Mechanics of Breathing 2 Flashcards
What is functional residual capacity (FRC) and what is it dependent on?
It is the volume of air present in the lungs after passive expiration
It is dependent on the compliance of the lungs and chest wall
What happens at FRC?
The opposing elastic recoil forces of the lungs and chest wall are in equilibrium
There is no exertion by the diaphragm or other respiratory muscles
What is compliance?
The change in lung volume per unit change in intrathoracic pressure
What does compliance measure?
It is a measurement of the ability of the lungs and chest wall to stretch and expand
What are the 2 types of compliance?
static compliance and dynamic compliance
What is the equation for compliance?
C = change in volume/change in pressure
What is static compliance?
The change in volume for any given applied pressure when there is no air flow
What is static compliance a function of?
A function of elastic recoil of the lung and surface tension of the alveoli
What is dynamic compliance?
The compliance of the lung at any given time during the actual movement of air
Why is dynamic compliance always less than static compliance?
Dynamic compliance includes the pressure required to generate flow by overcoming resistance forces
There is always a degree of airway resistance
What is an oesophageal balloon used to measure?
Intrathoracic pressure
This is the pressure that is present between the 2 layers of pleura
What is the assumption made when measuring intrathoracic pressure?
It is measured indirectly by assuming that the pressure in the thorax and the pressure in the chest wall are equal, at the end of either inspiration or expiration
Why are the lungs both compliant and elastic?
They are elastic to recoil and push air out during expiration
They are compliant to fill with air during inspiration
At the start and end of expiration, what is alveolar pressure?
At the start and end of expiration, alveolar pressure is in equilibrium with atmospheric pressure
What is recoil pressure of the lung?
The pressure that drives the lung to collapse
The lung has a natural tendency to collapse on itself
How is recoil pressure determined?
Determined by the different between alveolar pressure and intrapleural pressure
At maximal expiration, what is alveolar pressure and why?
Breathing all the way out keeps the glottis open so alveolar pressure is zero
What is maximal intrapleural pressure and recoil pressure after maximal expiration?
Maximum intrapleural pressure is -3 cm of water
Recoil pressure is +3 cm of water
What is the equation for recoil pressure?
Alveolar pressure - intrapleural pressure
At maximal expiration, what lung volume is present?
+3 cm of water is at the residual volume
After expiration and during normal tidal breathing, what lung volume is present?
Functional residual capacity is the volume of air left in the lungs
After expiration and during normal tidal breathing, what is intrapleural pressure?
around -5 cm of water
After expiration and during normal tidal breathing, what is recoil pressure?
alveolar pressure is still 0 as the glottis is open so
0 - - 5 = +5
recoil pressure is +5 cm of water
At peak inspiration, what is the intrapleural pressure?
A very negative pressure is generated in the chest during maximal inspiration
intrapleural pressure is -30 cm of water
What is recoil pressure at peak inspiration and what lung volume is this equivalent to?
+30 cm of water
total lung capacity
Which 3 points are needed to draw a lung compliance curve?
- residual volume
- functional residual capacity
- total lung capacity
When is compliance low and high and what does this mean?
It is low when the lung is expanded - a lot of effort is required to expand the alveoli more
It is high after expiration meaning less effort is needed to expand the alveoli
Which pressures determine chest wall compliance?
Intrapleural pressure and barometric pressure
How is recoil pressure of the chest wall calculated?
intrapleural pressure - barometric pressure
What is the value of atmospheric pressure?
It is zero unless the altitude is altered
How do you remove the effect of the lung when measuring chest wall compliance?
Breathe all the way out, close the glottis and relax the muscles
Closing the glottis prevents air from entering the lungs
After maximal expiration, what is the recoil pressure of the lungs?
minimum recoil pressure is generated after maximal expiration
this is -30 cm of water and is residual volume
Once the glottis is open, what is the recoil pressure (&intrapleural pressure) of the chest wall?
-5 cm of water
this is the functional residual capacity
During maximal inspiration, how is total lung capacity reached?
The glottis is closed and the muscles are relaxed
The volume of air in the lung is retained by closing the g,lottis
What is recoil pressure of the chest wall at maximal inspiration?
The highest possible intrapleural pressure of +3 cm of water is reached
Chest wall compliance is +3 cm of water
What does the overall effect of compliance take into account?
Both chest wall and lung compliance
What is the overall effect of compliance at maximal expiration?
the lungs are compliant
the resistance to compliance caused by the chest wall overrides the stretchiness of the lungs
What is significant about functional residual capacity on the compliance curve?
It is a relaxation and equilibrium point
The chest wall and lung recoil pressures are equal but opposite
Where does FRC appear on the compliance curve?
where the curve crosses the zero pressure point on the y axis
What is the problem with having a lower FRC?
Normal tidal breathing becomes closer to residual volume
There is less expiratory reserve volume so if there is a need to increase ventilation, the capacity to increase the volume of ventilation is diminished
During tidal breathing, why is there a difference between compliance during inspiration and expiration?
It is not caused by a difference in compliance as the lung elasticity is the same
It is caused by changes in the calibre of the airways
Why does hysteresis occur?
the elastic nature of the tissues and airway resistance
What is hysteresis?
any process where the future state of a system is dependent on its current and previous state
it means that the compliance of the lung is different in inspiration and expiration
What is the difference in lung volumes during inspiration and expiration?
Lung volume at any given pressure during inspiration is less than the lung volume at the same pressure during expiration
Why does hysteresis occur in dynamic compliance curves?
It occurs due to airway resistance, which is a function of flow rate
In a dynamic compliance curve, when is flow rate maximal?
flow rate (and resistance) is maximal at the beginning of inspiration and the end of expiration
Why does hysteresis occur in a static compliance curve?
There is no resistive component
Hysteresis occurs due to viscous resistance of surfactant and the lungs
how is a heavy smoker defined?
someone who smokes at least 1 cigarette each day
What happens in pulmonary fibrosis?
elastic tissue becomes replaced with fibrous scar tissue
this causes lung tissue to become thickened and stiff
Where does scarring normally occur in pulmonary fibrosis?
A network of tissue called the interstitium, which surrounds the alveoli
What is the effect of scar tissue developing in pulmonary fibrosis?
Scar tissue is not elastic
Lungs lose their ability to transfer oxygen into the bloodstream as they cannot take in as much oxygen from the air during inspiration
what are the main symptoms of pulmonary fibrosis?
Breathing requires more effort so patients are breathless just from walking
a persistent cough
feeling tired all the time
what are the main causes of pulmonary fibrosis?
- being exposed to certain types of dust including wood, metal dust or asbestos
- being exposed to allergens
- side effect of a drug
- for most cases, a specific cause cannot be found
What is kyphoscoliosis?
What is it caused by?
caused by a decrease in chest wall compliance
restrictive lung disease that causes extrapulmonary restriction of the lungs
What do the terms “kyphosis” and “scoliosis” describe?
Kyphosis - anterior-posterior curvature of the spine
Scoliosis - lateral displacement of the spine
What is the effects of kyphoscoliosis?
restriction of the lungs leads to an impairment of pulmonary functions and respiratory failure
the ribs cannot articulate properly with the spinal column
What is the main cause of kyphoscoliosis?
80% of cases are idiopathic - the cause is unknown
What is a circumferential burn?
a burn that goes all the way around the body
what happens to the tissue after a circumferential burn?
the tissue becomes burnt and forms scar tissue
this is not elastic and restricts the expansion of the chest
What is the effect of a circumferential burn?
reduced chest wall compliance which hinders ventilation
How is scar tissue removed in a circumferential burn?
a surgical method called an escharotomy
what occurs in emphysema?
the alveolar membranes are destroyed causing the alveoli to fuse together
this means there is less lung tissue
How does emphysema affect gas exchange?
surface area of the alveoli are reduced so the volume of oxygen diffusing into the blood is also reduced
how does emphysema affect compliance of the lung?
elasticity and compliance are increased
Why is compliance in different parts of the lung not the same?
due to the effect of gravity
Why is compliance greatest at the base of the lung?
the same pressure is applied to every part of the lung as intrapleural pressure is the same
the bottom of the lung has a greater increase in volume as ventilation is greater at the bottom
What is closing capacity of the lung?
It is the point during expiration when small airways begin to close
it increases with age and decreased pulmonary blood flow
What affect does age have on the alveoli?
as you get older, some of the alveoli become closed during expiration
closed alveoli have low compliance
What happens if closing capacity exceeds FRC?
Alveoli in dependent parts of the lung are poorly ventilated
They will be closed or collapsed during normal tidal breathing
What is the problem with increased closing capacity?
It decreases the compliance of the lung
What would happen to the alveoli if there was no surfactant and why?
The alveoli are interconnected by bronchioles so all of the air would empty into the larger alveolus
All the alveoli are different sizes
What is the Law of Laplace?
P = 2t/r
P = pressure t = surface tension r = radius of alveolus
What does the Law of Laplace state?
The pressure needed to be applied to a sphere to prevent it from collapsing is inversely proportional to the radius
What is the role of surface tension?
Surface tension opposes pressure and acts inwards to create a collapsing force
A greater surface tension leads to a greater collapsing motion
What is the role of surfactant?
There is a tendency for smaller alveoli to collapse and empty into larger alveoli
This motion is overcome by altering the surface tension within the alveolus by producing surfactant
What cells produce surfactant?
specialised alveolar epithelial cells called type II cells
What is surfactant made from?
It comprises 90% phospholipid and 10% protein
Which disease shows a deficiency in surfactant?
What happens?
infant respiratory distress syndrome
collapsing pressure is not equalised so the smaller airways collapse
What are the 3 roles of surfactant?
- increases pulmonary compliance
- prevents atelectasis
- aids alveolar recruitment and minimises alveolar fluid
What happens in atelectasis?
there is collapse or closure of a lung resulting in reduced or absent gas exchange
the alveoli are deflated down to no or little volume
How does surfactant prevent alveolar collapse in alveoli of different sizes?
each alveolus produces the same amount of surfactant
the surfactant becomes more dispersed as alveolar volume increases
it equalises pressure between alveoli of different sizes
In which size alveoli does surfactant have the greatest effect on reducing surface tension?
smaller alveoli
in larger alveoli, the surfactant is more dispersed around the inner wall
How is the potential energy of the lungs generated?
the energy used to overcome elastic forces in inspiration is stored as potential energy
potential energy is dissipated in expiration
How is work expended during breathing?
Work is expended in the form of heat to overcome resistance forces of air passing through the airways
How is the work of breathing calculated?
work = volume x pressure
What is airway resistance?
The opposition to the flow os air caused by forces of friction
What is the resistance to flow like in laminar flow?
resistance is quite low
only a small driving pressure is required to produce a certain flow rate
What is the resistance to flow like in turbulent flow?
resistance is much larger
a much larger driving pressure is required to produce the same flow rate
What factors increase resistance to air flow?
reducing the radius of the airway
when the airways begin to branch
What % of energy expenditure is taken up by breathing in a healthy person?
2 - 5 %
At maximal hyperventilation, what % of energy expenditure is taken up by breathing?
30%
how is the work of breathing minimised in restrictive and obstructive lung conditions?
restrictive - taking rapid shallow breaths
obstructive - taking large volume, slow breaths
