Gas Movement within the airways Flashcards
Gas Laws
Gas movt is dependent on the gas laws:
- gas will move down a pressure gradient from higher to lower pressure (bulk flow)
- gas will move down a partial pressure (concentration) gradient from higher to lower concentration (diffusion - more slowly than bulk flow)
Gas movts b/w airways + atmosphere
Gas movt from the atmosphere to the lungs occurs in two phases:
- movt b/w the atmosphere + the upper airways: largely due to bulk flow
- movt b/w the upper airways + the alveoli: largely due to diffusion, although there is some bulk flow of gas into + out of alveoli
- Changes in lung volume are responsible for the pressure gradients down which the gases move.
Diffusion is a slow process so:
- changes in alveolar gas composition occur slowly
- the alveolar gas is not replaced on each breath
- In order for the process of diffusion to continue, the concentration gradients must be maintained by bulk flow.
EXAMPLES (JUST FOR ME TO LOOK AT)
Bulk Flow: movt down a pressure gradient
- 800mmHg → 600 mmHg
Partial pressure gradient
- 800 mmHg 50% O2 50% CO2
- 800 mmHg 20% O2 80% CO2
- Diffusion means that O2 diffuses from 50% to 20%
- CO2 diffuse from 80% to 20%
- Pressure + volume are inversely related
- i.e. as volume increases the pressure decreases
Why is muscular effort needed
- During normal breathing muscular effort is required to increase lung volume.
- The muscular effort is needed because:
- the lung + the chest wall must be stretched = the lung + chest wall are elastic - this opposes inflation - as lung volume increases the elastic forces become greater
- the changes in lung volume must generate sufficient pressure to overcome the resistance to gas movement = the column of gas in the airways has inertia = must be overcome to initiate gas movt,
+ the airways represent a resistance to gas flow - the impedance - the flow rate (determined by frequency) determines the impedance.
- The total work of breathing is the work required to overcome resistive forces.
Inspiratory muscles (Involves, exercise, respiratory disease)
Inspiratory muscles = contraction increases the volume of the thorax
INVOLVES:
- contraction of diaphragm
- lowers dome - predominates when supine
- lifts + flares ribs
- parasternal + scalenes also active on standing = are also intercostal muscles
- diaphragm shortens, parasternals + scalene more activated
WITH EXERCISE
- inspiratory muscles more activated w/ increasing min ventilation (VE)
RESPIRATORY DISEASE
- additional recruitment of other rib cage muscles + neck inspiratory muscles if there are high lung volumes and/or high resistance
Expiratory muscles
Expiratory Muscles = quiet at supine rest, activated on standing
Recruitment at:
- high VE = min ventilation
- high lung volume
- high resistance
- When standing
Expiration is generally passive - relying on elastic recoil of the energy stored during inspiration.
FORCES ACTING ON THE LUNGS (general info)
- respiratory muscles can only move the thoracic wall. - Movt of the thoracic wall changes the lung volume because various forces functionally link the chest wall and the lungs
FORCES ACTING ON THE LUNGS
- ALVEOLAR PRESSURE (pressure in the alveoli): pressure tends to keep the lungs inflated
- NEGATIVE INTRAPLEURAL (space in b/w the 2 pleural layers = space immediately outside the lungs) PRESSURE: the pressure in the space b/w the two pleural layers is subatmospheric: this tends to suck the lungs outwards and the chest wall inwards
- INTRAPLEURAL SURFACE TENSION: the pleural space is filled w/ fluid. Surface tension makes the outer layer of the lungs loosely adhere to the inner wall of the chest
- ELASTIC FORCES: elastic tissue in the lung opposes inflation + facilitates deflation
forces on inspiration/expiration (>..
Subatmospheric intrapleural pressure + alveolar pressure + surface tension = elastic force
Inspiration:
Intrapleural + alveolar + surface > elastic forces
Expiration:
Intrapleural + alveolar + surface < elastic forces
