Session 2-Ventilation and Lung Mechanics Flashcards
Which two forces is the lung subjected to at rest?
- lung’s elasticity and surface tension favouring small lung volume
- muscles of rib cage have elasticity and these favour outward movement of chest wall
What is the net effect of the two forces acting on the lung?
Balance each other and create negative pressure within the intrapleural space relative to atmosphere
Describe the mechanism of quiet inspiration
- diaphragm contracts and moves down
- external intercostals contract and elevate ribs
- thoracic cavity expands so pressure inside lungs falls below atmospheric pressure
- air flows in
Describe the mechanism of quiet expiration
- muscles relax
- elastic recoil of lungs -> thoracic cavity and lung return to original equilibrium position and resting end-expiratory level
Describe the mechanism of forced inspiration and the accessory muscles involved
- maximum increase in capacity of thoracic cavity and every muscle that can raise ribs is bought into action
- SCM, scalene, pectoralis minor and major and trapezius
Describe the mechanism of forced expiration and the accessory muscles involved
-active process bought on by forced contraction of muscles of anterior abdominal wall and internal intercostals
What happens if the pleural seal is broken?
Ipsilateral lung shrinks down
Describe the pleural pressure at rest
Negative because of opposing forces of lungs and chest wall
Describe the pleural pressure during inspiration
More negative than at rest due to expansion of thorax and returns to resting at the end of quiet expiration
How does pneumothorax occur?
When air or gas is present within the pleural cavity -> removal of surface tension -> reduction of lung expansion
How does pneumothorax result in lung collapse?
Loss of pleural cavity means that lungs and chest wall are no longer attached so the lung’s natural elastic recoil leads to a collapsed lung
What is compliance?
Stretchiness of the lungs; relationship between pressure and volume
What is the equation for compliance?
compliance = change in volume / change in pressure
What are the factors affecting compliance?
- elastin, reduced in ageing so lungs become slacker, also in emphysema
- fibrosis (disease) - lungs become stiffer
- surface tension
True or false: surface tension makes inflation of alveoli easier
FALSE - makes it harder
What is surfactant?
Surface active agent - mixture of lipids and proteins secreted by alveolar cells
What does surfactant do?
- Reduces surface tension and therefore increases lung compliance
- stabilises lung by preventing small alveoli collapsing into larger ones
How does surfactant work?
Hydrophilic ends lie in alveolar fluid and hydrophobic end projects into alveolar gas so they float on the surface between molecules, disrupting the interaction between surface molecules and therefore reducing surface tension
Where is the main site of resistance in the respiratory tract and why?
Upper respiratory tract because total cross-sectional area of bronchioles > trachea
What are the factors affecting airway resistance?
- increased mucus
- hypertrophy of smooth muscle
- oedema
- loss of radial traction
True or false: airway resistance increases as lung volume decreases
TRUE
Define tidal volume
volume of air which enters and leaves the lungs with each breath
Define inspiratory reserve volume
maximal amount of additional air that can be drawn into lungs
Define expiratory reserve volume
additional amount of air that can be expired from the lungs
Define residual volume
volume of air still remaining in the lungs after the most forcible expiration
Define inspiratory capacity
From end of quiet expiration to maximum inspiration (inspiratory reserve + tidal volume)
Define functional residual capacity
Volume of air in lungs at the end of quiet expiration
Define vital capacity
Inspiratory capacity + expiratory reserve OR
inspiratory reserve volume + tidal volume + expiratory reserve volume
Define total lung volume
Vital capacity + reserve volume
Define anatomical dead space
Upper resp tract between mouth and resp bronchioles
Define alveolar dead space
Where alveoli are ventilated but not perfused, or very poorly perfused
Define physiological dead space
Anatomical and alveolar dead space
How is pulmonary ventilation calculated?
Tidal volume x resp rate
How is alveolar ventilation calculated?
(Tidal volume - dead space) x resp rate
What is respiratory distress syndrome in newborns?
Surfactant is absent from alveoli and without it, surface tension of the alveolar sacs is high -> increased tendency of alveoli to collapse
What are the signs of respiratory distress syndrome in newborns?
Cyanosis, grunting, intercostal and subcostal recession
How is respiratory distress syndrome in newborns treated?
Surfactant replacement and supportive (O2 and assisted ventilation)
Why do newborn’s with respiratory distress syndrome get intercostal recession?
Baby’s chest wall is more pliable and less compliant so doesn’t have strength to resist the recoil of the lung
Describe the lung compliance in fibrosis
Low (stiffer)
Describe the lung compliance in emphysema
High (slacker)
