mechanics of breathing Flashcards
what are the lungs and chest wall? 5
- elastic structures
- lungs are stretched when we inhale
- they recoil on exhalation
- this is balanced by the chest wall tendency to recoil in the opposite direction
- at the end of quiet expiration, the pressures balance
what are the inspiratory muscles? 3
- diaphragm- 75% of volume changes- lowers the floor of the thoracic wall- ribs move upwards and outwards
- external intercostals-bucket handle-elevate ribs when they contract
- accessory muscles-scalene-sternomastoid
what is inspiration? 6
- an active process
- contraction of inspiratory muscles increases the intrathoracic volume
- this causes a decrease in intrapleural pressure as the volume of the pleural cavity increases
- lungs are pulled into a more expanded position and the pressure in the airways becomes negative compared to that in the mouth and air will move in
- at the end of inspiration, the pressures are equal
- recoil of lungs and chest will then occur
what is transpulmonary pressure? 3
- 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 air flow inwards
- this distending pressure is called the transpulmonary pressure (Ptp)
- chest wall expansion is done by muscles
explain the pressures of the lung under physiological conditions? 3
- the Ptp is always positive
- the Plp is always negative
- for any given lund, the transpulmonary pressure is equal and opposite to the elastic recoil pressure of the lung
explain the lung volumes: TV IC RV TLC
TV= tidal volume- normal quiet breathing
IC-inspiratory capacity- when we take a deep breath in
RV= residual volume= all gas left in the lung after expiration
- TLC= total lung capacity= show all gas from full inspiration to the end of expiration and gas trapped in lungs at the end of forced expiration
what is compliance? 5
- static measure of lung stretchiness
- different to resistance (dynamic, accounting for airflow resistance)
- volume changer per unit pressure change
- balance point (where lung and chest are in equilibrium after exhaling= FRC= functional residual capacity
- endpoints= total lung capacity and residual volume
when is the transpulmonary pressure positive?
between the RV to the TLC so the lungs always tend to collapse
- the lungs are a spring which can only be stretched
what are the properties of the chest wall? 2
- the chest wall is a spring that can be compressed or distended
- transthoracic pressure is negative at RV and FRC so the chest wall tends to spring out
what is special about the FRC?
the tendency of the recoil of the chest wall and the collapse of the lung is equal
what does lung compliance depend on? 3
- how inflated or not it is
- the lung is less compliant at higher volumes
- the compliance curves are different for inspiration and exhalation- this difference hysteresis (frictional resistance changes)
explain altered lung compliance in lung disease? 4
- in emphysema the pressure volume curve demonstrates lungs with increased compliance
- loss of elastic recoil- easy to inflate but difficult to exhale
- in pulmonary fibrosis the pressure volume curve demonstrates stiff lungs
- increase in elastic recoil means its difficult to inflate the lungs
what is exhalation? 4
- exhalation occurs when the distending pressures are released
- built up in the form of increased elastic recoil- passive relaxation of alveoli- decrease in alveolar volume - increase in PalV - outward flow
- active exhalation occurs when expiratory respiratory muscles are engaged, but short of exercise and disease, we don’t normally need to call upon these
- loss of elastic recoil impairs efficient and effective exhalation
what is surface tension? 6
- cohesive forces between molecules
- molecules on the surface have no atoms above them
- results in stronger attractive forces on the nearest neighbours on the surface
- liquid surface area becomes as small as possible (sphere)
- tends to collapse the alveolar
- surface tension can increase with emphysema and age
what does surfactant do? 4
- type 2 alveolar cells extract fatty acids from blood and synthesise surfactant
- major component is DPPC
- hydrophilic and hydrophobic ends repel each other and interfere with liquid molecule attraction
- this lowers the surface tension
