2. Lung mechanics Flashcards
What are the three different pressure concentrations important in ventilation?
- Atmospheric (barometric) pressure
- Intra-alveolar pressure (intrapulmonary pressure)
- Intrapleural pressure (intrathoracic pressure)
Atmospheric pressure at sea level
760 mm Hg
What happens to atmospheric pressure as altitude increases?
It decreases
What is intra-alveolar pressure?
The pressure of air in alveoli (open system)
Intra-alveolar pressure during inspiration/expiration
–During inspiration = negative (less than atmospheric)
–During expiration = positive (more than atmospheric)
What drives ventilation?
Difference between Palv and Patm drives ventilation
What is intrapleural pressure?
The pressure inside the pleural sac (closed system)
Intrapleural pressure at rest
756 (or -4) mm Hg
What is lower alveolar or intra-alveolar or intra-pleural pressure?
Intrapleural is always lower
Under normal conditions at rest, is intrapleural pressure always positive or negative?
Always negative
Why is there negative intrapleural pressure at rest?
Elasticity in lungs and chest wall
Structures working to create negative intrapleural pressure
– Lungs recoil inward
– Chest wall recoils outward
– Opposing pulls on intrapleural space
– Surface tension of intrapleural fluid hold wall and lungs together (H2O molecules are polar, attract each other)
– Sub-atmospheric P: due to vacuum in the pleural cavity
FRC
Functional Residual Capacity
Functional Residual Capacity (FRC)
Volume of air in lungs between breaths (defined as rest);
Transmural pressure gradient across lung wall =
Intra-alveolar pressure minus intrapleural pressure.
Transmural pressure gradient across thoracic wall =
Atmospheric pressure minus intrapleural pressure
Pneumothorax meaning
Punctured lung
Diagram traumatic pneumothorax
Diagram spontaneous pneumothorax
Air flow =
Mechanics of breathing
• Atmospheric pressure constant (during breathing cycle)
• Therefore, changes in alveolar pressure create/change gradients
• Boyle’s Law: pressure is inversely related to volume in an airtight container (closed system)
• Thus – can change alveolar pressure by changing its volume
• R = resistance to air flow
– Resistance related to radius of airways and mucus
Factors determining intra-alveolar pressure
- Quantity of air in alveoli
- Volume of alveol
Inspiration
- Lungs expand – alveolar volume increases
- Palv decreases
- Pressure gradient: air into lungs
- Quantity of air in alveoli rises
- Palv increases
Expiration
- Lungs recoil – alveolar volume decreases
- Palv increases
- Pressure gradient: air out of lungs
- Quantity of air in alveoli decreases
- Palv decreases
