Topic 1: Lung Volumes and Gas Transport
Tidal Volume: volume of air inspired and expired with a normal breath Vt
Residual volume: volume of air remaining after a maximal expiration effort
Total Lung Capacity: volume of gas contained within the lungs at the end of a max inspiration
Total Dead Space: volume of gas that does not eliminate CO2 and is made up of:
-
Anatomical Dead Space: volume of the conducting airway
-
Alveolar Dead Space: volume in the alveoli that are not or poorly perfused
Alveolar Volume: volume of gas per unit of time that reaches the alveoli, the respiration portions of the lungs
where gas exchange occurs.
Minute Volume: amount of gas expired per minute
Inspiratory Reserve Volume: extra volume of air that can be inspired after a normal Vt inspiration
Expiratory Reserve Volume: extra volume of air that can be expired after a normal Vt expiration
Vital Capacity: volume of air that can be expired following a max inspiration
Functional Residual Capacity: volume of gas remaining in the lungs at the end on a normal expiration
Inspiratory capacity: the maximum volume of gas that can be inspired from resting end-expiratory level
Topic 2: Functions of the Lung and Lung Mechanism
Basic functions of the lung
1.
Gas exchange: simple diffusion of oxygen from atmosphere into bloodstream and CO2 from
bloodstream into atmosphere: high partial pressure to low partial pressure
2.
Defence: protective function, able to deal with particles and microorganism
3.
Reservoir for blood
4.
Filtering of blood
5.
Metabolism
Lung Mechanism – Chest Wall
The properties of the rib cage and abdomen (elastic walls) determine the mechanical characteristics of the chest
wall as a whole. A number of factors must be overcome to enable air to move into/out of lungs:
1.
Elastic recoil of chest wall and lungs
2.
The friction resistance of the lungs and chest wall tissue and the frictional resistance of the airways to
the flow of air
3.
The inertia
Lung Mechanism – The Respiratory Muscles
Respiration is dynamic
Normal quiet breathing: diaphragm major inspiratory muscle. It acts on the rib cage by changes in pleural or
abdominal pressure. The inspiratory muscles are responsible for the anterio-posterior diameter of the rib cage
and stabilize the chest wall.
Diaphragm contracts to enlarge thoracic cavity: reduce intrathoracic pressure so that the lungs may expand to fill
their alveoli.
Expiration: passive, as the lung and chest wall are elastic and have a tendency to return to their equilibrium
position. Abdominal wall muscles most expiratory important. Contraction causes intra-abdominal pressure to
increase, and the diaphragm is pushed upwards.
Lung Mechanism – Elastic Properties of the Lungs
As pressure increases, lung volume increases. The lung distends easily at long lung volumes, however at high
lung volumes, the dispensable components of alveolar walls already have been stretched out and large
increases in pressure produces only small increases in volume.
Compliance: the ease with which something can be stretched or distorted
Elastance: the tendency for something to oppose stretch or distortion, as well as to its ability to return to its
original configuration
Lung Mechanism – Pressures during respiratory cycle
Airflow occurs as a result of change in pressure
1.
Intrapleural pressure: pressure between the visceral and parietal pleura (negative pressure)
2.
Alveolar pressure: pressure inside the alveoli
3.
Transmural pressure: pressure difference across the airway or across the lung wall
Transpulmonary pressure: difference in pressure between inside and outside of the lungs: always positive
during normal breathing
Transairway pressure: pressure difference between alveolar pressure and intrapleural pressure: important to
keep the airways open during expiration
At rest: alveolar pressure = atmospheric pressure (no airflow)
Alveolar pressure > intrapleural pressure (no airflow)
Transmural P = alveolar p (0 cmH2O) – Intrapleural pressure (-5cm H2O) = + 5cm H2O
Inspiration:
intrapleural pressure becomes more negative
Alveolar pressure < atmospheric pressure
= increase in transmural pressure
Transmural P = alveolar P (-1 cmH2O) – (-8 cmH2O) = +7 cmH2O
Interdependence: factor of alveoli: if an alveolus starts to collapse, the surrounding alveoli are stretches and
then recoil exerting expanding forces in the collapsing alveolus to open it.
Important for alveoli stabilization. Each alveolus supports each other.
Factors responsible for keeping the alveoli open:
1.
Surfactant
2.
Interdependence
3.
Functional residual capacity
Surfactant
-
Reduces muscular effort of breathing
-
Reduces elastic recoil of the lungs at low volume
-
Maintains the equality of size of alveoli during inflation/deflation
-
Lowers surface tension during deflation
Topic 4: Mechanics/Compliance/Airflow Resistance/WOB
Compliance
Work of breathing is determined by elastic recoil of the lungs and chest wall, and resistance to airflow
Compliance is the ease of which the lung is expanded. > volume change per unit of pressure change
Cl = lung compliance
Ccw = chest compliance
Increased compliance:
-
Floppy airways: inflate easily and does not take much change in pressure to get lots of volume
-
High compliance at Residual volume
Decreased compliance:
-
Stiff airways: they are harder to inflate and need bigger changes in pressures to get big changes in
volume
-
Furthermore, at low lung volumes, the lungs are relatively stiff, as well as at high volumes > ‘S-shaped
curve’
-
Decreased compliance at TLC: because the distensible components of alveolar walls have already been
stretched and therefore the recoil pressure is high as the lungs want to spring back
Factors affecting compliance:
1.
Lung volume
2.
Surfactant
3.
Pulmonary blood flow
4.
Age
5.
Disease
1.
Lung volume
At high lung volumes, the compliance is decreased because more pressure is required to stretch the already
stretched elastic tissues: elastic recoil is high
At low lung volumes, the compliance is decreased
-
Closed airways
-
Collapsed alveoli
2.
Surfactant
Surfactant is the substance secreted by the cells lining the alveoli, which lowers the surface tension of the
alveolar lining layer. This leads to:
-
Lower surface tension
-
Increased compliance
-
Work of expanding the lungs is reduced
-
Promotes stability of the alveoli
-
Keeps alveoli dry
3.
Pulmonary Blood Flow
Contributes to the stiffness of the lungs
-
Increased capillary blood flow leads to decreased compliance
Need good blood flow around the alveoli to exchange O2 and CO2. However, engorgement of blood around the
alveoli can create it to collapse, which will reduce lung volume back to the bottom of the S-shaped curve. >
Decreased compliance due to collapsed alveoli
4.
Age
As you increase in age, your chest wall stiffens > reduces chest movement > decreased chest wall compliance
With age, you lose elasticity > increase lung compliance
Therefore, floppy airways with a stiff thorax
5.
Disease
All diseases that have increased compliance (floppy airways) and early airway closure:
-
Obstructive Lung Disease
o
Emphysema (destroys elasticity)
o
Chronic Bronchitis
o
Asthma
o
Bronchiectasis
o
Cystic Fibrosis
All disease that have decreased compliance (stiff airways) and have a problem with expansion:
-
Restrictive Lung Disease
o
Interstitial lung disease > idiopathic pulmonary fibrosis
o
Scoliosis
o
Diabetes
Effects of Altered Compliance
1.
Lung Compliance affects diameter of airways
-
Increase Lung Compliance:
•
Less Elastic Support
•
Early airway closure
a.
Closing capacity is the lung volume at which some of the small
airway begin to close. The closing volume = closing capacity –
residual volume. Occurs at very low lung volumes in younger;
occurs at higher lung volumes in elderly due to the loss of its elastic
recoil (intrapleural pressure is less negative)
2.
•
Reduced airway diameter
•
Increase resistance to flow
•
Obstructive
Lung compliance and Chest wall compliance affect airflow
-
Decrease in chest wall and lung compliance:
•
Decrease airflow
•
Preferential ventilation of compliant lung units: air will follow path of least resistance
Airflow Resistance
Airflow resistance is the resistance of the respiratory tract to airflow during inspiration and expiration
Factors affecting airflow resistance:
1.
Character of airways
2.
Pattern of airflow
3.
Density/viscosity of gas
4.
Lung volume
1.
Airway character
-
Increase length of tube > increase resistance
-
Decrease diameter of tube > increase resistance
High resistance in trachea due to the sum of the diameters
Low resistance in alveoli due to the sum of the diameter
Progressively lower resistance in bronchioles from trachea down
Causes of airway narrowing:
1.
Inside the lumen: partial occlusion by secretions or foreign materials
2.
Wall of lumen: hypertrophy of mucus glands, oedema of bronchial wall, contraction of bronchial smooth
muscle
3.
Outside airways: loss of radial traction caused by destruction of lung parenchyma, local compression,
peri-bronchial oedema
2. Pattern of flow
Laminal > minimal resistance
Turbulent > greatest resistance (caused by sputum)
Transitional > nose, glottis, carina
4.
Lung Volume
High lung volumes; airways more distended
Low lung volumes: airway closure > increased resistance
Effects of altered resistance
-
Increased resistance > decreased airflow
-
Airflow takes the path of least resistance
Dynamic hyperinflation: is a compensatory mechanism: occurs when a new breath begins before the lungs
have reached the static equilibrium volume.
-
Increase CO2
-
Increase resistance > decrease expiratory flow and increase expiratory time
-
Vm maintained by increasing respiratory rate > decrease expiratory time > increase functional
residual capacity
Increased work of breathing
Due to increased resistance and decreased compliance
-
Increased oxygen consumption
-
Increased CO2 production
-
Increase work of respiratory muscles
•
Fatigue of respiratory muscles
•
Respiratory failure
Topic 5: Ventilation and Perfusion
Ventilation
Ventilation: process by which air moves into the lungs = tidal volume x frequency
Regions of the lungs individual lungs are referred to as dependent and non-dependent.
•
Dependent: compressed
•
Non-dependent: expanded
Ventilation preferentially distributed to the dependent regions
Intrapleural pressure more negative in non-dependent regions,
which results in greater distending pressure, thus greater
volumes > decreased compliance
Factors affecting distribution of ventilation:
1.
Weight of the lung
2.
Elastic properties of the lung
3.
Gravity
1.
Weight of the lung
Dependent region > compressed = increased compliance
-
Decreased volume
-
Bottom of ‘S-shaped curve’