Mechanics of breathing

Question 1

The diaphragm is the most important muscle for inspiration , when it contracts the abdominal contents are pushed down and the ribs are lifter upward and outward. What do these changes do to thoracic volume and intrathoracic pressure?

Answer 1

Increase thoracic volume

Decrease intrathoracic pressure

Question 2

Expiration is normally a passive process. How is it different during exercise or in patients with obstructive disease pathologies?

Answer 2

In these cases the expiratory muscles may aid the expiratory process.

(abdominals, internal intercostals)

Question 3

What does lung compliance describe?

Answer 3

The change in lung volume for a given change in pressure.

Question 4

The compliance of the lungs is Inversely correlated with what?

Answer 4

The elastic properties of the lungs

Question 5

What is transmural pressure?

In the lungs what is the transmural pressure we will be dealing with?

Answer 5

The pressure across a structure.

Transpulmonary pressure (P_TM)

Question 6

What is the transpulmonary pressure equal to?

Answer 6

P_TM = Alveolar pressure (Palv) - Intrapleural pressure (P_PI)

Question 7

Lung elastic recoil is equal and opposite to?

Answer 7

Transpulmonary pressure

Question 8

Where is the intrapleural space?

Answer 8

Between the lungs and chest wall

Question 9

What are lung pressures referred to? (zeroed to)

Answer 9

Atmospheric pressure

Question 10

What is lung compliance equal to?

Answer 10

Lung Compliance = ΔV/ΔP

Question 11

During respiration when is lung compliance high/low?

Answer 11

High during expiration, low during inspiration

Question 12

Why is compliance different for the lung during different parts of the respiratory cycle?

Answer 12

Because of surface tension at the liquid-air interface of the air filled lung…

Question 13

Under normal circumstances, how would one describe the pressure of the intrapleural space?

Answer 13

Negative

Question 14

What creates the negative intrapleural pressure?

Answer 14

Two opposing elastic forces -

1. Chest wall with it’s elastic forces tends to spring outward
2. Lungs, with their elastic properties pull inward

These two opposing forces pull on the intrapleural space and generate a negative pressure - or a vacuum.

Question 15

What happens if the intrapleural space is punctured by a sharp object and the pressure inside becomes equal to P_atm?

Answer 15

Pneumothorax…

Without the negative intrapleural pressure to hold the lungs open, they collapse.

Similarly, the chest wall springs outward.

Question 16

Collapsing force of the lungs is equal to the expanding force of the chest wall at…

Answer 16

FRC

Question 17

Why does compliance of the lungs increase in patients with emphysema?

Answer 17

Lost of elastic fibers in the lungs.

Question 18

Why does a patient with emphysema (or other obstructive lung disease) have increased FRC?

Answer 18

At the normal value for FRC in a healthy person the pressures on the lungs and chest wall are in balance.

In a patient with emphysema however, at original FRC the tendancy for the lungs to collapse will be less than the force of the chest wall to expand. As a result, volume must be added to the system to increase their collapsing force.

Essentially the lung/chest wall system seeks a new higher FRC where the two opposing forces can be balanced, due to increased engagement of elastic fibers.

Question 19

Describe what causes the change seen to FRC in a patient with fibrosis of the lungs.

Answer 19

Fibrosis leads to an increased elastic character of the lungs, and consequently, at normal FRC volume the tendancy of the lungs to collapse is much stronger than the tendancy of the chest wall to expand.

As a result the system seeks a new FRC at a lower lung volume.

Question 20

What is laplace’s law for alveoli?

Answer 20

P = 2T/r

P = collapsing pressure on alveolus

T = Surface tension

r = radius of alveolus

Question 21

Characterise the collapsing pressure large and small alveoli experience respectively.

Answer 21

Large alveoulus will experience a low collapsing pressure (requires only minumal effort to hold it open)

Small alveolus will experience a high collapsing force (requires greater effort to hold open)

Question 22

Small alveoli are not ideal due to their tendancy to collapse, but for gas exchange they must be as small as possible to increase surface area. How is this problem solved?

Answer 22

Surfactant

Question 23

What is the term for a collapsed alveolus?

Answer 23

Atelectasis

Question 24

What impact does surfactant have on lung compliance?

Answer 24

It increases it.

Question 25

What does one see in infants who lack adequate production of surfactant?

Answer 25

Atelectasis, leading to hypoxemia. Decreased lung compliance leading to higher required work to inflate lungs.

Question 26

What is the formula for airflow: Q

Answer 26

Airflow (Q) = change in pressure (delta P)/ Airway Resistance (R)

Question 27

Medium size bronchi (generations 5-7) are the highest sites of resistance, why aren’t the smaller bronchioles the highest?

Answer 27

The smaller bronchioles arise in parralell, and yield a very high aggregate cross sectional area, resulting in very low total resistance.

Question 28

What are the three primary ways to influence airway resistance?

Answer 28

1. autonomic nervous system
2. Lung volume
3. Viscosity of inspired air (less common, deep sea diving and inspiring helium)

Question 29

What is the impact of large lung volume on airway resistance? How does this happen?

Answer 29

At high lung volumes, airway resistance is substantially reduced due to increased radial traction on the airways by lung parenchyma.

Question 30

What happens to airway resistance at low lung volumes?

Answer 30

At low lung volumes, the radial traction exerted by the lung parenchyma is reduced, and leads to increased airway resistance. (sometimes to the point where they collapse)

Question 31

What compensatory mechanism do patients with asthma use to overcome airway resistance?

Answer 31

They breath at higher lung volumes and partially offset the high airway resistance of the disease.

Question 32

What are the phases of the breathing cycle?

Answer 32

1. Rest (period between breaths)
2. Inspiration
3. Expiration

Question 33

What force is there on the lung if transpulmonary (AKA transmural) pressure is positive?

Answer 33

An expanding force

Question 34

What force is there on the lung if transpulmonary force is negative? When does one see this in normal breathing?

Answer 34

A collapsing force.

In the normal breathing cycle the transmural pressure should be positive at all times, ensuring the lungs always remain open. You would see this in a pneumothorax though.

Question 35

What is alveolar pressure during the resting phase?

Answer 35

Equal to atmospheric pressure. (zero)

Question 36

during inspiration the intrapleural pressure becomes even more negative. Explain the two reasons for this.

Answer 36

1. As lung volume rises, the elastic recoil of the lungs also increases and pulls more forcefully on the intrapleural space.
2. airway and alveolar pressures become negative

Question 37

Discuss the impact of forced expiration on…

Lung/airway pressure

intrapleural pressure

In a healthy person do the airways collapse under forced expiration?

Answer 37

Forced expiration causes substantial increases in boht lung/airway pressure and intrapleural pressure.

In healthy people the transmural pressure remains positive, so the airways do not collapse, even with the substantiall increased pressures generated by forced expiration.

Question 38

How does forced expiration impact someone with an obstructive condition like emphysema?

Answer 38

Since a patient with emphysema has increased lung compliance (reduced lung elasticity), the alveoli and airways have lower pressure than in a normal person. When they forcefully expire, their intrapleural pressure reaches the same value as a normal person but the alveoli/airways with their diminished recoil and low pressure lead to a negative transmural pressure, thus collapsing the airways.

Question 39

How do patients with emphysema tend to compensate for their condition?

Answer 39

They learn to expire slowly and with pursed lips, raising airway pressure and preventing the reversal of the transmural pressure gradient across airways and preventing their collapse.