Respiratory system-Mechanism of breathing

The mechanism of breathing, also known as ventilation, involves the process of inhaling and exhaling air to facilitate the exchange of gases (oxygen and carbon dioxide) between the body and the environment. The respiratory muscles and the changes in thoracic (chest) cavity volume play a key role in this process. Let’s explore the mechanism of breathing in detail:

**1. Inspiration (Inhalation):

• Muscle Contraction:
  • Diaphragm: Contracts and flattens, increasing the vertical dimensions of the thoracic cavity.
  • External Intercostal Muscles: Contract, elevating the ribs and expanding the chest cavity from front to back and from side to side.
  • These muscle contractions collectively increase the volume of the thoracic cavity.
• Volume Increase and Pressure Decrease:
  • The increase in thoracic cavity volume decreases intrathoracic pressure (pressure within the chest cavity) compared to atmospheric pressure.
  • This pressure gradient causes air to rush into the lungs through the airways (nose, pharynx, trachea, bronchi, and bronchioles).
• Gas Exchange:
  • Oxygen (O2) from inhaled air diffuses across the alveolar membranes into the bloodstream, while carbon dioxide (CO2) diffuses from the blood into the alveoli to be exhaled.

**2. Expiration (Exhalation):

• Muscle Relaxation:
  • Diaphragm: Relaxes and returns to its dome shape due to elasticity.
  • Internal Intercostal Muscles: Contract (in forced expiration) to decrease the ribcage’s width.
• Volume Decrease and Pressure Increase:
  • The reduction in thoracic cavity volume increases intrathoracic pressure, making it higher than atmospheric pressure.
  • This pressure gradient forces air out of the lungs and into the environment through the airways.
• Gas Exchange:
  • Carbon dioxide (CO2) from the bloodstream diffuses into the alveoli to be exhaled.

**3. Analogy – Mechanism of Breathing as a Bellows:

• Imagine the mechanism of breathing as operating a bellows:
  • Inspiration (Inhalation):
    • Pulling the handles of the bellows apart (expanding the chest cavity) causes a vacuum inside (decreased intrathoracic pressure).
    • This vacuum draws air into the bellows (lungs) through the nozzle (airways), allowing for gas exchange (oxygen in, carbon dioxide out).
  • Expiration (Exhalation):
    • Releasing the handles of the bellows (relaxing the muscles) compresses the bellows (reducing thoracic volume).
    • The compression increases the pressure inside the bellows, forcing air out through the nozzle (exhaling).

In this analogy, the bellows represents the thoracic cavity, illustrating how changes in volume and pressure in the thoracic cavity facilitate inhalation and exhalation, similar to how a bellows operates to move air.