4. Ventilation and Compliance 2 Flashcards
What inwardly directed force has to be overcome during inspiration?
Lung’s natural tendency to recoil due to its elasticity (they’re even stretched slightly open at the end of a relaxed expiration)
what 2 cells types make up the alveolar wall?
- type 1 pneumocytes (cells)
2. type 2 pneumocytes (cells)
what do type 1 pneumocytes do?
permit gas exchange
what do type 2 pneumocytes do?
specialised to secrete surfactant fluid (detergent-like fluid)
what is the main function of surfactant?
reduces surface tension on alveolar surface membrane thus reducing tendency for alveoli to collapse
How does surface tension occur?
occurs wherever there is an air-water interference and refers to attraction between water molecules (like droplets on a window)
what surface does surfactant cover in a thin layer?
alveoli
what is found inside an alveoli?
gaseous space with fully/highly saturated air (with water droplets surrounding it)
where does surfactant sit on an alveoli?
between water molecules surrounding the alveoli which reduces the tendency for the alveoli to collapse
What is an advantage of partially inflated alveoli, which is due to surfactant?
they are much easier to inflate than completely collapsed alveoli
what are the 3 main benefits of surfactant in the lungs?
- increases lung compliance (distensibility, stretchiness)
- reduces lung’s tendency to recoil
- makes work of breathing easier
why is surfactant more effective in small alveoli than large alveoli?
Because surfactant molecules come closer together and are therefore more concentrated
What does compliance mean in terms of lungs?
stretchiness but not necessarily recoiling (like plasticine- which can be stretched but it doesn’t recoil as there’s no elasticity)
CHANGE IN VOLUME RELATIVE TO CHANGE IN PRESSURE (ie how much does volume change for any given change in pressure)
what does the law of LaPlace show?
The larger the vessel radius, the larger the wall tension required to withstand a given internal fluid pressure
what is the law of LaPlace equation?
P = 2T/r
where T= surface tension
and r= radius
Without surfactant, where would be pressure be greatest in, the small or large alveoli?
the small
What does the surfactant do in terms of pressure in large and small alveoli?
it equalises the pressure in the large and small alveoli as the surface tension is reduced
At what stage of gestation/pregnancy does surfactant production start?
~25 weeks of gestation
At what stage of gestation/pregnancy is the surfactant production complete?
~36 weeks of gestation (40 weeks is full term)
What 2 substances stimulate the production of surfactant?
- thyroid hormones
- cortisol
(both increase towards end of pregnancy)
What respiratory condition can premature babies suffer from due to insufficiency in surfactant production ?
Infant/neonatal Respiratory Distress Syndrome (IRDS)
What is the pressure difference usually in large and small alveoli?
pressure is 2 x higher in small alveoli than large (making them more likely to collapse without surfactant)
Why are small alveoli so crucial if they can collapse more easily?
Because they increase the SA significantly and make gas exchange much more efficient
How is synthetic surfactant administered to premature babies?
Through aerosol (inhalation), which saves many lives
Why is less change in pressure required to inflate an in-utero lung than a post-birth lung?
Because in-utero lung doesn’t need to overcome surface tension (no air-water interference) so volume changes in an in-utero lung happen much quicker
What does HIGH compliance mean in terms of lung volume and pressure?
large increase in lung volume for small decrease in itrapleural pressure
What does LOW compliance mean in terms of lung volume and pressure?
small increase in lung volume for large decrease in intrapleural pressure
Why is low compliance never healthy?
Because muscles in respiration need to work very hard to increase the volume of chest cavity but get little volume of air in.
What is emphysema?
disease which causes overinflation of the alveoli causing impaired gas exchange. Lungs lose their elasticity, trapping the air inside the lung.
What is a common example of an emphysema condition?
COPD
Does compliance decrease with age?
Yes
Does surfactant abolish surface tension?
No, it only REDUCES it
What 2 things need to be overcome in the early part of inspiration and becomes easier in the later part of inspiration?
-elastic recoil
-surface tension
(which is why it becomes negative and requires a greater change in pressure from Functional Residual Capacity to reach a particular lung volume at the start to overcome this)
Why is there little movement in volume of air at the start of expiration?
Because we need to overcome the resistance of the narrower airways which get constricted as we breathe out as airways get compressed and pressure is needed to overcome this
How is inspiration effort (work) recovered?
as elastic recoil during expiration (which is passive)
What happens during emphysema? what does this result in?
Loss of elastic tissue which means expiration is more difficult (requires more effort) due to loss of elasticity
What happens during fibrosis?
inert/weak fibrous tissue means inspiration is more difficult (effort increases) due to loss of compliance
Name examples of when forced expiration would be necessary. (4)
- coughing
- sighing
- during exercise
- blowing out candles
Why can patients with emphysema often become weak and without much energy?
Energy is invested for each breath to expel the air out of the lungs making breathing more difficult due to loss of elasticity
Which lung region has the greatest compliance?
at the base of the lung
Which lung region has the least compliance?
at the apex of the lung
Why does the base of the lung have the greatest compliance?
- Lungs hang in the thoracic cavity so alveoli at the bottom are “squashed” by having the lung above it.
- Alvoeli at the top remain further inflated while alveoli at the bottom have more pressure put on them.
- Alveoli at the top have less opportunity to expand as they are already expanded. Whereas alveoli at the base of the lung have a much bigger range for inflation and can expand more during insiration than alveoli at the top.
- all dependent on gravity
Which region of the lung has the greatest alveolar ventilation?
base of the lung (alveolar ventilation declines with height from base to lung)
Which region of the lung has the greatest compliance?
Base (compliance declines with height from base to lung due to alveoli at apex being more inflated FRC;functional residual capacity)
Does a small change in intrapleural pressure bring about a larger change in volume at base or apex of the lung?
at the base (bigger effect on the base); changes more apparent as base does most of the work
What is obstructive lung disease?
obstruction of air flow, especially on expiration
What is restrictive lung disease?
restriction of lung expansion
What are examples of obstructive lung disease? (2)
- Asthma
- COPD
What is an example of a restrictive lung disease?
fibrosis (fibrous tissue grown on elastic fibres which prevent expansion elasticity)
What are the 2 types of COPD?
- chronic bronchitis (inflammation of bronchi)
2. emphysema (destruction of the alveoli, loss of elasticity)
How many people worldwide suffer from moderate to severe to COPD?
80 million people
How prevalent is COPD in men over the age of 75?
10%
What 5 are common features of restrictive lung disorders?
- loss of lung compliance
- fibrosis
- infant respiratory distress syndrome (insufficient surfactant production)
- oedema (fluid building up around alveoli, restricting inflation)
- pneumothorax
What are some features of loss of lung compliance? (2)
- lung stiffness
2. incomplete lung expansion
What are some fibrosis features? (3)
- formation or development of excess fibrous connective tissue
- idiopathic (cause unknown)
- asbestosis
What is spirometry?
- technique commonly used to measure lung function
What can the measurements in spirometry be classed as? (2)
- static
2. dynamic
What is a static measurement in a spirometer?
where the only consideration made is the VOLUME EXHALED (amount of air)
What is a dynamic measurement in a spirometer?
where the TIME taken to exhale a certain volume is what is being measured
Which measure is most useful in a spirometer?
dynamic measure
Which measures can be directly measured by a spirometer?
- tidal volume
- inspiratory reserve volume
- expiratory reserve volume
- inspiratory capacity
- vital capacity
What is forced expiratory volume (FEV)?
how much someone can expire in 1 second (on forced expiration)
What is forced vital capacity? (FVC)
how much someone can physically expire in total (maximum)
What is the usual FEV (forced expiratory volume) in a fit, healthy male in L?
4L
What is the usual FVC (forced vital capacity) in a fit, healthy male in L?
5L
What is the usual FEV/FVC ratio in a healthy, fit male?
around 80% (4/5 = 80%)
What is the usual FEV/FVC ratio in a patient suffering from obstructive lung disease?
approx. 42% (decreased significantly)
Why is FEV/FVC ratio in obstructive lung disease decreased significantly?
because the amount they can expire in 1 second becomes much smaller (FEV) -due to obstruction
What is the usual FEV/FVC ration in restrictive lung disease patients?
approx 90% (can be higher than the normal ratio in a healthy individual)
Why is the FEV/FVC ratio in restrictive lung disease sometimes even bigger that normal?
FEV and FVC have a closer value, BUT their numbers are significantly lower in both FEV and FVC than in a healthy individual because of the restriction in inspiration (both inspiration and expiration affected, not only expiration like in obstructive)
Describe why ratio of FEV:FVC is reduced in obstructive pulmonary disease by comparing it to normal. (e.g. in COPD)
- rate at which air is exhaled is much/significantly slower (FEV)
- total volume is also reduced (FVC)
- major effect is on airways so FEV is reduced to a greater extent than FVC
- therefore both FEV and FVC fall (FEV falls more)
- Therefore ratio is reduced
Describe why ratio of FEV:FVC is reduced in restrictive pulmonary disease by comparing it to normal (e.g. in pulmonary fibrosis).
- absolute rate of airflow is reduced ( since less air can be breathed in in the first place)- both FEV and FVC are reduced
- total volume is reduced due to limitations to lung expansion (FVC)
- ratio remains CONSTANT or can increase as large proportion of volume can be exhaled in the first second
Why is a normal FEV/FVC ratio not always indicative of health?
Because in restrictive pulmonary disease, both FEV and FVC fall so ratio remains much the same despite severe compromise of lung function
What is Forced Expiratory Flow (FEF)?
- measures the average expired flow over the middle of an FVC
- correlates with FEV but changes are generally with FEV but changes are generally more striking
- looks at how much air is expired in 25%-75% of movement
Is the “normal” range greater or smaller in FEF?
greater
When can FEF (forced expiratory flow) be used?
can help predict people who are starting to develop lung disease but don’t have established symptoms
What is ventilation in simple terms?
- volume of air moved into and out of lungs.
- driven by chest wall movement and changes in thoracic movement
What is anatomical dead space?
- volume of air taken in during each breath that does not mix with air in the alveoli.
- it’s the measure of the volume of the conducting airways
What is PHYSIOLOGICAL dead space?
includes alveoli that are ventilated but not perfused with blood (no gas exchange)
What is pulmonary (or minute) ventilation the product of?
it’s the product of tidal volume and respiratory frequency
What is alveoli ventilation?
the volume available for exchange in the alveoli
What must the pressure required to inflate the lung overcome?
it must overcome airway resistance and expand elastic the elastic elements of the chest as measured by compliance
What 3 things determine compliance?
- elastic forces
- surface tension at the alveolar air-liquid interference
- by air resistance
How does surfactant effect alveolar surface tension and compliance?
- alveolar surface tension is greatly reduced
- compliance is increased
Why does alveolar ventilation decline with height from base to apex of an upright lung?
due to changes in compliance (compliance is greater at base)
What measurement effect can be used in diagnosing lung disease?
spirometry
Does spirometry have limitations?
Yes
What parts of ventilation are associated with air flow obstruction in obstructive lung disease?
expiration
What parts of ventilation are associated with restricted lung expansion in restrictive lung disease?
inspiration (and expansion)
Why is intrapleural pressure always less than alveolar pressure?
- ribs will always be pushing outwards so to compensate we need a force going inwards
- force= difference in pressure x surface
- therefore intrapleural pressure needs to be be less than alveolar pressure so force vector goes the right way
- if they were equal, there would be no inwards force so chest would continue expanding and lung would collapse
