Ventilation: Physics of Breathing Flashcards
(47 cards)
Define pulmonary ventilation.
Movement of air into and out of the lungs
Briefly explain the role of volume and pressure in breathing.
Δ volume → Δ pressure → movement of air, i.e. airflow follows a change in pressure resulting from the change of volume of the lungs. Air flows from high pressure area to low pressure area. To lower the pressure inside the lungs we expand the size of the chest and lungs.
Identify and describe the pressures that are significant in the process of breathing.
Intrapulmonary pressure = pressure within the alveoli (falls and rises over one respiratory cycle). Expressed relative to atmospheric pressure.
Intrapleural pressure = pressure within pleural cavity. Always more negative than alveolar. Elastic nature of lung tissue versus ribcage and thorax trying pull apart visceral from parietal pleura. Expressed relative to atmospheric pressure.
Atmospheric pressure = 760 mmHg or 101.325 kPa
What are the values for intapulmonary pressure, intrapleural pressure and collapsing force of the lungs, at the end of expiration ?
Intrapulmonary P = 760 mmHg (=0 mmHg relative to atmospheric P)
Intrapleural P = 756 mmHg (= -4 mmHg relative to atmospheric P)
Collapsing force = 4 mmHg (=absolute difference between intrapulmonary and intrapleural P)
What is the implication of the negative intrapleural pressure relative to atmospheric pressure ?
Slight vacuum in pleural cavity
Describe the distribution of pressures in different areas of the lung.
Pressure changes from top of the lung to the bottom of it.
Describe the action of the main muscles responsible for inspiration, and how exactly they lead to inspiration.
- Diaphragm: main muscle of inspiration. Contraction flattens domes. Abdominal wall relaxes to allow abdominal contents to move downwards
- External intercostals: with first rib fixed, two movements, forward movement of lower end of sternum, and upward and outward movement of ribs.
- Trapezius: accessory muscle in forced inspiration (e.g. respiratory distress)
Flattening of diaphragm + outward expansion of rib cage by external intercostals = increase in V = decrease in P
Describe the changes in pressure in inspiraton.
Increases volume of thorax by about 500 ml – normal tidal volume
Intrapleural pressure drops to ~ -6 mmHg (in normal tidal volume)
Decreases intrapulmonary pressure by ~1 mmHg (becomes -1 mmHg relative to atmospheric pressure, i.e. 759 mmHg) (in normal tidal volume)
Describe the action of the main muscles responsible for expiration, and how exactly they lead to expiration.
- Passive – no direct muscle action normally
- Cessation of muscle contraction
- Elastic recoil – drives air out of lungs (decreased V, increased P, air moves down pressure gradient)
Describe the changes in pressure in expiration.
- Thoracic volume decreases by 500 ml
* Intrapulmonary pressure increases
Describe the action of the main muscles responsible for forced expiration, and how exactly they lead to forced expiration.
Contraction of abdominal walls, forces abdominal contents up against diaphragm, and internal intercostals – pull ribs downwards (decreases V, increases P, air forced down pressure gradient)
Graph the fluctuations in intrapleural and intrapulmonary pressures during inspiration, and expiration.
Refer to graph on slide 7 in lecture on “Ventilation: Physics of Breathing”
Graph the change in V of breath during inspiration and expiration.
Refer to graph on slide 7 in lecture on “Ventilation: Physics of Breathing”
Define transpulmonary P.
Difference between intraplueral and intrapulmonary P.
What actions in breathing is energy required for ?
Energy is required to:
♦ contract the muscles of inspiration – in quiet breathing contraction of the diaphragm comprises 75% of energy expenditure.
♦ stretch elastic elements
♦ overcome airway resistance
♦ overcome frictional forces arising from the viscosity of the lung and chest wall
♦ overcome inertia of the air and tissues
Explain the significance and relevance of airway resistance in breathing. What are factors which can affect airway resistance ?
Airway resistance is the most significant non-elastic source of resistance in breathing.
F (air flow) = ΔP/R (resistance)
Resistance greater when turbulent flow (turbulent flow likely to occur with high velocities, and large diameter airways.)
Which part of the respiratory tract most contributes to airway resistance ?
Upper airways (1/3)
Describe the differences of resistance in different areas of the lung.
In different parts of lungs, resistance changes (in some parts, turbulent in other parts laminar).
Where is the greatest resistance to airflow found ? Why ?
Segmental bronchi, because cross sectional area is relatively low and airflow is high and turbulent. (hence they are the ones to contract in asthma)
How much resistance is there in alveoli? Why ?
Flow is laminar and the resistance is small (there is a large total cross- sectional area due to large number of small airways combined)
Describe any changes in airway resistances during respiration. Is this significant in health ?
In inspiration, airway resistance decreases
In expiration, airway resistance increases
No, insignificant in health, unless disease state where airway resistance is increased:
- In asthma, inflammatory mediators change smooth muscle tone – narrowing airways – increases resistance.
- patients with COPD tend to have over-inflated chests (barrel-chested) (COPD also increases resistance)
Define compliance in the context of breathing.
Distensibility or ease of stretch of lung tissue when external force applied, or the ease with which the lungs expand under pressure (i.e. change in volume of the chest that results from a given change in intrapleural pressure. High compliance means there is a large change in volume for a given change in pressure)
What are the major determinants of compliance in the lungs ?
Elastic components and alveolar surface tension
In a healthy individual, what is the compliance ?
Compliance in a healthy individual is about 1 L per kPa (1 L per 7.5 mmHg)
