Mechanics of Breathing

Breathing Cycle

Breathing, or ventilation, consists of two stages: inhalation (breathing in) and exhalation (breathing out).
During inhalation, the diaphragm and the intercostal muscles contract. This increases the size of the thoracic cavity, decreasing the internal air pressure and drawing air into the lungs.
Exhalation, on the other hand, is a generally passive process. It occurs when the diaphragm and intercostal muscles relax, reducing the size of the thoracic cavity, increasing the internal air pressure and expelling air from the lungs.

The diaphragm is a dome-shaped muscle situated at the base of the lungs. Its contraction and relaxation are key to the breathing process.
When the diaphragm contracts, it flattens and lowers, increasing the volume of the thoracic cavity and reducing the pressure within it, resulting in inhalation.
During relaxation, the diaphragm returns to its dome shape, decreasing the volume of the thoracic cavity and increasing the pressure, which leads to exhalation.

The intercostal muscles, situated between the ribs, also play a crucial role in the breathing cycle.
These muscles consist of external and internal layers. The external intercostal muscles aid in inhalation, while the internal intercostal muscles aid in exhalation.
During inhalation, the external intercostal muscles contract, pulling the ribs upward and outward which further increases the volume of the chest cavity.
During exhalalation, the internal intercostal muscles relax, allowing the ribs and chest to lower and helps push the air out.

Boyle’s Law explains the relationship between the pressure and volume of a gas: if the volume of a gas increases, the pressure decreases, and vice versa.
During inhalation, the expansion of the thoracic cavity decreases the internal air pressure allowing high-pressure air from the environment to rush into the lungs.
Conversely, during exhalation, the compression of the thoracic cavity increases the internal air pressure, forcing the low-pressure air out of the lungs.

The lungs are fully enclosed by two layers of protective pleural membranes, filled with a thin layer of pleural fluid.
These membranes and the fluid allow the lungs to move smoothly against the chest wall during the breathing cycle.
The fluid also creates a pressure gradient that assists lung expansion.