Mechanics of breathing

Question 1

What does Boyle’s Law state?

Answer 1

At a given temperature, the pressure and volume of an ideal gas are inversely proportional

Question 2

Describe the 2 anatomical structures involved in quiet breathing (inspiration).

Answer 2

Contraction and flattening of the diaphragm – increases thoracic volume

Contraction of external intercostals – rib cage moves upwards and outwards

Question 3

Name the 4 accessory muscles of forced inspiration.

Answer 3

Sternocleidomastoid

Scalene muscles

If the arms and shoulders are fixed in a tripod position – pectoralis major and latissimus dorsi

Question 4

Name the 3 muscles of forced expiration.

Answer 4

Abdominal muscles – contract to increase intra-abdominal pressure and force diaphragm upwards

Internal intercostal muscles

Innermost intercostal muscles – pulls ribs downwards

Question 5

Is the pleural space a higher or negative pressure compared to atmospheric pressure?

Answer 5

Negative

Question 6

What prevents our lungs from collapsing?

Answer 6

The negative pressure in the pleural cavity (negative intrapleural pressure)

Question 7

Which 3 factors create a negative intrapleural pressure?

Answer 7

Elastic recoil of the lungs

Surface tension

Outward recoil of the chest wall

Question 8

What is the definition of transpulmonary pressure?

Answer 8

The pressure difference between intrapulmonary pressure and intrapleural pressure

Question 9

What is the usual value of transpulmonary pressure?

Answer 9

Transpulmonary pressure = intrapulmonary pressure – intrapleural pressure. 0 – (-4) = 4mmgHg

Question 10

As the chest expands during inspiration, what effect does this have on intrapleural pressure and why?

Answer 10

Decreases intrapleural pressure since volume of the pleural cavity is increasing

Question 11

As the chest expands during inspiration, what effect does this have on transpulmonary pressure?

Answer 11

Increases transpulmonary pressure

Question 12

What is the definition of lung compliance?

Answer 12

The distensibility or ease of stretch of lung tissue when an external force is applied to it

Question 13

How is lung compliance calculated?

Answer 13

Compliance = change in volume / change in transpulmonary pressure

Question 14

What is elasticity?

Answer 14

Resistance to stretch

Question 15

The elastic properties of the lung are due to which 2 components?

Answer 15

Elastin and collagen fibres in the lung

Surface tension within the alveoli

Question 16

Where in the lungs is intrapleural pressure more negative, the apex or base? What causes this?

Answer 16

Apex

The effect of gravity pulling the lung downwards, causing higher volume at the apex of the lung and therefore less pressure than at the base

Question 17

Where in the lungs is there a greater transmural pressure gradient, the apex or base?

Answer 17

Apex

Question 18

Describe the apex and base alveoli in terms of size and compliance.

Answer 18

Apex – larger alveoli, less compliant

Base – smaller alveoli, more compliant

Question 19

Where in the lungs is ventilation greatest, apex or base?

Answer 19

Base

Question 20

Describe what is meant by surface tension.

Answer 20

Cohesive forces via hydrogen bonds between surface molecules are stronger, allowing the surface of a liquid to resist external force

Question 21

Surface tension at the air-fluid interface in the alveoli creates what in the alveoli?

Answer 21

An inward collapsing pressure

Question 22

Describe the equation of Laplace’s Law.

Answer 22

Pressure = 2 x surface tension / radius

Question 23

What size alveoli are harder to inflate?

Answer 23

Smaller alveoli

Question 24

Which 2 factors help to stabilise alveoli?

Answer 24

Alveolar interdependence – mechanical support alveoli provide to each other, including pores of Kohn and canals of Lambert + surfactant

Question 25

How and where is surfactant secreted from?

Answer 25

Via exocytosis from type II pneumocytes

Question 26

By what mechanism does surfactant decrease surface tension?

Answer 26

Disrupts hydrogen bonds between surface water molecules

Question 27

Define the work of breathing.

Answer 27

The energy consumed by the respiratory muscles during the respiratory cycle

Question 28

The work of breathing consists of which 2 components?

Answer 28

Elastic work
Resistive work

Question 29

What is elastic work in breathing?

Answer 29

The energy required to overcome the elastic recoil of the lungs and chest wall during inspiration

Question 30

What happens to some of the energy used in elastic work?

Answer 30

Stored as potential energy and used in expiration

Question 31

What is resistive work in breathing?

Answer 31

The energy needed to overcome airway resistance during ventilation

Question 32

What happens to the energy used in resistive work?

Answer 32

Energy is ‘wasted’ as heat and sound

Question 33

What element(s) of work of breathing is affected in obesity and how?

Answer 33

Increases elastic work of breathing – opposes the outward elastic recoil of the chest wall

Question 34

What element(s) of work of breathing is affected in pulmonary fibrosis and how?

Answer 34

Increases elastic work of breathing – lung tissue is replaced by scar tissue which means the lungs are stiffer and reduces lung compliance. Increases resistive work of breathing – increased amount of lung tissue increases tissue resistance

Question 35

What element(s) of work of breathing is affected in asthma and how?

Answer 35

Increases resistive work of breathing – airway narrowing due to bronchoconstriction and mucosal oedema increases airway resistance

Question 36

What are the 2 main factors affecting airway resistance?

Answer 36

Turbulent air flow

Change in airway radius

Question 37

Describe dynamic airway compression.

Answer 37

During forced expiration, intrapleural pressure can become positive, causing collapse of airways without cartilage in their walls e.g. bronchioles. This can be worsened in disease which cause airway narrowing or loss of elastic tissue