Surfactant, Compliance and lung function tests Flashcards
What type of cell makes up alveolar walls?
Thin walled “Type I” cells which permit gas exchange Specialised “Type II” cells which secrete surfactant fluid
What is the function of surfactant?
Reduces surface tension on alveolar surface membrane thus reducing the tendency for alveoli to collapse. Increases lung compliance - distensibility Reduces lung’s tendancy to recoil Makes work of breathing easier
What is surface tension?
The attraction that occurs between water molecules that occurs whenever there is an air-water interface and refers to the attraction between water molecules.
When is surfactant most effective?
In small alveoli than large alveoli because surfactant molecules come closer together and are therefore more concentrated.
What force has to be overcome during inspiration?
Inward force caused by the elasticity of the lungs - they are stretched slightly open even at the end of a relaxed expiration - natural tendency to recoil (contract inwards)
What type of alveoli has the greatest pressure?
The smallest ones
When does surfactant production start? Finish?
25 weeks gestation Complete by 36 weeks
What stimulates surfactant production?
Thyroid hormones and cortisol which increase towards the end of pregnancy
Why is less change in pressure required to inflate lung in utero?
Don’t need to overcome surface tension (no-air water interface)
What is meant by compliance?
Change in volume relative to change in pressure
What is high compliance?
Large increase in lung volume for small decrease in ip (intrapleural) pressure
What is low compliance?
Small increase in lung volume for large decrease in ip (intrapleural) pressure
It requires a greater change in pressure (from FRC) to reach a particular lung volume during inspiration, than to maintain that volume during expiration Why?
Normally effort (work) of inspiration is recovered as elastic recoil during expiration (hence expiration is passive).
What is emphysema?
Toxins in smoke activate elastase causing loss of elastic tissue - means expiration requires effort
What is fibrosis?
Inert fibrous tissue means effort of inspiration increases
What is the law of LaPlace?
The pressure of alveolus = 2 times the surface tension divided by the radius
Which part of the lung changes more significantly in volume when the pressure increases?
The base, the volume change is much greater for a given change in pressure
What is the relationship between height from base to apex and compliance/alveolar ventilation?
Decrease as height increases
Why does compliance decrease with height?
Alveoli at the apex are more inflated at FRC. At the base the alveoli are compressed between the weight of the lung above and the diaphragm below and hence more compliant on inspiration. Alveoli at the top are further inflated than the alveoli at the base of the lung for the duration of the breathing cycle - hence they are able to inflate less
What is an obstructive lung disease?
Obstruction of air flow, especially on expiration
What is restrictive lung disease?
Restriction of lung expansion
What is transpulmonary pressure?
The relationship between the alveolar pressure and the intrapleural pressure - always positive.
Give examples of obstructive lung disorders
Asthma COPD (chronic obstructive pulmonary disease)
Give examples of restrictive lung disorders
Involve loss of lung compliance (lung stiffness, incomplete lung expansion) Fibrosis (formation or development of excess fibrous connective tissue Idiopathic (cause unknown); 1:100,00 per year Asbestosis (and other occupational origins e.g. coal dust) Infant Respiratory Distress Syndrome: (insufficient surfactant production) Oedema - fluid builds up around alveoli creating pressure which requires more force to inflate Pneumothorax
What is used to measure lung function?
Spirometery
What is a static measurement in spirometry?
Where the only consideration made is the volume exhaled
What is a dynamic measurement in spirometry?
Where the time taken to exhale a certain volume is what is being measured
What measurements can be measured directly by spirometery?
Tidal volume Expiratory reserve volume Inspiratory reserve volume Inspiratory capacity Vital capacity
What is the average FEV1 for a healthy male?
4 litres
What is the forced vital capacity for a healthy male?
5 litres
How do you calculate the FEV1/FVC?
Divide the forced expiratory volume in one second by the forced vital capacity
What are the FEV and FVC for an obstructive respiratory disorder?
FEV decreases (significantly) FVC is also reduced FRC may be increased Ratio is reduced
What are the FEV and FVC for an restrictive respiratory disorder?
Absolute rate of airflow is reduced (FEV redcued) Total volume is reduced due to limitations to lung expansion (FVC) Ratio remains constant or can increase as a large proportion of volume can be exhaled in the first second.
What is a forced expiratory flow?
Average expired flow over the middle of a forced vital capacity
