Lecture 17 resp physiology work of breathing / lung volumes Flashcards
Work of Breathing
- Expand chest / lungs
2. Overcome airways resistance
what do you need to overcome to expand chest / lungs?
Stiffness / elasticity of lung/chest wall - Compliance
AND
Surface tension
(How hard is it to expand lungs?)
what is compliance lung stiffness?
measure of how easy it is to change lung volume
increase pressure = increase lung volume
Fibrosis
Low compliance = “stiff lungs”
Need to do more work to expand
increase pressure
Fluid lining alveoli
exerts surface tension which causes them to contract / resist expansion
-> need to overcome this force to expand lung
Surface tension reduced by
SURFACTANT
Surfactant
reduces surface tension in alveoli
i.e makes them easier to expand
Lack of surfactant/failure
results in stiff lungs
fibrosis
Surfactant
where produced?
by alveolar type II pneumocytes.
Major constituent is phospholipids
Lowers surface tension.
surfactant
why do you lower surface tension?
Reduces attractive forces between fluid molecules lining alveoli.
Easier to increase lung size i.e. increased compliance
what does Lack of surfactant/failure to produce
adequate surfactant result in?
stiff lungs
Surfactant is absent in premature infants what syndrome does it result in?
development of respiratory distress syndrome (RDS)
what air flow is during quiet breathing?
laminar flow
what determines resistance to flow?
Poiseuille’s Law
Poiseuille’s Law
R = 8nl / πr4
R=resistance;
n = viscosity;
l = length;
r = radius
R inversely proportional to r4
R inversely proportional to r4
what does 1/2 r =?
what does 2r =?
16 x R
1 / 16 x R
what are some important elements in airways resistance (e.g. asthma)?
Bronchoconstriction /Bronchodilation
Bronchi
Main area airway resistance
where does most of the resistance to airflow rise?
in upper airways and the first 6 generations of the lower airway
do small airways contribute a lot or very little to airway resistance and why?
Very little
due to high CSA
Ventilation → Pressure gradient →
Airflow
Need to do work to generate a big enough pressure gradient to:
(also = work of breathing?
Expand lung - overcome lung stiffness
Move air - overcome airways resistance
How big a breath can you take?
Measurement of Lung Function
Spirometry
what are some Measurement of Lung Function tests?
Pulmonary Function tests, Lung Function tests
Spirometry
- Lung volumes – How much?
2. Air flow rates – How fast?
Pulmonary Function Tests (Spirometry)
Mechanical / Digital
Common, simple test
Can measure how much and how fast
Test response to therapy
Explain what Pulmonary Function Tests (Spirometry) Mechanical / Digital is?
Breathe into/out of a hollow bell inverted over water – measures volume inspired / exhaled
Spirometry: How Much?
Divide the air in lungs into four volumes: what are they?
Tidal volume
Inspiratory reserve volume
Expiratory reserve volume
Residual volume
whats Tidal volume?
500 ml (VT). Volume of air moved in and out during normal quiet breath
whats Inspiratory reserve volume?
3300ml (IRV).
Extra volume that can be inspired with maximal inhalation - external intercostal muscles
whats Expiratory reserve volume?
1000ml (ERV).
Extra volume that can be exhaled with maximal effort - internal intercostal and ab muscles
whats Residual volume?
1200ml (RV).
Volume remaining in lungs after maximal exhalation.
Lung capacities
These volumes can then be grouped into capacities:
what are they?
Vital capacity
Total lung capacity
Inspiratory capacity
Functional residual capacity
whats Vital capacity?
~4800ml.
Maximal breath in to maximal out – volume of air you can shift in/out of lungs
whats Total lung capacity?
~6000ml.
Total volume in lungs when maximally full = VC + RV
whats Inspiratory capacity?
Tidal volume + IRV
whats Functional residual capacity?
~2200ml.
Volume at end of normal breath out
(Equilibrium point for thorax / lungs)
Spirometry: how fast?
Forced measurements give info about flow.
what are they?
Forced Vital capacity (FVC)
Forced expiratory volume in one second FEV1
whats Forced Vital capacity (FVC)?
maximum breath into maximum breath out (VC) - forced out as hard as possible
whats Forced expiratory volume in one second FEV1?
How much of FVC comes out in first second
Reduced with diseases causing resistance to airflow
(airways obstruction e.g. asthma) or small lungs
what diseases causes resistance to airflow?
airways obstruction e.g. asthma or small lungs
reduce FEV1
whats FEV1/FVC ratio?
Normal ~ 80%
< 0.70 indicates airways obstruction
what can Spirometry identify?
Obstruction and restriction airflow diseases
whats an Obstructive disease?
Increase Resistance to airflow
Asthma
Chronic obstructive lung disease (Chronic bronchitis and Emphysema)
whats a restrictive disease?
decrease lung volume (reduced lung capacity)
Fibrosis (reduced lung compliance)
Chest wall abnormalities
Respiratory muscle disease
how much do we breathe in / out?
Volume of single breath = Tidal Volume = VT = 500 ml (0.5L)
Respiratory rate =
Frequency (f) = 10 – 12 / minute
Total (mouth) ventilation (Respiratory minute volume - VE) = Frequency x Tidal volume
Respiratory minute volume (VE)
= frequency (f) x Tidal Volume (VT)
VE = Frequency (f) x VT units
VE = ml / min f = breaths / min VT = ml / breath
whats DEAD SPACE (VD)?
150ml
Some of the inhaled air never gets to the alveoli so cannot gas exchange
About 150 ml of each breath just fills upper airways and is of no use for gas exchange
What’s really important is the amount of air that gets into the alveoli. whats this called?
….Alveolar Ventilation VA
whats Alveolar Ventilation VA?
measures the flow of
fresh gases into and out of the alveoli–
VA = f x (VT - VD)
Hypoventilation
Low VA
Hyperventilation
High VA
whats the equation for pulmonary ventilation?
Pulmonary ventilation VE = frequency x tidal volume
whats the equation for Alveolar Ventilation?
Alveolar ventilation VA = frequency x (tidal volume – dead space)
