17. Mechanics of Breathing Flashcards
What is Boyle’s Law?
When the temperature of a gas is constant, the pressure of the gas varies inversely with volume of the gas container.
Pressure (P) is inversely proportional to volume (V).
P = 1/V
What is Pulmonary Ventilation?
Inspiration and expiration of air between atmosphere and lungs (alveoli).
What is External (pulmonary) Respiration?
Exchange of gases between alveoli and blood in pulmonary capillaries.
- Blood gains oxygen and loses carbon dioxide.
What is the Internal (tissue) respiration?
Exchange of gases between blood in systemic capillaries and tissue cells.
Blood loses oxygen and gains carbon dioxide. Carbon dioxide is generated from cellular respiration.
Describe the mechanics of inhaling.
- Diaphragm and external intercostals contract
- Rib cage rises
- Thoracic cavity volume increases
- Intrapulmonary volume increases
- Intrapulmonary pressure decreases
- Atmospheric pressure is greater than intrapulmonary pressure. Air enters lungs down pressure gradient until intrapulmonary pressure = atmospheric pressure.
Describe the mechanics of exhaling.
- Diaphragm and external intercostals relax.
- Rib cage falls
- Thoracic cavity volume decreases
- Intrapulmonary volume decreases
- Intrapulmonary pressure increases
- Intrapulmonary pressure is greater than atmospheric pressure. Air leaves lungs down pressure gradient until intrapulmonary pressure = atmospheric pressure.
How is the Thoracic Capacity Increased?
- Flattened diaphragm
- Contraction of the external intercostal muscles
- Increasing the anteroposterior diameter
- Contraction of the pectoralis major and minor muscles
- Surface tension of the plural fluid
What is Intrapleural pressure?
- Pressure within the pleural cavity
- Always lower than atmospheric and intrapulmonary pressures
- Created by elastic recoil of the lungs
- 5 cm H20 at rest
- This pressure can be undermined by a pneumothorax
- Prevents lungs from collapsing
What is Intrapulmonary Pressure?
- Pressure inside the respiratory tract
- Pressure within the alveoli
- Increases when exhaling
- Decreases when inhaling
What is Transpulmonary Pressure?
- 5 cm H2O at rest
- Difference between respiratory tract pressure and pleural cavity pressure
- A pressure that is always positive throughout the normal breathing cycle
- Elastic recoil pressure of the lung
Increasing when inhaling
Describe Surface tension.
- Caused by intermolecular forces between molecules in a liquid
- Air-fluid interface surface of fluid is under tension like a thin membrane being stretched
○ like the thin fluid layer between the alveolar cells and the air
Describe Laplace’s Law.
- Describes the relationship between Pressure (P), surface tension (T) and the radius (r) of an alveolus (bubble)
At equilibrium, the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse it.
How does pulmonary surfactant affect surface tension?
Pulmonary surfactant greatly reduces surface tension increasing compliance.
It equalises the pressure differences between small and large alveoli.
What is the purpose of surfactant in the alveoli?
Surfactant helps keep uniform alveolar size
- More concentrated in smaller alveoli (per mm s. area)
- Lower surface tension helps equalise pressure among alveoli of different sizes
- Easier to inflate smaller alveoli
- Work needed to expand alveoli with each breath greatly reduced
What is Neonatal respiratory distress syndrome?
- Lack of surfactant secretion in premature babies (28-32 weeks gestation)
- Reduced compliance
- Alveoli collapse on exhalation
- Difficult to inflate lungs
- 50% die without rapid treatment
