Breathing involves the movement of air into and out of the lungs and is carried out with the help of respiratory organs and muscles such as the diaphragm and intercostal muscles. The volume of air exchanged during breathing can be measured using a spirometer. The mechanism of breathing includes two main stages: inspiration (inhalation) and expiration (exhalation). Air movement occurs due to the pressure difference between the lungs and the atmosphere.
A healthy human breathes about 12–16 times per minute. Breathing movements and lung capacity can be measured using a spirometer.
Inspiration
Inspiration is the process of taking air into the lungs. Air passes from the nasal cavity to the pharynx, larynx, trachea, bronchi, bronchioles, and finally reaches the alveoli. In the alveoli, oxygen diffuses into the blood. Inspiration occurs when the pressure inside the lungs is lower than atmospheric pressure. This happens due to:
- Contraction of the diaphragm, which moves downward and flattens
- Contraction of external intercostal muscles, which lift the ribs and sternum
These actions increase the volume of the thoracic cavity, which in turn increases lung volume and decreases intrapulmonary pressure. As a result, air enters the lungs.
Expiration
Expiration is the process of expelling air from the lungs. It occurs when the pressure inside the lungs becomes higher than atmospheric pressure. During expiration:
- The diaphragm relaxes and moves upward
- The intercostal muscles relax, causing the ribs to move downward
This reduces the volume of the thoracic cavity and lungs, increasing intrapulmonary pressure and forcing air out of the lungs. Normal expiration is a passive process, but during forced expiration, abdominal muscles and internal intercostal muscles help expel air more forcefully.
Role of Alveoli in Gas Exchange
Inside the lungs are tiny air sacs called alveoli, where the actual exchange of gases takes place. Oxygen diffuses from the alveoli into the blood and Carbon dioxide diffuses from the blood into the alveoli. This exchange occurs due to differences in concentration (diffusion).
