Theme 4: Lecture 1 - Mechanics of breathing Flashcards
Compliance
Stretchiness of lungs
What is elastic recoil balanced by
Chest wall tendency to recoil in the opposite direction
What happens regarding pressures at the end of quiet expiration
The pressures balance
What are the inspiratory muscles
- Diphragm (75% of change in volume)
- External intercostals
- Accessory muscles (Scalene and sternomastoids)
Describe the process of inspiration
- 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
Is inspiration an active or passive process
Active
Is expiration an active or passive process
Generally passive
Trans pulmonary 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
- It is the difference between the alveolar pressure and the intrapleural pressure in the pleural cavity
- This distending pressure is called the trans pulmonary pressure (Ptp).
- Chest wall expansion is done by muscles
Relationship between transpulmonary pressure and elastic reocoil
For a given lung volume, the transpulmonary pressure is equal and opposite to the elastic recoil pressure of the lung.
Elastic recoil
The recoil on exhalation
Tidal volume (TV)
Normal breathing
Functional residual capacity (FRC)
The volume remaining in the lungs after a normal passive exhalation
Expiratory reserve volume (ERV)
the volume of extra air, above-normal volume, exhaled during a forceful breath out
Inspiratory capacity (IC)
The amount of air that can be inspired in a deep breath in
Vital capacity (VC)
- the maximum amount of air a person can expel from the lungs after a maximum inhalation.
- Inspiratory reserve volume plus tidal capacity plus expiratory reserve volume
Residual volume (RV)
The volume of air left in the lung after a forced expiration
Total lung capacity (TLC)
the volume of air in the lungs upon the maximum effort of inspiration
Compliance
- Static measure of lung “stretchiness” (lung and chest recoil)
- Volume change per unit pressure change
How is compliance different to resistance
Compliance is a static measure whereas resistance is dynamic, accounting for airflow resistance
What is the balance point of compliance
- Where the lung and chest are in equilibrium after exhaling
- Functional residual capacity
Why do lungs always tend to collapse
Transpulmonary pressure is positive from residual volume to total lung capacity so the lungs always tend to collapse
What is positive pressure
- a pressure within a system that is greater than the environment that surrounds that system
- therefore if there is a leak, gas will leak out into the surrounding environment
What is negative pressure
- a pressure within a system that is lower than the environment that surrounds the system
- Therefore if there is a leak, gas will get sucked into the system
Describe the lungs like a spring
The lungs are like a spring that can only be stretched
Describe the chest wall like a spring
The chest wall is like a spring which can be compressed or distended
Why does the chest wall tend to collapse
Transthoracic pressure is negative at residual volume and functional residual volume so the chest wall tends to spring out
What is transthoracic pressure
- The transthoracic pressure gradient is the difference between the pressure in the pleural space and the pressure at the body surface
- represents the total pressure required to expand or contract the lungs and chest wall.
What does lung compliance depend on
- How inflated the lungs are
- Lungs are less compliant at higher volumes
What is the difference in compliance curves for inspiration and expiration called
Hysteresis
Describe lung compliance in emphysema
- In emphysema the P-V curve demonstrates lungs with increased compliance
- ‘Loss of elastic recoil therefore easy to inflate, but difficult to exhale’
Describe lung compliance in pulmonary fibrosis
- In pulmonary fibrosis the P-V curve demonstrates stiff lungs
- ‘Increase in elastic recoil therefore difficult to inflate the lungs’
Compliance equation
change in volume divided by change in pressure
Describe the process of exhalation
- Exhalation occurs when the distending pressure is released
- Built up potential in the form of increased elastic recoil leads to passive relaxation of alveoli leads to decrease in alveolar volume leads to increase in Palv (to be > Patm) leads to 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
Describe surface tension
- Cohesive forces between molecules
- Molecules on the surface have no atoms above them which results in stronger attractive forces on nearest neighbours on the surface
- Liquid surface area becomes as small as possible i.e. sphere
What does surface tension do to the alveolus
Tends to collapse the alveolus
What does surface tension increase with
Emphysema and age
What does surfactant do
Reduces surface tension
Which cells make surfactant
Type II alveolar cells
What is the major component of surfactant
Dipalmitoyl phosphatidylcholine
How do type II alveolar cells make surfactant
By extracting fatty acids from the blood
How does surfactant reduce surface tension
- Hydrophilic and hydrophobic ends repel each other and interfere with liquid molecule attraction
- Lowers surface tension
What does Laplace’s law tell us
It tells us that pressure in a spherical compartment is:
- proportional to the tension and,
- inversely proportional to the radius of that sphere
infant/neonatal respiratory distress syndrome
- Premature babies (< 30 weeks) have surfactant deficiency
- 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 does airway resistance originate from
Friction between air and mucosa
Airway resistance equation
Pressure difference between alveoli and mouth divided by the flow rate
What is pulmonary resistance the sum of
Total tissue resistance and airway resistance
What causes tissue resistance
Lung and chest wall sliding over each other
What happens to the cross section as you go further down into the small airways
Increases a lot (trumpet shaped graph)
What does Reynold’s number predict
Helps predict when laminar flow converts to turbulent flow
Describe laminar flow
Laminar flow is smooth flow. Resistance generated is proportional to the radius (r4).
Describe turbulent flow
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
Factors affecting airway resistance
- Inflammation
- Mucus
- Bronchodilators
- Steroids
- Gas density
What is the 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.
What causes greater work of breathing due to elastic work
Decreased elasticity in restrictive diseases
What causes greater work of breathing due to non elastic work
Obstructive diseases lead to greater work of breathing to overcome increased airway resistance
Poiseuille’s equation
- Resistance to flow is described by this law
- Resistance = (8 x viscosity x length)/(pi x radius to the power 4)
- Resistance is inversely proportional to the 4th power of the radius
- Flow is inversely proportional to the viscosity of the fluid
- ‘It’s hard to squeeze honey through a long thin tube’
Why does airway resistance decrease as you go down into the smaller airways even though the radius of the airways is decreasing
- There are more airways the further down the generations you go
- The airways of a given generation are in parallel with each other.
- All airways in a generation contribute to that generation’s total resistance (Rt)
- The Rt of a system in parallel will always be less than any individual airway resistance
