Mechanics of Breathing Flashcards
Work of breathing
The effort required to pump the respiratory system
What are the muscles contract in inspiration?
In expiration?
-
Inspiration
- Diaphragm
- External intercostals–> pulling ribs out
- Sternocleidomastoid & scalene (increased work)
-
Expiration: Elastic recoil of lungs & chest wall
- Internal intercostals & abdominals (increased work)
What factors create elasticity of the lungs?
- Collagen
- Elastin
- Alveolar interdependence
- Surface tension (decreased by surfactant)
Pneumothorax
Air enters the pleural space, compromising the negative pressure
- Lungs collapse
- Chest wall expands
Lower lung ventilates more than upper lung, so transpulmonary pressure is typically negative.
At FRC (volume in the lung after normal expiration), transpulmonary pressure is ____ negative
less negative.
At FRC, lower alveoli are smaller than alveoli at the top.
Why does the lower lung have better ventilation than the upper lung?
At FRC, lower alveoli are smaller than alveoli at the top.
–> During inspiration, expansion of the thoracic cavity will cause greater expansion of the lower, smaller alveoli compared to the upper alveoli
–> More ventilation in the lower
(however, V/Q decreases as you go down because blood flow increases more rapidly than ventilation)
Explain the effect of airway closurewith aging
AIr trapping/airway closure: Smaller airways within the lung may collapse, trapping gas in the alveoli.
Older people lose elastic recoil –> airway closure in the lowermost regions of the lungs occurs at higher volumes and may be present at functional residual capacity.
What are the opposing forces that set functional residual capacity (FRC)?
Chest wall expansion
vs
Lung contraction
What impact does obesity have on the FRC & work of breathing?
How?
Obesity increases chest wall elasticity (decreases its compliance).
–> Decreases FRC & increases the work of breathing
Pulmonary fibrosis, pulmonary edema, and pneumonia all decrease the FRC and increase the work of breathing by
increasing the elasticity of the lungs
How do flow patterns change as you increase the flow rate?
As you go from slow flow rates to increasing velocity, air flow becomes
laminar -> transitional (eddy at branch points) -> turbulent
Increased airway resistance, such as from asthma, bronchitis, or COPD exacerbation, __creases the work of breathing
increases the work of breathing
Airway resistance contributes to
the negative pressure in alveoli needed for breathing
&
making intrapleural pressure even more negative
Factors that determine airway resistance:
Autonomic innervation & Physical properties of gas
-
Autonomic innervation
- Sympathetics cause relaxation, decreasing the diameter and reducing resistane to airflow
- Parasympathetics cause bronchoconstriction and increased respiratory secretions
-
Gas density
- Ex) At great depths under sea, atmospheric pressure and density are super high and increases work
- Ex) “Heliox” (helium+oxygen) is very light and decreases the work of breathing
Performing spirometry
Patient is asked to make a forced expiratory effort from total lung capacity (TLC) all the way down to residual volume (RV)
Generates a flow-volume curve
Dynamic compression of airways
A form of air-trapping that gets worse with the forced expiratory effort required during exertion.
When a lung-diseased person tries to exhale forcefully, the compression of the airways causes the walls to collapse proximally to the gas being exhaled.
What changes during dynamic compression of the airways?
- Normal passive expiration:
- Intrapleural pressure is -8 but the transmural pressure across the alveolar wall is +10 because of elastic recoil of lung tissue
- Alveolar pressure is 10 - 8 = +2, so it will leave into the atmosphere where pressure is 0.
- Forced expiration:
- Intrapleural pressure increases to +25 due to muscular contraction of the thoracic cavity, but transmural pressure is still +10.
- Alveolar pressure is 10 + 25 = +35, so it leave into the atmosphere where pressure is 0
- But at some point, the pressure keeping the airway open = intrapleural pressure forcing it closed -> airway collapses
- Lungs get smaller and smaller during exhalation while the equal pressure point moves down the airway.
Negative pressure breathing (normal)
We expand the thoracic cavity to increase the volume of the lungs –> Pressure drops (gets more negative) in the airways –> air enters the lungs
Since the negative pressure originates in the intrapleural pressure, the outer alveoli pull the alveoli open progressively from the outside-in
Positive pressure ventilation
Trying to increase pO2 or ventilation by applying a facemask or an endotracheal tube
Pressure moves from the inside out
Describe the characteristics of an exhale in obstructive disease on a flow-volume loop
- Higher TLC (which is where exhalation starts)
- Duller slope of the initial exhalation
- Normal sharp peak, but the slope drops off sooner
- Curve during exhalation is concave upwards,
- Residual volume is higher due to arrow closure
Characteristics of restrictive disease on a flow-volume curve
- Lower TLC & RV
- Duller slope after the peak
Explain the causes of uneven ventilation in various lung units of diseased lungs.
- Alveolus A: completely full by the end of inspiration and starts to shrink at the onset of expiration
- Alveolus B: thickening on walls from pulmonary fibrosis finished filling early on in the inspiratory cycle; isn’t getting ventilated optimally
- Alveolus C: airflow obstruction; pressure is still negative here when A & B arestarting to exhale
