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
explain pressure in a spherical compartment? 3
- proportional to tension
- inversely proportional to the radius of that sphere
- surfactant causes tension to be lower in smaller spheres
explain premature babies and surfactant?
have a surfactant deficiency and are at risk for infant/neonatal respiratory distress syndrome from alveolar collapse to die due to high tension
why is surfactant important? 4
- increases lung compliance because surface forces are reduced
- promotes alveolar stability
- prevents alveolar collapse as small alveoli are prevented from getting smaller and large alveoli are prevented from getting larger
- surface tension tends to suck fluid from capillaries into the alveoli- reduction of surface tension reduces hydrostatic pressure in the tissues outside the lungs and keeps them dry
what is airway resistance? 7
- force needed to inflate the lung is greater than elastic recoil
- airway resistance originates from friction between the air and mucous
- it is the pressure difference between the alveoli and mouth divided by the flow rate
- resistance= pressure1- pressure2/flow
- V=IR
- pulmonary resistance= tissue + airway
- tissue forces are the lung and chest wall sliding over each other
what is the airway trumpet? 2
- massive increase in cross section in small airways
- as the airway generation gets smaller, there is a massive increase in the total cross sectional airway
what are the 2 types of flow?
- laminar flow= smooth, resistance is proportional to the radius
- turbulent flow= irregular, chaotic with eddy currents, good for transferring heat, but the resistance is high
what does Reynolds number predict? 2
- when laminar flow converts to turbulent flow
- Re=2(length)(density of gas)(velocity of gas)/ viscosity of gas
what is resistance described by? 2
- by the law equating resistance and radius
- inversely proportional to the 4th power of the radius
what happens to resistance in lower generations? 2
- the airways of each generation are parallel to each other
- all airways in a generation contribute to that generations total resistance
what factors cause airway resistance? 6
- inflammation
- mucous
- bronchodilators
- steroids
- gas density
- heliox/diving
what is work breathing? 2
- work is required to stretch elastic tissues of the chest wall and lungs, moving inelastic tissues and air through the tubes
- it is the amount of energy of O2 consumption needed by the respiratory muscles to produce enough ventilation and respiration to meet the metabolic demands of the body
what is elastic work? 2
- decreased elasticity in restrictive diseases
- greater wOB
what is non-elastic work?
obstructive diseases lead to greater wOB to overcome increased airway resistance
