Ventilation: Physics of Breathing

Question 1

Define pulmonary ventilation.

Answer 1

Movement of air into and out of the lungs

Question 2

Briefly explain the role of volume and pressure in breathing.

Answer 2

Δ volume → Δ pressure → movement of air, i.e. airflow follows a change in pressure resulting from the change of volume of the lungs. Air flows from high pressure area to low pressure area. To lower the pressure inside the lungs we expand the size of the chest and lungs.

Question 3

Identify and describe the pressures that are significant in the process of breathing.

Answer 3

Intrapulmonary pressure = pressure within the alveoli (falls and rises over one respiratory cycle). Expressed relative to atmospheric pressure.

Intrapleural pressure = pressure within pleural cavity. Always more negative than alveolar. Elastic nature of lung tissue versus ribcage and thorax trying pull apart visceral from parietal pleura. Expressed relative to atmospheric pressure.

Atmospheric pressure = 760 mmHg or 101.325 kPa

Question 4

What are the values for intapulmonary pressure, intrapleural pressure and collapsing force of the lungs, at the end of expiration ?

Answer 4

Intrapulmonary P = 760 mmHg (=0 mmHg relative to atmospheric P)
Intrapleural P = 756 mmHg (= -4 mmHg relative to atmospheric P)
Collapsing force = 4 mmHg (=absolute difference between intrapulmonary and intrapleural P)

Question 5

What is the implication of the negative intrapleural pressure relative to atmospheric pressure ?

Answer 5

Slight vacuum in pleural cavity

Question 6

Describe the distribution of pressures in different areas of the lung.

Answer 6

Pressure changes from top of the lung to the bottom of it.

Question 7

Describe the action of the main muscles responsible for inspiration, and how exactly they lead to inspiration.

Answer 7

• Diaphragm: main muscle of inspiration. Contraction flattens domes. Abdominal wall relaxes to allow abdominal contents to move downwards
• External intercostals: with first rib fixed, two movements, forward movement of lower end of sternum, and upward and outward movement of ribs.
• Trapezius: accessory muscle in forced inspiration (e.g. respiratory distress)

Flattening of diaphragm + outward expansion of rib cage by external intercostals = increase in V = decrease in P

Question 8

Describe the changes in pressure in inspiraton.

Answer 8

Increases volume of thorax by about 500 ml – normal tidal volume

Intrapleural pressure drops to ~ -6 mmHg (in normal tidal volume)

Decreases intrapulmonary pressure by ~1 mmHg (becomes -1 mmHg relative to atmospheric pressure, i.e. 759 mmHg) (in normal tidal volume)

Question 9

Describe the action of the main muscles responsible for expiration, and how exactly they lead to expiration.

Answer 9

• Passive – no direct muscle action normally
• Cessation of muscle contraction
• Elastic recoil – drives air out of lungs (decreased V, increased P, air moves down pressure gradient)

Question 10

Describe the changes in pressure in expiration.

Answer 10

• Thoracic volume decreases by 500 ml

* Intrapulmonary pressure increases

Question 11

Describe the action of the main muscles responsible for forced expiration, and how exactly they lead to forced expiration.

Answer 11

Contraction of abdominal walls, forces abdominal contents up against diaphragm, and internal intercostals – pull ribs downwards (decreases V, increases P, air forced down pressure gradient)

Question 12

Graph the fluctuations in intrapleural and intrapulmonary pressures during inspiration, and expiration.

Answer 12

Refer to graph on slide 7 in lecture on “Ventilation: Physics of Breathing”

Question 13

Graph the change in V of breath during inspiration and expiration.

Answer 13

Refer to graph on slide 7 in lecture on “Ventilation: Physics of Breathing”

Question 14

Define transpulmonary P.

Answer 14

Difference between intraplueral and intrapulmonary P.

Question 15

What actions in breathing is energy required for ?

Answer 15

Energy is required to:
♦ contract the muscles of inspiration – in quiet breathing contraction of the diaphragm comprises 75% of energy expenditure.
♦ stretch elastic elements
♦ overcome airway resistance
♦ overcome frictional forces arising from the viscosity of the lung and chest wall
♦ overcome inertia of the air and tissues

Question 16

Explain the significance and relevance of airway resistance in breathing. What are factors which can affect airway resistance ?

Answer 16

Airway resistance is the most significant non-elastic source of resistance in breathing.
F (air flow) = ΔP/R (resistance)

Resistance greater when turbulent flow (turbulent flow likely to occur with high velocities, and large diameter airways.)

Question 17

Which part of the respiratory tract most contributes to airway resistance ?

Answer 17

Upper airways (1/3)

Question 18

Describe the differences of resistance in different areas of the lung.

Answer 18

In different parts of lungs, resistance changes (in some parts, turbulent in other parts laminar).

Question 19

Where is the greatest resistance to airflow found ? Why ?

Answer 19

Segmental bronchi, because cross sectional area is relatively low and airflow is high and turbulent. (hence they are the ones to contract in asthma)

Question 20

How much resistance is there in alveoli? Why ?

Answer 20

Flow is laminar and the resistance is small (there is a large total cross- sectional area due to large number of small airways combined)

Question 21

Describe any changes in airway resistances during respiration. Is this significant in health ?

Answer 21

In inspiration, airway resistance decreases
In expiration, airway resistance increases
No, insignificant in health, unless disease state where airway resistance is increased:

• In asthma, inflammatory mediators change smooth muscle tone – narrowing airways – increases resistance.
• patients with COPD tend to have over-inflated chests (barrel-chested) (COPD also increases resistance)

Question 22

Define compliance in the context of breathing.

Answer 22

Distensibility or ease of stretch of lung tissue when external force applied, or the ease with which the lungs expand under pressure (i.e. change in volume of the chest that results from a given change in intrapleural pressure. High compliance means there is a large change in volume for a given change in pressure)

Question 23

What are the major determinants of compliance in the lungs ?

Answer 23

Elastic components and alveolar surface tension

Question 24

In a healthy individual, what is the compliance ?

Answer 24

Compliance in a healthy individual is about 1 L per kPa (1 L per 7.5 mmHg)

Question 25

How can compliance be reduced ?

Answer 25

♠ replacing elastic tissue with non-elastic tissue (e.g. age, pulmonary fibrosis – lungs become stiffer)
♠ blocking smaller respiratory passages
♠ increasing alveolar surface tension
♠ decreasing the flexibility of the thoracic cage or its ability to expand
♠ in general, smaller compliance with higher lung volumes (hence, if breath out, lung V low so can expand it maximally; if breath in deeply then lungs already expanded so cannot expand lungs much further, low compliance)

Question 26

How can compliance by increased ?

Answer 26

♠ Pulmonary emphysema. Due to alveoli rupture, creating larger air space and thus reducing surface area of lung. Impaired elastic recoil leads to poor deflation, trapping more air (unable to expire all the air).
♠ higher compliance with smaller lung volumes

Question 27

Describe the main implication of the indirect relationship between compliance and lung volume.

Answer 27

Ventilation difference between apex and base of lung:
Lung volume at base is less because it is compressed compared to apex. For the same change in intrapleural pressure at inspiration the base of the lung expands more than apex.

Question 28

Why is there surface tension on the alveolar surface ? How is this addressed ?

Answer 28

Surface tension due to polar nature of water. If he lungs were lined with pure water, they would collapse.

This is addressed by presence of surfactant, which reduces surface tension.

Question 29

What is surfactant produced by ? What is it made of ? What is its function ?

Answer 29

• Produced by the type II alveolar cells
• Surfactant is made up of phospholipids
• Prevents alveolar collapse + Increases lung compliance by reducing surface tension – allows greater expansion for a given change in pressure.

Question 30

Identify the main Respiratory Volumes and Pulmonary Function Tests.

Answer 30

• Spirometry and vitalograph
• Peak flow meters (breath out as much as you can, gives indication about lung capacity)
• Alveolar ventilation and minute ventilation

Question 31

What are the main volumes in the lungs to consider ?

Answer 31

Tidal Volume
Inspiratory Reserve Volume
Expiratory Reserve Volume
Residual Volume

Question 32

Define Tidal Volume (TV). What is the average TV in a healthy individual ?

Answer 32

Volume of air breathed in and out in a single breath (0.5 l)

Question 33

Define Inspiratory Reserve Volume (IRV). What is the average IRV in a healthy individual ?

Answer 33

Volume breathed in by max inspiration at end of normal inspiration (3.3 l)

Question 34

Define Expiratory Reserve Volume (ERV). What is the average ERV in a healthy individual ?

Answer 34

Volume of air expelled by max effort at the end of normal expiration (1 l)

Question 35

Define Residual Volume. What is the average RV in a healthy individual ?

Answer 35

Volume of air in lungs at the end of maximum expiration ( 1.2 l)

Question 36

What are the main capacities in the lungs to consider ?

Answer 36

Inspiratory Capacity (IC)
Functional Residual Capacity (FRC)
Vital Capacity (VC)
Total Lung Capacity (TLC)

Question 37

Define inspiratory capacity (IC). How do we calculate it? What is the average IC in a healthy individual ?

Answer 37

Volume or air breathed in by max inspiration at the end of a normal expiration (3.8 l)
TV + IRV

Question 38

Define functional residual capacity (FRC). How do we calculate it ? What is the average FRC in a healthy individual ?

Answer 38

Volume of air left in lungs at end of normal expiration. Buffer against extreme changes in alveolar gas levels in each breath (2.2-2.4 l)
ERV+RV

Question 39

Define vital capacity (VC). How do we calculate it ? What is the average VC in a healthy individual ?

Answer 39

Volume of air that can be breathed by max inspiration following a max expiration (4.8 l)
IRV+TV+ERV

Question 40

Define total lung capacity (TLC). How do we calculate it ? What is the average TLC in a healthy individual ?

Answer 40

“Volume of air contained in the lung at the end of a maximal inhalation” (6 l)
VC+RV

Question 41

Do we use all of total lung capacity in breathing ?

Answer 41

No, we only use a fraction of it.

Question 42

Graph all volumes and capacities of the lung.

Answer 42

Refer to slide 19 in lecture on “Ventilation - Physics of Breathing”

Question 43

What is a spirometer ?

Answer 43

Used to measure and record volumes of inspired and expired air. Graph produced = spirogram.
Asked to take a very deep breath and blow out as fast as you can into a mouthpiece until no more air comes out.

Question 44

What volumes/capacities can you measure and which can you not measure, using a spirometer ?

Answer 44

Cannot measure Residual Volume (and therefore FRC and TLC) but can measure others. Can measure RV with other methods.

Question 45

What does a vitalograph measure ? Graph the result of a vitalograph for a normal subject and a subject with obstructed airway.

Answer 45

How much air one can breath out rapidly in one second (i.e. percentage of vital capacity exhaled in one second).
Refer to slide 21 in lecture on “Ventilation: Physics of Breathing”

Question 46

Identify the dead spaces in the respiratory tract.

Answer 46

Anatomical dead spaces- areas of airway not involved gaseous exchange including nose and mouth, pharynx, larynx, trachea, bronchi, bronchioles (150 ml on average)

Alveolar dead spaces- areas where gas exchange is suboptimal (i.e. less ventilation) (5 ml)

Physiological dead spaces- anatomical + alveolar (in normal person about the same as anatomical due to low alveolar dead space)

Question 47

What factors may cause the physiological dead space to increase ?

Answer 47

Lung disease, resulting in decreased ventilation:perfusion ratio (e.g. due to inhalation of foreign body/tumour/growth causing occlusion and thus lower ventilation) and thus resulting in increased alveolar dead space.