Mechanisms of Breathing I & II Flashcards
How does air move within the respiratory system?
The respiratory system achieves movement of air due to the ability of gases to naturally move from (connected) areas of higher to lower pressure, until an equilibrium is reached again
What are the two ways to quantify pressure?
PV = nRT
P ∝ n/v
if n remains constant, inc. volume = decreased pressure
Explain the effects of change in lung volume
Change in lung volume induces changes in alveolar pressure
=> generates pressure gradients between alveoli and
atmosphere, causing air flow
Describe the movement of the respiratory system during inspiration
Diaphragm contracts
Thoracic cavity expands
Alveolar pressure decreases
Air moves in
What movement occurs within the respiratory system during expiration?
Diaphragm relaxes
Thoracic cavity volume decreases
Alveolar pressure increases
Air moves out
How is lung pressure altered to create pressure gradients during the mechanism of breathing?
Inhalation: atm > lung
Exhalation: atm < lung
How is a pressure gradient created within the lungs?
Achieved by manipulating lung volume
What causes lung volume to be altered within the respiratory system?
Contraction of respiratory muscles induces volume changes in the thoracic cavity, causing pressure changes in the lungs
- pressure gradient created between atm and lungs
=> air moves
When is equal pressure reached in the respiratory system?
At the end of expiration, P(alveoli) = P(atm) so there is no movement of air as no pressure gradient
Describe the pressure gradient during inspiration
Outer (parietal membranes) surfaces of the lung are pulled outwards (expansion)
- increases volume
- decreases pressure
P(alveoli) < P(atm)
air flows in from atm to low pressure in lungs
Describe the pressure gradient during expiration
Air in the lungs is compressed
- decreased volume
- increased pressure
P(alveoli) > P(atm)
Air flows out of high pressure lungs to low pressure in atm
What is the role of the pleural cavity?
Indirectly attaches the lungs and chest wall
What is the pleural cavity?
Fluid filled space between membranes (pleura); lines each lung and the chest wall
What is the function of the pleura?
Help reduce friction to aid movement of the lungs
What causes negative pressure to build up in the pleural cavity?
The opposing elastic recoil of the chest wall (outwards) and lungs (inwards), generates negative pressure within the pleural cavity
sealed cavity + increased vol. = decreased pressure
What is the effect of the negative pressure in the pleural cavity?
Pulls the two membranes together
more negative the pressure = more force pulling pleurae together
What is the stability of pressure like between neighbouring spaces?
Differences in pressure between neighbouring spaces are unstable
What is the effect of insufficient opposing force in the pleural cavity?
Equilibrium will be re-established via either:
- movement of liquid / gas
- collapse / expansion of volume at the expense of
surrounding structures
What is negative pressure?
Lower no. of molecules per volume (relative to surroundings)
-> generates collapsing force by pulling surfaces of
contained spaces together
What is positive pressure?
Increased no. of molecules per volume (relative to surroundings)
-> generates an expanding force by pushing surfaces of
contained spaces apart
What is the effect of equal pressures?
Equilibrium reached
Explain the effect of increasing the volume of the pleural cavity
Decreases pressure further = negative pressure
- Collapsing force exerted on lung at expense of lung
volume; draws air in
What is the effect of positive pressure on on the lungs and pleural cavity?
Increasing pressure of the lungs (decreased vol.) compresses the pleural membrane
Expanding force exerted on the lungs (inwards) so air moves out
What determines the degree of expansion and retraction of the lungs?
Expansion and retraction of the lung is determined by the relative size of the opposing forces
What is the pressure gradient like at the end of expiration?
At the end of expiration, the system is in equilibrium
- forces are equal
Why is intrapleural pressure slightly negative after expiration?
At the end of expiration, the intra-pleural pressure is slightly negative due to minor stretching of the cavity by the opposing recoil forces acting on each pleura
What occurs in the intrapleural membranes for lung volume to change?
Either forces acting on one of the pleura needs to become greater than the other
Describe the events occurring during inspiration in the pleural cavity and lungs
Muscular contraction (e.g. diaphragm) Parietal pleura pulled outwards Cavity stretches - inc. vol, decreased P(intrapleural)
What is the effect of decreased P(intrapleural)?
The decreased (more negative) P(intrapleural) creates a greater level of force acting to pull the visceral pleura outwards
What enables the parietal pleura to be held outwards?
Parietal pleura held outwards by muscular contraction
What causes the parietal pleura to pulled outwards?
Outward force generated by P(intrapleural) > Inward force generated by lung recoil, expanding the lung
Describe the events occurring during passive expiration
Relaxation of previously contracted muscles
Reduces outwards force on parietal pleura
Reduces degree of cavity stretch
P(intrapleural) increases
Explain the effect of the increased P(intrapleural)
Increased P(intrapleural) no longer generates sufficient force to overcome elastic recoil of visceral pleura inwards Lung volume decreases - air moves out
What happens during a forced expiration?
Other respiratory muscles also contract (e.g. abdominal and intercostal)
provide further inward force on parietal pleura
Pleural cavity compressed further increasing P(intrapleural) forcing an increased lung volume decline
How is movement of air into / out of lungs achieved?
By altering the volume of the thoracic cavity
Outline the events within the thoracic cavity during inspiration
- Respiratory muscles contract
- Thoracic cavity volume increases
- P(intrapleural) = more negative
- Outward force on visceral pleura > inward recoil force
- Lungs expand, increasing Volume
- Palv < Patm
- air moves into alveoli expanding lungs
Which mechanisms are involved in expiration?
Involves the elastic recoil of lungs and muscle relaxation
What causes lung compression during expiration?
Increased P(intrapleural) - during forced expiration
Why does passive expiration not require an increased P(intrapleural)?
Elastic recoil is sufficient enough to decrease lung volume
What is an open pneumothorax?
an unsealed opening in the chest wall impairing respiratory mechanisms => air moves in from atm
What is the atmospheric pressure value?
100kPa
What is the normal intrapleural pressure, relative to atmospheric pressure?
Intrapleural pressure is naturally sub-atmospheric ~99.5 kPa due to opposing elastic recoil of chest wall and lungs
What is the consequence of piercing either of the pleural membranes?
Will cause air to enter pleural cavity from either the lungs (closed) or the atmosphere (open) depending on the injury => pneumothorax
What is a closed pneumothorax?
When air / gas enters pleural cavity from lungs, without any external injury - normally occurs in CF / cancer patients (when lung already damaged)
What is the consequnece of pneumothorax?
Negative P(intrapleural) lost
Chest wall and lungs no longer indirectly attached
- will recoil in opposing directions causing lung tissue
collapse => atelactasis
Describe the effect of entry of air into a pneumothorax
Increased pleural cavity volume
at expense of lung volume (decreases)
Reduces P(intrapleural) changes during inspiration
- prevents proper lung expansion
pneumothorax causes lung collapse - limits gas exchange
Outline how breathing occurs
- Lung volume increases
- Palv < Patm
- air moves into lungs
Why is the difference in pressure between atm and alveoli not negated immediately ?
The difference in pressure aren’t instantly negated as there’s a delay due to the time taken for air to move
What is speed of airflow increased by?
- Decreased resistance
- Increased Radius
- Increased Pressure
R ∝ 1/r⁴
How does air reach gas exchange surfaces?
To reach gas exchange structures, air must pass through a series of increasingly narrow and numerous airways
What causes impaired airway function?
Due to insufficient ventilation
What does rate of airflow depend upon?
The rate of airflow depends on the pressure gradients and level of airway resistance
Outline Ohm’s Law
Airflow = ΔP / resistance
↑ΔP = ↑ airflow
↑ resistance = ↓airflow
Explain how a reduction in airway radius effects airflow
As airway radius decreases, Resistance increases, reducing airflow dramatically
What type of airflow increases resistance?
Turbulent flow
How does turbulent airflow increase airway resistance?
Air moves in different directions increasing resistance reducing speed of airflow
Describe the structure of alveoli in healthy individuals
Elastin in surrounding alveoli provides radial traction to splint bronchioles against positive Palv
How does COPD affect alveoli structure and airflow?
Without radial traction, bronchioles collapse causing obstruction
What causes a loss of airway patency in COPD?
Loss of airway patency due to degradation of structure causing airway obstruction
Airways collapse due to lack of structural integrity
How can we calculate the total % of lung capacity an individual can exhale in the first second?
100 x FEV₁ / FVC = %
FEV₁ - Forced expiratory volume (in 1 sec)
FVC - Forced vital capacity
(<80% is indicative of obstructive airways disease)
Describe the results for a patient with obstructive COPD
FEV₁ / FVC < 70%
e. g. asthma
- increased resistance
What would the results of a patient with restrictive COPD look like?
FEV₁ / FVC > 70%
FVC < 80%
e.g. Fibrosis
- decreased compliance
Outline how you’d calculate Transpulmonary pressure Pₜₚ
Pₜₚ = Pₐₗᵥ - Pᵢₚ
What is transpulmonary pressure?
The level of force acting to expand the lung
What is lung compliance?
Lung compliance is the relationship between transpulmonary pressure and lung volume (higher lung CL, steeper gradient)
How does scoliosis muscular dystrophy + obesity affect lung CL?
Decreases lung compliance interfering with chest wall mechanisms
How does NRDS (newborn respiratory distress syndrome) affect compliance?
Affects alveolar surface tension - decreasing CL
What is the effect of fibrosis and COPD on lung compliance?
Affect the elastin fibres:
- Fibrosis : ↓CL due to collagen deposition
- COPD: ↑CL due to elastin degradation
How does emphysema affect lung compliance?
↑CL due to elastin degradation
How do air liquid interfaces resist inflation?
Generate surface tension which resist inflation
Give examples of air liquid interfaces
Alveoli are lined with fluid to enable gas exchange - the gas molecules dissolve into the water before diffusing.
Why does tension arise within the bubble formed by water-air interfaces?
Tension arises due to H-bonds between water molecules exerting a collapsing force towards the centre of the bubble
How does Laplace’s law relate to the respiratory system?
Describes the pressure generated by surface tension within a bubble
- collapsing force generates pressure, the amount within
a specific bubble is described by Laplace’s
Outline the equation used by Laplace’s Law to calculate pressure generated
P = 2T/r
p - pressure
2T - surface tension
r - bubble radius (alveoli)
How does the size of alveoli affect the pressure generated at air liquid interfaces?
if T remains constant, P ∝ 1/r
- smaller the alveoli, the larger the pressure generated
How is alveolar surface tension reduced?
Reduced by the presence of pulmonary surfactant, secreted by type II pneumocytes
What is the role of pulmonary surfactant?
Pulmonary surfactant act to equalise pressure and volume across varying alveoli
How does [surfactant] change as gas exchange occurs?
As alveoli expand, the [surfactant] decreases - increasing surface tension
How are the lungs kept inflated?
Larger alveoli tend to collapse into smaller ones, helping consistent inflation of the lungs
What role does pulmonary surfactant play in alveolar oedema?
Pulmonary surfactant help prevent alveolar oedema by reducing surface tension
How does surface tension affect alveolar oedema?
Surface tension produced at air-liquid interface also reduces hydrostatic pressure
- fluid pulled out of surrounding capillaries -> alveoli
What is NRDS?
neonatal respiratory distress syndrome
What causes NRDS?
Insufficient surfactant production often due to:
- premature birth
- maternal diabetes
- congenital development issues
Explain the effects of NRDS and how they occur
Premature birth / congenital defects ↓ Insufficient surfactant production ↓ Stiff (low CL), alveolar collapse, oedema ↓ Respiratory failure ↓ Pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary & cerebral haemorrhage
How is NRDS treated?
- artificial surfactant supplementation
- maternal glucocorticoid supplementation
