action straightens and lowers the diaphragm and increases the size of the thoracic cavity from below.
- With the increase in the size of the thoracic cavity, the lungs expand simultaneously.
- As the lungs expand, the air pressure in them drops (it falls below the atmospheric pressure) and results the atmospheric air to rush into lungs through the air passages just to equalize the outer and inner air pressures.
Image source: microbiology notes
B) Expiration or exhalation
- It takes place when the size of the thoracic cavity is reduced and the air pressure is increased.
- The process involves the following movements:
- The internal intercostal muscles contract and external intercostal muscles relax.
- As a result of contraction of internal intercostal muscles, ribs are pulled back to their normal original position.
- Diaphragm relaxes and rises to become its original dome shaped.
- As a result, the size of the thoracic cavity is reduced and lungs become compressed and air pressure raises.
- Due to rise in the air pressure in lungs, air is forced out of the lungs through air passages.
Mechanism of breathing