Module 3 Section 2 (Mechanics of Breathing)

Question 1

Describe the different pressures involved in the mechanics of breathing.

Answer 1

Atmospheric Pressure - (PB):

• Also called barometric pressure
• It’s the pressure exerted by the weight of the air in the atmosphere on the Earth’s surface.
• At sea level = 760 mm Hg and this value decr as you gain altitude.
• Given that, even when standing, there is not enough difference in height b/w the lungs and the nose/mouth -> PB is the same in both places so we effectively treat it as if it was = 0.

Alveolar Pressure - (PA):

• This is the pressure in the alveoli
• Also called intrapulmonary pressure.
• At the end of inspiration, PA is the same as PB at 0 cmH2O.

Pleural Pressure - (Ppl):
- This is the pressure in the pleural space.
- Also called the intrapleural pressure and it closely
approximates the intrathoracic pressure.
- It’s negative to PB and is normally around -5 cmH2O.
- It is negative b/c the lungs want to collapse yet the chest wall wants to expand.

Transpulmonary Pressure - (Ptp):

• This pressure is the difference b/w the PA and PpI.
• Also referred to as lung recoil pressure (Pl) or transmural pressure.

Question 2

Applying the law of Laplace, describe why alveoli do not collapse.

Answer 2

The division of the lung into millions of alveoli creates a very large surface area for gas exchange, but the differences in alveolar size can cause problems for alveolar stability.

The inward forces of alveolar surface tension are working to collapse the alveoli.

The law of LaPlace states that the magnitude of this collapsing pressure is directly proportional to the surface tension and inversely proportional to the radius of the alveoli, as seen in the following equation:
2T (surface tension) / r (alveolar radius) = P (collapsing pressure)

Question 3

Describe the pressure changes that occur to enable inhalation and expiration.

Answer 3

Inhalation and exhalation are the result of changes in pleural and alveolar pressure.
- Before inspiration
(at the end of the preceding expiration), there is no flow as the PA = PB.
- During inspiration, a pressure gradient is established from atmosphere to alveoli which results in inspiratory flow.
- Inspiration ends as the contraction of inspiratory muscles decreases, allowing lung recoil pressure to “catch up to” and equal pleural pressure.

Expiration starts when inspiratory muscles have stopped contracting. Lung recoil pressure (Pl) is now > than pleural pressure (Ppl), resulting in a + alveolar pressure and, therefore, expiratory flow.

Question 4

Describe the pressure-volume relationship of the lung.

Answer 4

PI = As lung volume incr, its Pl incr from about 0 cmH2O at residual volume to about 30 cmH2O at total lung capacity.

Pw = In opposition, the pressure of the chest wall (Pw) functions more like a spring in that bellow about
65% of total vital capacity, the compressed spring exerts negative (inflating) pressures yet at 100% total vital capacity, the chest wall, now a stretched spring, wants to collapse.

Prs = If we combine Pl and Pw, we get Prs, which represents the pressure-volume relationship of the
respiratory system.

Pressures are negative so lungs want to inflate

Pressures are positive so lungs want to deflate

Question 5

The pressure gradient is the driving force of air flow. It’s used to overcome the elastance (stiffness) of the resp system, the resistance to flow, and the inertia of the system. How is this done?

Answer 5

For air to flow -> into the alveoli, the pressure in the alveoli must be < than the pressure in the nose.

For air to flow -> out of the alveoli, the pressure in the alveoli must be > than the pressure in the nose.

Question 6

Pressures can be expressed in many different units. What are they?

Answer 6

For respiratory physiology, we use traditional units based on the height of a column of mercury (mm Hg) or water (cmH2O) :

• mm Hg for the partial pressures of gases when discussing diffusion
• cmH2O when discussing bulk flow (convection)

For the latter, we use cmH2O rather than mm Hg b/c the pressures needed to generate flow are
typically small (only a few cmH2O).
- Ex: a pressure of 5 cmH2O (which can easily cause a high
flow) =~3.7 mm Hg (3.7 mm is too small to be measured accurately when using a mercury manometer)
- 1 mm Hg = 1.36 cmH2O)

In addition, pressures related to convective flow are expressed relative to atmospheric pressure.
- Thus, if atmospheric pressure = 1034 cmH2O (usually given as 760 mm Hg), an alveolar pressure of 1029 cmH2O is referred to as - 5 cmH2O.

Question 7

True or false: “negative” pressures do not exist.

Answer 7

True

They are negative
only b/c they are less than the barometric pressure to which they are referred.

Question 8

The lungs have an intrinsic tendency to deflate following

inflation. This is because of 2 things. What are they?

Answer 8

Elastin fibres and surface tension.

Question 9

What are elastin fibres

Answer 9

They are connective tissues within the lung that contain lots of elastin fibres that are arranged in a meshwork that enhances their elastic behaviour.

When the lung is stretched during inhalation, this
elastic recoil which causes the lung to deflate.

Question 10

What is surface tension?

Answer 10

This is the force exerted by the liquid lining the inside of the alveoli and accounts for about 70% of the
elastic recoil properties of the lung.

Question 11

Surface tension has a two-fold effect on elastic recoil. What are those effects?

Answer 11

1) The liquid layer resists any forces that try to incr its surface area. This is due to the water molecules resisting being pulled apart.
2) The surface area of the liquid shrinks as much as it possibly can. This is due to the water molecules being so strongly attracted to each other.

Because of surface tension of the liquid lining the alveoli, in the absence of expanding forces, the alveoli shrink as much as possible and expel alveolar gas.

Question 12

True or false: the elastic recoil of the lungs is counteracted by the chest wall, preventing the lungs from collapsing

Answer 12

True

Question 13

Why then, if surface tension is such a strong factor, do alveoli not collapse?

Answer 13

If the surface liquid was water alone, they actually would collapse and the forces, or pressure, required to open them upon inspiration would be much greater. The alveoli do not collapse b/c of pulmonary surfactant and alveolar interdependence.

Pulmonary Surfactant:
- A complex mix of lipids and proteins secreted by type II alveolar cells.
- These secretions help
disperse the O2 molecules on the surface of the alveoli.
- By dispersing the O2 molecules, the water-water attractions decr, which causes alveolar surface tension to decr.
- This reduced surface tension decr the effort needed for inflation (incr compliance) and it reduces the surface tension of smaller alveoli more so than it does larger alveoli.

Alveolar Interdependence:
- Each alveolus is connected to its surrounding alveoli by connective tissues. If 1 alveolus starts to
collapse, it is supported by its neighbours.
1) When an alveolus in a group of interconnected alveoli starts to collapse, the surrounding alveoli are stretched by the collapsing alveolus.
2) As the neighbouring alveoli recoil, they pull outward on the collapsing alveolus. This helps prevent the alveolus from collapsing.

Question 14

Using this equation, if the surface tensions of 2 alveoli, Alveolus 1 and Alveolus 2, were equivalent, but Alveolus 2 had double the radius of Alveolus = 1, which alveolus would have the larger inward directed pressure?

a) Alveolus 1
b) Alveolus 2
c) They will have the same collapsing pressure

Answer 14

a) Alveolus 1

From the law of LaPlace, we can see that by decr the size of an alveolus, we incr the collapsing pressure. Therefore, when 2 alveoli have the same surface tension but different radii, the alveolus with the small radium will have a larger collapsing pressure. In the case that these alveoli are connected by the same terminal airway, the smaller alveolus would collapse and empty its contents into the large alveolus.

Question 15

When 2 alveoli with the same surface tension but different radii are connected by the same terminal airway, the smaller alveolus will collapse and empty into the larger alveoli.
- However, this would be detrimental to lung function if it were to occur.

How do you think this might be prevented?

a) Small alveoli contain more elastin fibres
b) Small alveolus will secrete more surfactant
c) Small alveoli are always surrounded by many other alveoli

Answer 15

b) Small alveolus will secrete more surfactant

In order to prevent a collapse, small alveoli secrete more surfactant. And since we just learned that pulmonary surfactant decr surface tension, we can use the equation to see that this decr the collapsing pressure. If we look back to Alveolus 1 and Alveolus 2 as used in the last example, we now know that due to the higher proportion of surfactant, Alveolus 1 must have half the surface tension of alveolus 2. This means the collapsing pressures b/w the 2 alveoli are comparable, preventing alveolar collapse.

Question 16

What is the difference in collapsing pressure b/w a small alveoli w/ a surface tension of 2 and a radius of 4 and a larger alveoli with a surface tension of 2 and a radius of 8?

Explain in your own words what pulmonary surfactant is and how it helps to prevent alveolar collapse. What other factors contribute to the maintenance of alveolar structure?

Answer 16

Pulmonary surfactant is a compound composed of a mixture of lipids and proteins, and is produced and secreted by alveolar cells onto the surface of alveoli.
- The hydrophobicity of surfactant enables it to interfere with the attractive intermolecular forces b/w the O2 molecules found lining the alveoli, thus reducing surface tension.

Each alveolus is able to regulate the amount of pulmonary surfactant it secretes, allowing each alveoli to moderate its surface tension. This means that all alveoli, even with differences in size, are able to equalize their collapsing pressures.
- This is essential for the lung to maintain max gas exchange surface area at all times.

Other than surfactant, alveolar interdependence (the supportive recoil of neighbouring alveoli) helps to maintain alveolar structure and prevent collapse.

Question 17

Using what you have learned in this Module, match the correct acronym to match the
pulmonary pressure: 
- Ptp
- Ppl
- PA
- PB

1) The pressure in the alveoli. It is 0 c m H2O.

2) The pressure exerted by the weight of the air in the atmosphere on the Earth’s surface. For
simplicity, it is 0 cmH2O.

3) This is the pressure in the pleural space and it closely approximates the intrathoracic pressure. It is normally around -5 c m H2O.

4) The difference between the alveolar pressure and the pleural pressure. It is also referred to as
lung recoil pressure (Pl).

Answer 17

1) The pressure in the alveoli. It is 0 c m H2O.
- PA

2) The pressure exerted by the weight of the air in the atmosphere on the Earth’s surface. For
simplicity, it is 0 cmH2O.
- PB

3) This is the pressure in the pleural space and it closely approximates the intrathoracic pressure. It is normally around -5 c m H2O.
- Ppl

4) The difference between the alveolar pressure and the pleural pressure. It is also referred to as
lung recoil pressure (Pl).
- Ptp

Question 18

True or false: for air to flow into the alveoli, alveolar pressure (PA) must be < than atmospheric pressure (PB) and for air to flow -> out of the alveoli, PA must be > than atmospheric pressure.

Answer 18

True

However, PB is essentially fixed at 0 mm Hg, so PA must change in order to generate the pressure gradient necessary for air flow.

Question 19

Use the equation to see how it can be rearranged to deduce alveolar pressure.

Alveolar Pressure (PA) - Pleural Pressure (Pp l) = Lung Recoil Pressure (Pi)

Answer 19

Lung Recoil Pressure (Pl) + Pleural Pressure (Ppl) = Alveolar Pressure (PA)

Therefore, to change PA we need to change either Pl, Ppl, or both.
- However, the recoil pressure of the lung (Pl) depends on lung volume. We can therefore not change lung volume by changing a pressure
that reflects lung volume. This effectively means that to change PA, you have to change Ppl. This is where the muscles in the chest wall come in.

Activating the inspiratory muscles decreases Ppl, decreasing PA, allowing for air to flow into the alveoli.

Activation of the expiratory muscles has the opposite effect allowing you to expel air from the alveoli.

Question 20

Discuss the onset of inhalation.

Answer 20

Immediately before inhalation, PA = PB.
- Air flows neither in nor out of the lungs.
- Contraction of the inspiratory muscles causes the Ppl to decr and the thoracic cavity enlarges. This decreases PA and air flows down its pressure gradient
into the lungs and inflates the alveoli.
- This continues until the increasing PA again = that of PB.

The changes in PA are relatively small as it generally decreases by only 1 cmH2O.

The change in pleural pressure is not linear. At the onset of inspiration, there is an increased resistance to flow that pleural pressure must overcome.

Question 21

Discuss the onset of exhalation.

Answer 21

At the end of inspiration, the inspiratory muscles relax.

• This incr Ppl and therefore incr PA.
• Air flows from the lungs until PA = PB.

Question 22

True or false: activation of expiratory muscles is not necessary for normal expiration due to the strong recoil forces.

Answer 22

True

Question 23

In healthy persons at rest, expiration is passive. However, it is possible to empty the lungs faster, or more forcefully, using active expiration. Give an example of when it would be used.

Answer 23

An example of this would be during exercise when high levels of ventilation are necessary.

Activation of expiratory muscles reduces the end-expiratory lung volume, which incr the tidal volume (the V of air inhaled and exhaled) independent of the inspiratory muscles

Question 24

How does active exhalation occur during routine exercise?

Answer 24

To forcefully breath out, PA must be increased by more than is accomplished by decreased excitation of inspiratory muscles and elastic recoil.
- This is accomplished by the expiratory
muscles of the abdominal wall.
- Upon contraction, the incr abdominal pressure is transferred to the pleural space, thus increasing Ppl.
- Contraction of the internal intercostals helps by pulling the ribs downward and inward to decr the size of the thoracic cavity.

Question 25

Define end-expiratory lung volume.

Answer 25

The basal level of inflation that lungs tend to return to during relaxed
breathing.

Also called functional residual capacity (FRC).

Question 26

Define the equal pressure point.

Answer 26

The point at which the pressure within the airways = the pleural pressure.

Question 27

How does active exhalation occur during forced expiration?

Answer 27

Suppose a person activates the expiratory muscles to generate a high Ppl for forced

expiration.
- As expiratory flow continues, the pressure decr b/c of energy lost due to resistance.
- At some point along the airways, the equal pressure point will be reached.
- Past this point, the transpulmonary pressure (Ptp) becomes negative (inside pressure < outside pressure), meaning the airway is compressed. This means that, at a given V, once the equal pressure point has been established, trying to breathe out harder does not incr flow b/c any pressure incr is offset by a proportionate incr in airway resistance due to compression of the airways.
- At this point, the pressure gradient generating the air flow is the recoil pressure of the lungs (PA - Pl).

Question 28

Using what you know about pleural pressure and the formula PA = Pl + Ppl, explain in your own words why the lungs do not collapse during active expiration.

Answer 28

During active expiration, Ppl becomes + due to the increased abdominal pressure, but the
lungs do not collapse.
- This occurs b/c the PA incr correspondingly.
- Also, any pressure incr in the Ppl is offset by a proportionate incr in airway resistance due to the compression of the airways. This blocks further outflow and, as such, active expiration never results in a person exhaling past their physiological residual volume (which would collapse the bronchioles).

Question 29

Define compliance.

Answer 29

The ability of the lung to stretch so at functional residual capacity, it is easy to move air in or out of the lungs.

Question 30

What is the significance of compliance?

Answer 30

From a pressure-volume curve we can derive compliance, which is the slope of the curve.

Compliance is greatest at functional residual capacity, which means the amount of work, or pressure needed, to either inhale or exhale is at its minimum.

Question 31

What does low compliance mean? What may cause it?

Answer 31

Low compliance means more pressure is required to move air in or out.

Compliance is affected by lung diseases.
- Ex: emphysema, in which case compliance is decr so that even at functional residual volume, the work required for breathing is significantly greater.

~Watch vid on slide 18~