ventilation and compliance Flashcards
define tidal volume
TV
500ml
the volume of air breathed in/out of the lungs at each breath
only uses a fraction of our lung capacity
define expiratory reserve volume
ERV
1.1L
the max vol of air which can be expelled from the lungs at the end of a normal expiration
define inspiratory volume
IRV
3L
The maximum volume of air which can be drawn into the lungs at the end of a normal inspiration
define residual volume
RV
1.2L
the volume of gas in the lungs at the end of a normal expiration
cannot be voluntarily moved out and remains in the alveoli
what is the purpose of RV
important for gas exchange to continue at all times between breaths
helps prevent the alveoli collapsing (requires much more effort to inflate the alveoli from a collapsed state rather than a partially inflated state)
define vital capacity
VC 4.5L TV + IRV + ERV max amount of air you can move on one breath good value to use in lung function tests
define total lung capacity
TLC
VC + RV
define inspiratory capacity
IC
TV + IRV
define functional residual capacity
FRC
ERV + RV
volume of air left in the lungs after a normal expiration
requires a greater change in pressure from FRC to reach a particular lung volume during inspiration than to maintain that volume during expiration
define forced expired volume in 1s
FEV1
4.0L in healthy, fit, young adult males
define fraction of forced VC expired in 1s
FEV1:FVC
FVC in healthy young males = 5L
FEV1/FVC = 80%
absolute values decline slightly with age
define anatomical dead space
volume of gas occupied by the conducting airways and this gas isnt available for exchange
what is surfactant
detergent like fluid secreted by type II pnuemocytes
what is the role of surfactant (4)
reduces surface tension on the alveolar surface membrane
increases lung compliance
reduces lungs tendency to recoil
makes the work of breathing easier
how does alveoli reduce surface tension
reduces the attraction between the water molecules lining the alveoli
these create inwardly directed pressure
surfactant reduces the inwardly directed pressure which reduces the tendency of the alveoli to collapse
where is surfactant most effective
in smaller alveoli
the surfactant molecules come together and are more concentrated
when does surfactant production begin
~25wks gestation and is complete ~36wkss
stimulated by thyroid hormones and cortisol
what is the relationship between premature babies and surfactant
premature babies suffer IRDS (infant respiratory distress syndrome)and are often given synthetic surfactant to help them breathe and survive
law of laplace
P = 2T/r pressure = 2 x surface tension/radius
what is an obstructive lung disease
obstruction of air flow, especially on expiration
increased airway resistance
what is a restrictive lung disease
restriction of lung expansion
loss of lung compliance (lung stiffness, incomplete expansion)
give 2 examples of obstructive lung diseases
asthma
COPD
give 5 examples of restrictive lung diseases
fibrosis IRDS oedema pneumothorax traumatic injury
why is asthma an obstructive lung disease
over-reaction of the bronchiole smooth muscle which restricts air movement
why is COPD an obstructive lung disease
- chronic bronchitis (inflammation of the bronchi)
- emphysema (destruction of alveoli, loss of elasticity)
why is fibrosis a restrictive lung disease
formation of XS fibrous connective tissue
idiopathic or asbestosis
restricts expansion of the lungs
why is oedema a restrictive lung disease
fluid build up around the alveoli reduces their ability to expand
why is pneumothorax a restrictive lung disease
lung can’t expand and has collapsed
no longer associated with the chest wall
how can traumatic injuries restrict lung expansion
pain
what is a common technique used to measure lung function
spirometry
what are the 2 types of measurements in spirometry
static - volume exhaled
dynamic - time taken to exhale a certain volume
what alters normal spirometry values
height, weight, age
what values can spirometry measure
things you can voluntarily do yourself
TV, IRV, ERV, IC, VC
what happens to FEV1/FVC in obstructive lung disease
reduced e.g. 45%
rate at which air is expelled is much slower
FVC is reduced
FRC may be increased
major effect is on airways so FEV is reduced to a greater extent that FVC
ratio is significantly reduced
what happens to FEV1/FVC in restrictive lung disease
increases e.g. 90%
absolute rate of airflow is reduced
total volume is reduced due to limitations on lung expansion
ratio remains constant or can increase as a large proportion of volume can be exhaled in the first second
limitations of using FEV1/FVC
obstructive: both FEV and FVC fall but FEV more so, ratio is reduced
restrictive: both FEV and FVC fall so ratio remains the same despite severe compromise of function
normal ratio isnt always indicative of health
FEF25-75
forced expiratory flow
average expired flow over the middle of an FVC
correlates w/ FEV1 but changes are generally more striking
normal range is greater
what is pulmonary ventilation
total air movement in/out of the lungs
relatively insignificant in functional terms
TV x breaths/min
L/min
what is alveolar ventilation
FRESH AIR getting to the alveoli and therefore available for gas exchange functionally more significant (tidal dead space)/breaths/min L/min declines with height from base to apex
how efficient is normal breathing
70%
30% gets stuck in the dead space
dead space has a huge impact on alveolar ventilation
what is the most important determinism of alveolar ventulation
depth of breathing due to the impact of dead space
hypoventilation
reduced alveolar ventilation
ml/min
hyperventilation
increased alveolar ventilation
ml/min
just because someone is breathing quickly doesn’t mean they are hyperventilating
how does the pressure volume curve vary across the lungs
varies between the apex and the base
at the base the given volume change is greater for a greater change in pressure
a small change in intrapleural pressure brings about a larger change in volume at the base compared with the apex
how does compliance change across the lungs
declines with height from base to apex due to alveoli at the apex being more inflated at FRC
at the base the alveoli are more compressed between the weight of the lung above and the diaphragm below and hence are more compliant on inspiration
Dalton’s law
total pressure of a gas mixture is the sum of the pressure in the individual gases
define partial pressure
the pressure of a gas in a mixture of gases is equivalent to the % of that gas in the mixture multiplied by the pressure of the whole gaseous mixture
partial pressure increases with increasing [gas] in the mixture
what are the normal partial pressures in the alveoli
100mmHg for oxygen
40mmHg for carbon dioxide
these are lower than in the atmosphere as the gas mixture is diluted down (dead space, residual volume and water vapour)
under normal conditions, these resting values remain fairly constant
what can alter alveolar PO2 and PCO2
hyper and hypoventilation
what changes occur during hyperventilation
PO2 rises - 120mmHg
PCO2 falls - 20mmHg
it is difficult to hyperventilate for a long period of time as we rely on carbon dioxide to drive the breathing cycle
what changes occur during hypoventilation
PO2 falls - 30mmHg
PCO2 rises - 100mmHg
commonly seen in pathology
define compliance
change in volume relative to change in pressure
represents the stretchability of the lungs (how easy it is to get air in)
changes in disease state/age
define high compliance
large increase in lung volume for a small decrease in pressure
healthy lungs
in some conditions lungs can be highly compliant but lose their elasticity
define low compliance
small increase in lung volume for a large decrease in intrapleural pressure
not good, common in conditions like fibrosis
work of respiration
normally effort of inspiration is recovered due to elastic recoil during expiration (expiration is passive)
emphysema - loss of elastic tissue means expiration requires effort
fibrosis - inert fibrous tissue means effort of inspiration increases
