How is air drawn into the lungs?
By expanding the volume of the thoracic cavity
What is work done during breathing doing?
Moving the structures of the lungs and thorax to overcome the resistance to flow of air through the airways
What is the pleural space?
The space bewteen the lungs and thoracic wall
What is the pleural space normally filled with?
A few millimetres of fluid
What is the purpose of the fluid in the pleural space?
The surface tension of which forms a pleural seal holding the outer surface of the lungs to the inner surface of the thoracic wall
What is the result of the pleural fluid holding the lungs to the thoracic wall?
The volume of the lungs changes with the volume of the thoracic cage
What happens if the integrity of the pleural seal is broken?
The lungs will tend to collapse
What happens in a pneumothorax?
Air gets in between the two layers of the pleura, fluid surface tension is lost and the lungs collapse
What is meant by lung compliance?
The 'stretchiness' of the lungs
What is lung compliance defined as?
The volume change per unit pressure change
What does high compliance mean?
Lungs are easy to stretch
How is compliance measured?
By measuring the change in lung volume for a given pressure
What does a greater lung volume mean for compliance?
What is it more usual to calculate than compliance?
Why is it more usual to measure specific compliance?
Becasue, even with the constant elasticity of lung structures, compliance will also depend on the starting volume from which it is measured
How is specific compliance calculated?
Volume change per unit pressure change / Starting volume of lungs
Draw a diagram illustrating the compliance for;
- Elastic lungs
- Normal lungs
- Stiff lungs
What do the elastic properties of the lungs arise from?
- Elastic tissue in the lungs
- Surface tension forces of the fluid lining the alveoli
What is meant by surface tension?
The interactions between molecules at the surface of a liquid
What is the effect of surface tension on stretchiness?
It makes the surface resistant to stretching
What does a higher surface tension mean for compliance?
The higher the surface tension, the harder the lungs are to stretch and therefore the lower the compliance
What happens to the surface tension of the lungs at low lung volumes?
It is much lower than expected
Why is the surface tension of the lungs much lower than expected at low lung volumes?
Due to the disruption of interactions between surface molecules by surfactant
What produces surfactant in the lungs?
Type 2 alveolar cells
What is surfactant?
A complex mixture of phospholipid and proteins, with detergent properties
Where does the hydrophilic end of surfactant molecules lie?
In the alveolar fluid
Where does the hydrophobic end of surfactant molecules lie?
Projects into alveolar gas
What is the result of the position of the hydrophilic and hydrophobic ends of the surfactant molecule?
They float on the surface of the lining fluid, disrupting interaction between surface molecules
When does surfactant reduce surface tension?
When the lungs are deflated, but not when fully inflated
What is the result of surfactant only reducing surface tension when the lungs are deflated?
Little breaths are easy, and big breaths are hard, and it takes less force to expand small alveoli than it does large ones
What do the alveoli form?
An interconnecting set of bubbles
What is Laplace's law?
Pressure is inversely related to the radius of a bubble
What would happen if Laplace's law was applied to alveoli?
Large alveoli would 'eat' small ones
What happens as alveoli get bigger?
With respect to surface tension
The surface tension in their walls increases
Why does the surface tension in the walls of alveoli increase as they get bigger?
Because surfactant is less effective
What is the result of surface tension in the walls of large alveoli being higher?
Pressure stays high and stops them from 'eating' smaller alveoli
How must energy be expended in the lungs, in addition to work done against the elastic nature of the lungs?
To force air through the airways
What is true of the flow in most of the airways of the lungs?
It is laminar
What determines the resistance of an airway to flow, when flow is laminar?
What is Poiseulle's Law?
The resistance of a tube sharply increases with a falling radius
What is true of the combined resistance of small airways?
It is normally low
Why is the combined resistance of small airways normally low?
Because they are connected in parallel over a branching structure, where the total resistance to flow in the downstream branches is less than the resistance of the upstream branch
Where does most of the resistance to breathing reside?
In the upper respiratory tract
What is work done against in the lungs?
- The elastic recoil of the lungs and thorax
- Resistance to flow through airways
What produces the elastic recoil of the lungs and thorax?
- Elastic properties of the lungs
- Surface tension forces in the alveoli
When is resistance to flow through airways important to consider?
In disease, as it is often affected
Of little significance in health
What % of oxygen consumption does the work of breathing consume at rest?
0.1% - is very efficient
What do the bronchioles do during inspiration?
Use their smooth muscle to increase their radius
What is the purpose of bronchioles increasing their radius during inspiration?
It decreases their resistance (due to Poiseulle's Law), allowing air to be drawn easily through them into alveoli
What happens in spirometry?
The patient fills their lungs from the atmosphre, and breathes out as far and fast as possible through a Spirometer
What does simple spirometry allow for?
Measurement of many lung volumes and capacities
What measurement taken from spirometry is particularly significant?
What can be used to predict the vital capacity of an individual of known age, sex, and height?
Why may vital capacity be less than normal?
Because the lungs are not;
- Filled normally in inspiration
- Emptied normally in expiration
What is meant by forced vital capacity?
The maximum volume that can be expired from full lungs
What is meant by forced expiratory volume in one second (FEV1)?
The volume expired in the first second of expiration from full lungs
What is FEV1 affected by?
How quickly air flow slows down
When is FEV1 low?
If the airways are narrowed
When may the airways be narrowed?
Draw a diagram illustrating FVC and FEV1
How can restrictive and obstructive deficits be separated?
By asking patients to breathe out rapidly from maximal inspiration through a single breath spirometer, which plots volume expired against time
What is maximal filling of the lungs determined by?
The balance between the maximum inspiratory effort and the force of recoil of the lungs
When is a restrictive deficit produced?
If the lungs are unusually stiff, or inspiratory effort is compromised by muscle weakness, injury, or deformity
What is the effect of a restrictive deficit on FVC?
It reduces it
What will be true of FEV1 in a patient with a restrictive deficit?
FEV1 > 70% FVC
Draw a graph illustrating the difference between normal lungs, and lungs with a restrictive deficit
What happens to small airways during expiration, particularly when forced?
They are compressed
What is the result of the compression of small airways during expiration?
It increases flow resistance, eventually to the point where no more air can be driven out of the alveoli
What happens if the airways are narrowed?
Expiratory flow is compromised much earlier in expiration
What is produced when the airways are narrowed?
An obstructive deficit
What is the effect of an obstructive deficit on FEV1?
It is reduced
What is the effect of an obstructive deficit on FVC?
It is relatively normal
Draw a graph illustrating the difference between normal lungs, and lungs with a obstructive deficit
What a flow volume curves?
A graph of volume expired against flow rate
What are flow volume curves derived from?
A vitalograph trace
What is happening at points A-D on this flow volume curve?
- A - When the lungs are full, the airways are stretched so resistance is at minimum, so flow is therefore at peak expiratory flow rate (PEFR)
- B-D - As the lungs are compressed, more air is expired and the airways begin to narrow, so resistance increases and flow rate decreases
In normal individuals, what is peak flow most affected by?
The resistance of large airways
In disease processes, what can peak flow be affected by?
Severe obstruction of the smaller airways
Give an example of a disease where there may be severe obstruction of the small airways
What does mild obstruction of the airways produce?
A 'scooped out' expiratory curve
What affect will a severe obstruction have on PEFR?
It will reduce it
Draw a graph of flow against volume for inspiration and expiration that would be seen in;
- A normal person
- Early small airways obstruction
- Chronic obstructive disease
- Fixed large airway obstruction
- Variable extrathoracic large airway obstruction
- Restrictive disease
What is the Helium Dilution Test used to measure?
Functional Residual Capacity (FRC)
What is FRC used to calculate?
What is the advantage of using helium in tests?
- It is an inert, colourless, odourless, tasteless gas that is not toxic
- It cannot transfer across the alveolar-capillary membrane, and therefore is not contained within the lungs
How is a helium dilution test carried out?
- At the normal tidal expiration, the patient is connected to a circuit, which is connected to a container containing a gas mixture with a known helium concentration (C1) and volume (V1)
- The patient continues to rebreathe into the contained until equilibrium occurs
What is lung volume equal to at the end of tidal expansion?
Functional residual capacity
What is FRC equal to?
How long does it usually take for equilibrium to occur in a helium dilution test?
What is the new concentration of helium at equilibrium termed in a helium dilution test?
How is FRC calculated from a helium dilution test?
- C1 x V1 = C2 x V2
- V2 = V1 + FRC
- Since C1, V1, and C2 are known, FRC can be calculated
What is residual volume equal too?
FRC - ERV
How can ERV be measured?
What does the Carbon Monoxide Transfer Factor measure?
The rate of transfer of CO from the alveoli to teh blood in ml per minute per kPa (ml/min/kPa)
What is the purpose of the CO transfer factor?
It is a way of measuring the diffusion capacity of the lung
Why is the CO transfer factor a way of measuring the diffusion capacity of the lung?
Because the amount transferred will depend on how well gas diffusion takes place
Why is inhaled CO used in a transfer factor test?
Because of its very high affinity for Hb
In a CO transfer factor test, what can we assume the ppCO is?
Why can we assume the ppCO is 0 in a CO transfer factor test?
Because almost all of the CO entering the blood binds to Hb, very little remains in the plasma
What is the result of the ppCO being 0 in a CO tranfer factor test?
The concentration gradient between alveolar CO and capillary ppCO is maintained
What is the result of the maintainence of the concentration gradient between alveolar ppCO and capillary ppCO?
The amount of CO transferred from the alveolli to the blood is limited only by the diffusion capacity of the lung
How is a CO transfer factor test carried out?
- The patient performs a full expiration, followed by a rapid maximum inspiration of a gas mixture composed or air, a tiny fraction of CO and a fraction of inert gas such as helium
- The breath is held for 10 seconds
- The patient exhales, and gas is collected mid-expiration, to gain an alveolar sample
- Concentration of CO and inert gas is measured
- From these measurements, the CO Transfer Factor is measured
Why is only a tiny fraction of CO used in a transfer factor test?
Because it is toxic
Why is a fraction of inert gas used in a CO transfer factor test?
To make an estimate of total lung volume
What does a nitrogen wsahout test measure?
Serial (anatomical) dead space
How is a nitrogen washout test carried out?