The Mechanics of Ventilation in Human Lungs
Understanding Ventilation Basics
Ventilation is the mechanical process of moving air in and out of the lungs, commonly known as breathing. This process involves two main phases:
- Inspiration (breathing in)
- Expiration (breathing out)
Both phases rely on pressure differences between the atmosphere and the lungs, following the principle that air moves from areas of high pressure to areas of low pressure.
The Role of the Thoracic Cavity
The thoracic cavity houses our lungs and is surrounded by several key structures:
- Ribcage
- Intercostal muscles (external and internal)
- Diaphragm
- Pleural membranes (visceral and parietal)
Mechanics of Inspiration
During inspiration:
- The diaphragm contracts and flattens
- External intercostal muscles contract
- Ribs move upward and outward
- Thoracic cavity volume increases
- Lung volume increases
- Intrapulmonary pressure decreases below atmospheric pressure
- Air rushes into the lungs
Think of inspiration like pulling open an accordion - as you pull, the volume increases and air rushes in to fill the space.
Mechanics of Expiration
During normal, quiet expiration:
- Diaphragm relaxes and dome-shapes upward
- External intercostal muscles relax
- Ribs move downward and inward
- Thoracic cavity volume decreases
- Lung volume decreases
- Intrapulmonary pressure increases above atmospheric pressure
- Air is pushed out of the lungs
During forced expiration, the internal intercostal muscles and abdominal muscles actively contract to force more air out.
Pressure Changes During Ventilation
Key pressure measurements:
- Atmospheric pressure: 760 mmHg
- Intrapulmonary pressure during rest:
- Inspiration: approximately 758 mmHg
- Expiration: approximately 762 mmHg
Students often forget that it's the pressure difference, not the absolute pressure, that drives ventilation.
