Mechanics of breathing, pressure and work Flashcards
How does inspiration/expiration work?
- Lungs and chest wall elastic structures
- Lungs are stretched when we inhale (‘stretchiness’ is termed lung compliance)
- They recoil on exhalation (Term: elastic recoil)
- This is balanced by the chest wall tendency to recoil in opposite direction
- At end of quiet expiration pressures balance
What are the inspiratory muscles?
- Diaphragm (75% change in volume)
- External intercostals
- bucket-handle
- Accessory muscles
- scalene
- sternomastoids
How does inspiration work?
- Contraction of inspiratory muscles increases intrathoracic volume
- This causes a decrease in intrapleural pressure (usually -2.5 -6 mmhg)
- Lungs are pulled into more expanded position and the pressure in the airways becomes negative. Air moves in (Patm>Palv)
- At end of inspiration pressures are equal
- Recoil of lungs and chest wall then occur
Which part of the graph is inhalation and which part is exhalation?
What is transpulmonary pressure?
- The chest wall exerts a distending pressure on the pleural space, which is transmitted to the alveoli to increase its volume, lower its pressure, and generate airflow inwards
- This distending pressure is called the trans pulmonary pressure (Ptp).
What anatomical component is responsible for chest expansion?
Muscles
Under physiological conditions:
The Ptp is always _______
The Ppl is always ________
Positive
Negative
What is Ppl?
Note: Ppl is variably called pleural or intrapleural pressure
What is Ptp?
Transpulmonary pressure
What is the relationship between transpulmonary pressure and elastic recoil pressure?
For a given lung volume, the transpulmonary pressure is equal and opposite to the elastic recoil pressure of the lung.
Label which line is Total, Lung and Chest Wall Compliance
What graph is this?
Lung Compliance (pressure volume curve)
Label each line
What does the compliance of the lung depend on?
The compliance of the lung depends how inflated or not it is
What is hysteresis?
The compliance curves are different for inspiration and exhalation. This difference is called hysteresis (frictional resistance changes)
Is the lung more or less compliant at higher volumes?
lung is less compliant at higher volumes
What does each picture show?
What is compliance like in normal lung?
Compliance is just right. Good compliance for low work of inhalation, and good retention of elasticity of alveolar units to allow effective exhalation.
What is compliance like in interstitial fibrosis?
Compliance is decreased due to more stiff alveolar walls from scarring (called fibrosis).
What is compliance like in emphysema?
Compliance is increased due to loss of alveolar interdependence.
What does a P/V curve in emphysema show?
In emphysema the P-V curve demonstrates lungs with increased compliance
‘Loss of elastic recoil à easy to inflate, but difficult to exhale’
What does a P/V curve in pulmonary fibrosis show?
In pulmonary fibrosis the P-V curve demonstrates stiff lungs
‘Increase in elastic recoil -> difficult to inflate the lungs’
Complete the P/V curve labels
How does exhalation work?
- Exhalation occurs when the distending pressure is released
- Built up potential in the form of increased elastic recoil -> passive relaxation of alveoli -> decrease in alveolar volume -> increase in Palv (to be > Patm) -> outward air flow
When does active exhalation occur?
Active exhalation occurs when expiratory respiratory muscles are engaged, but short of exercise and disease, we don’t usually need to call upon these
What impairs efficient and effective exhalation?
Loss of elastic recoil
What is surface tension?
- Cohesive forces between molecules
- Molecules on the surface have no atoms above them
- Results in stronger attractive forces on nearest neighbours on the surface
How do alveoli collapse?
- Liquid surface area becomes as small as possible i.e. sphere - liquid tends to form spheres because of surface tension pressures
- Tends to collapse the alveolus
What 2 things does surface tension increase with?
- Emphysema
- Age
How is surfactant produced?
Type II alveolar cells extract fatty acids from blood and synthesise surfactant
What is surfactant made of?
Major component is dipalmitoyl phosphatidylcholine (DPPC)
What does surfactant cause?
Surfactant causes surface tension be low, and also to vary across the respiratory cycle
Which alveolus has higher pressure?
Which alveolus is more likely to collapse?
What is Laplace’s law?
It tells us that pressure in a spherical compartment is:
(1) proportional to the Tension and,
(2) inversely proportional to the radius of that sphere
What are the effects of surfactant on an alveolus?
Surfactant causes surface tension (which contributes to alveolar wall tension, along with elastic recoil), to be lower in smaller spheres
Which patients have surfactant deficiency and what does this cause?
Premature babies (< 30 weeks) have surfactant deficiency and are at risk for infant/neonatal respiratory distress syndrome from “alveolar collapse” due to high surface tension
Why is surfactant important?
- Increases lung compliance because surface forces are reduced
- Promotes alveolar stability
- Prevents alveolar collapse
- Small alveoli are prevented from getting smaller
- Large alveoli are prevented from getting bigger
•Surface tension tends to suck fluid from capillaries into alveoli
- Reduction of surface tension reduces hydrostatic pressure in tissue outside capillaries and keeps lungs dry
What 2 forces need to be overcome to inflate the lungs?
Force of elastic recoil
Force of airway resistance
What is airway resistance?
Is the pressure difference between the alveoli and mouth, divided by the flow rate
What is the equation for airway resistance?
Resistance = Pressure1-Pressure2 / Flow
(V=IR) (analogous to ohms law)
Where does airway resistance originate from?
Airway resistance originates from friction between air and mucosa
What is pulmonary resistance?
Pulmonary resistance = tissue + airway
Tissue forces are lung and chest wall sliding over each other (~ 20% of total)
Is there an increase or decrease in cross sectional area in small airways?
Massive increase in cross section in small airways ‘trumpet’
What is laminar flow and how is resistance generated?
Laminar flow is smooth flow. Resistance generated is proportional to the radius (r4).
What is turbulent flow and how is resistance generated?
Turbulent flow is irregular, chaotic, with eddie currents. It’s good for transferring heat (radiators try to enhance turbulent flow), but the resistance is high
Label the types of flow
What is the Reynold’s number?
The “Reynold’s number” helps predicts when laminar flow converts to turbulent flow
What is this?
Re = 2 (length) (density of gas) (velocity of gas)
viscosity of gas
The “Reynold’s number” helps predicts when laminar flow converts to turbulent flow
How is resistance worked out?
-Resistance is described by the law equating resistance and radius
(Hagen-Poiseuille equation): R = (8 n l) / (P r4)
- Resistance is inversely proportional to 4th power of radius
- Flow is inversely proportional to the viscosity of a fluid
Draw a graph showing airway resistance through different levels of the respiratory tree
How is a generations (level of respiratory tree) resistance worked out?
All airways in a generation contribute to that generation’s total resistance (Rt)
What are the factors affecting airway resistance?
- Inflammation
- Mucus
- Bronchodilators
- Steroids
- Gas density
- Heliox (lower Re)
- Diving
What is work of breathing?
Work required to stretch elastic tissues of chest wall and lungs , moving inelastic tissues and air through tubes
It is the amount of energy or O2 consumption needed by the respiratory muscles to produce enough ventilation and respiration to meet the metabolic demands of the body.
How does elastic work lead to greater work of breathing?
- Decreased elasticity in restrictive diseases
- Greater wOB
How does non-elastic lead to greater work of breathing?
Obstructive diseases lead to greater WOB to overcome increased airway resistance
What does failure of work of breathing lead to?
Ventilatory support
