Draw lung volumes and capacities chart:
Lung capacities are always the sum of:
two or more volumes
Volume in our lungs increases when:
pressure in the lungs decreases
Functional residual capacity (FRC):
- the volume of gas remaining in the lungs after a normal tidal expiration (passive expiration)
Is there any airflow into or out of the lungs at FRC?
No. Avleolar pressure = atmospheric pressure (0).
State of the lung and the chest wall at the end of expiration (FRC), when all the respiratory muscles are relaxed:
- inward elastic recoil of lung balanced by outward elastic recoil of chest wall.
- alveolar pressure equal to atmospheric pressure, no airflow occurs.
- transmural pressure negative.
Negative-pressure breathing:
- lowering alveolar pressure below atmospheric pressure to create air flow into the lungs
Positive-pressure ventilation:
- Air flow into the lungs caused by raising the pressure at the nose and mouth above alveolar pressure.
Steps in inspiration (muscles, pressures, etc.):
- contraction of muscles of inspiration.
- intrapleural pressure becomes more negative.
- transmural pressure difference increases.
- alveoli distend, decreasing alveolar pressure below atmospheric pressure, which causes air to flow into alveoli.
Transmural Pressure =
Transmural Pressure = Pinside - Poutside
(alveolar pressure - intrapleural pressure)
How does transmural pressure increase?
- inspiratory muscles contract, expanding the thoracic volume and increasing the outward stress on the lung, the intrapleural pressure becomes more negative.
- TP = alveolar pressure - intrapleural pressure
Boyle's Law in relation to breathing/alveoli:
P = 1/V
- as the alveolar volume increases, alveolar pressure decreases.
- when alveolar pressure drops below atmospheric pressure, inspiration occurs.
What causes negative intrapleural pressure at rest, and what is the resting negative intrapleural pressure value?
- Value: –3 to –5 cm H2O.
- Caused by the mechanical interaction between lung and chest wall.
- Elastic recoil of lung pulls inward.
- Elastic recoil of chest wall pulls outward.
Alveolar pressure equation:
intrapleural pressure + alveolar elastic recoil pressure
- creating a more negative intrapleural pressure will create a negative alveolar pressure and pull air into the alveoli from the atmosphere.
How do alveoli located centrally in the lung expand in response to a more negative intrapleural pressure?
- structural interdependence of alveolar units.
- Alveolar septa transmit the pressure difference across the outermost alveoli to inner alveoli.
Difference between inner and outer alveoli due to structural interdependence in negative-pressure breathing:
- mechanical stress transmitted from exterior alveoli (those closest to the chest wall) to more interior alveoli, so the exterior alveoli might be more distended.
Difference between inner and outer alveoli due to structural interdependence in positive-pressure ventilation:
- Lungs must push against diaphragm and rib cage to move them.
- Outermost alveoli might be more compressed than those more interior.
Status of the lungs at FRC:
- end of passive expiration.
- inspiratory muscles relaxed.
- alveolar pressure equal to atmospheric pressure (0).
- no air movement occurs.
The inward elastic recoil of lung is equal to:
- transmural pressure difference
- (Alveolar pressure - Intrapleural Pressure)
The muscles of inspiration include:
- diaphragm
- external intercostals
- sternocleidomastoid
- trapezius
- muscles of vertebral column
Primary muscle of inspiration:
diaphragm
When a person is in the supine position, the diaphragm is responsible for how much of the air that enters the lungs during normal quiet breathing (eupnea)?
2/3
When a person is standing or seated in an upright posture, the diaphragm is responsible for how much of the air that enters the lungs during normal quiet breathing (eupnea)?
1/3 to 1/2
Accessory inspiratory muscles and when they become activated:
- activated when tidal volume needs to be increased.
- sternocleidomastoid
- trapezius
- muscles of vertebral column
- sternocleidomastoid
- trapezius
- muscles of vertebral column
Expiration is mainly driven by:
- elastic recoil of alveoli.
- this decreases alveolar volume, which increases alveolar pressure above atmospheric and leads to air flowing out of alveoli.
PASSIVE PROCESS
Describe the dashed line and solid line in this graph:
- dashed line predicts the changes in intrapleural pressure necessary to overcome the elastic recoil of alveoli.
- solid line more accurate; includes additional pressure work that must be done to overcome resistance to airflow and tissue resistance.
Changes in intrapleural pressure during inspiration:
- Contraction of the inspiratory muscles causes intrapleural pressure to continuously become more negative as the lungs are pulled open and the alveoli are distended.
When is intrapleural pressure the most negative?
at the end of inspiration
Changes in alveolar pressure during inspiration:
- 0 at start of inspiration.
- becomes more negative, and then less negative.
- 0 at end of inspiration and start of expiration.
When is there no airflow into or out of the lungs?
- when alveolar pressure = atmospheric pressure.
- end of expiration.
- end of inspiration.
Lung compliance is:
- the ease at which the lung volume can be expanded.
Lung elastance is:
- the natural ability of the lung to return to its original shape after some external force which has acted upon it is removed.
Compliance equation:
C = ∆V/∆P
Draw lung pressure/volume graph for inspiration and expiration (isolated lungs):
Slope = compliance (∆V/∆P)
Difference in distensibility of the lungs at low and high volumes:
-
Low volumes: lung distends easily
-
High volumes: distensible components of alveolar walls have already been stretched, and large increases in transpulmonary pressure yield only small increases in volume.
Why is there a difference in curve for inflation and deflation of the lungs in the graph below?
DIFFERENCE IS CALLED HYSTERESIS.
- Surfactant has less effect on decreasing surface tension during inspiration.
- Recruitment of alveoli during inspiration requires more energy/work.
Equation to determine total compliance:
Transmural pressure difference for lungs:
alveolar pressure - intrapleural pressure
Transmural pressure difference for chest wall:
intrapleural pressure - atmospheric pressure
Draw compliance curves for normal lung, emphysema, and fibrosis:
- emphysema = more compliance
- fibrosis = less compliance
Emphysema increases the compliance of the lungs because:
- it destroys the alveolar septal tissue that normally opposes lung expansion.
- decreased alveolar elastic recoil.
Fibrosis decreases the compliance of the lungs becuase:
- it makes the lungs less compliant, or “stiffer.”
- increase alveolar elastic recoil.
Why does compliance change at different lung volumes?
- Alveoli are more compliant and have less elastic recoil at low volumes.
- Alveoli become less and less compliant as volume increases and also have more elastic recoil.
Woman inspires 500ml from spirometer.
Intrapleural pressure before inspiration (-5).
End inspiration (-10).
What is lung compliance?
Normal pulmonary compliance value:
0.1 - 0.2 L/cm H2O