3. The Mechanics of Breathing and Lung Function Testing

Question 1

How is air drawn into the lungs?

Answer 1

By expanding the volume of the thoracic cavity.

Question 2

Why must work be done to expand the thoracic cavity in breathing?

Answer 2

To overcome the resistance to flow of air through the airways.

Question 3

What is the pleural space?

Answer 3

The space between the lungs and thoracic wall.

Question 4

What is the pleural space normally filled with?

Answer 4

A few millilitres of fluid.

Question 5

What is the importance of the pleural space in terms of changing lung volumes?

Answer 5

Surface tension forms a pleural seal between the outer surface of the lungs and the inner surface of the thoracic wall. So when the volume of the thoracic cage changes, so does that of the lungs.

Question 6

What does loss of integrity of the pleural seal cause?

Answer 6

Pneumothorax.

Question 7

What is lung compliance?

Answer 7

The stretchiness of the lungs. The volume change per unit pressure change.

Question 8

How is lung compliance measured?

Answer 8

By measuring the change in lung volume for a given pressure.

Question 9

How is specific compliance calculated?

Answer 9

Volume change per unit pressure change/ starting volume of lungs.

Question 10

What gives the lung tissue its elastic properties?

Answer 10

The elastic tissues and the surface tension forces in the alveoli.

Question 11

What is surface tension?

Answer 11

Interactions between molecules at the surface of a liquid, making the surface resist stretching.

Question 12

How hoes surface tension affect compliance?

Answer 12

The higher the surface tension, the lower the compliance.

Question 13

How does the actual surface tension of the lungs at low lung volumes differ from the expected?

Answer 13

It is lower.

Question 14

Why is the tension lower at low lung volumes than expected?

Answer 14

Because of disruption of interactions between surface molecules by surfactant from type II alveolar cells.

Question 15

What is surfactant?

Answer 15

A mixture of phospholipids and proteins, with detergent properties.

Question 16

What is the orientation of the hydrophobic and hydrophilic parts of surfactent?

Answer 16

Hydrophilic ends lies in the alveolar fluid and the hydrophobic end projects into the alveolar gas.

Question 17

Why are little breaths easy but deep breaths hard?

Answer 17

Because surfactant reduces surface tension when the lungs are deflated, but not when fully inflated. So it takes less force to expand small alveoli than it does large ones.

Question 18

What is Laplace’s law?

Answer 18

Pressure is inversely related to the radius of the alveoli.

Question 19

What is the relevance of Laplace’s law to alveoli in an interconnecting set of bubbles?

Answer 19

Big bubbles eat smaller bubbles.

Question 20

How are big alveoli stopped from ‘eating’ small alveoli?

Answer 20

As the alveoli get bigger, the surface tension in their walls increases, as surfactant is less effective. This keeps the pressure the same for all alveoli and stops them morphing together.

Question 21

What is Poiseulle’s law?

Answer 21

The resistance of a tube increases sharply with falling radius.

Question 22

What does Poiseulle’s law determines?

Answer 22

The resistance of an airway to flow when the flow is laminar.

Question 23

Why is combined resistance of small airways normally low?

Answer 23

Because they are connected in parallel over a branching structure. So total resistance to flow in downstream branches is less than the resistance of upstream branches.

Question 24

What is work done against in respiration?

Answer 24

Elastic recoil of the lungs and thorax, and resistance to flow through airways.

Question 25

How do bronchioles allow air to be drawn in easily through them into the alveoli in inspiration?

Answer 25

The bronchioles use smooth muscle to increase their radius and hence reduce resistance.

Question 26

How is a spirometry reading taken?

Answer 26

The patient fills their lungs from the atmosphere and breathes out as far and fast as possible through a spirometer.

Question 27

What can simple spirometry measure?

Answer 27

Lung volumes and capacities.

Question 28

When might vital capacity of the lungs be less than normal?

Answer 28

When they are not filled normally in inspiration, emptied normally in expiration, or both.

Question 29

What is forced vital capacity?

Answer 29

The maximum volume that can be expired from full lungs.

Question 30

What is forced expiratory volume in one second (FEV1)?

Answer 30

The volume expired in the first second of expiration from full lungs.

Question 31

What is FEV1 affected by?

Answer 31

By how quickly air flow slows down, so it’s low if the airways are narrowed.

Question 32

How can restrictive and obstructive deficits by separated on a vitalograph?

Answer 32

By asking the patient to breathe out rapidly from maximal inspiration through a single breath spirometer, which plots volume expired against time. In restrictive deficits, FVC is reduced, and FEV1 is a normal proportion of FEV (>70%). In obstructive deficits, FEV1 is reduced but FEV is relatively normal.

Question 33

What is a restrictive deficit of the lungs?

Answer 33

When the lungs are unusually stiff, or inspiratory effort is compromised by muscle weakness, injury, or deformity.

Question 34

What is an obstructive deficit of the lungs?

Answer 34

The airways are narrowed so the flow resistance is increased so much so that no more air can be driven out of the alveoli.

Question 35

What is plotted on a flow volume curve?

Answer 35

Volume expired against flow rate, derived from a vitalograph trace.

Question 36

What is peak expiratory flow rate?

Answer 36

The rate of flow at the beginning of expiration as the lungs are full so airways are stretched and resistance is at a minimum.

Question 37

What type of deficit causes a scooped out expiratory curve on flow volume curves?

Answer 37

Mild obstruction of the airways.

Question 38

What can the helium dilution test be used to measure?

Answer 38

Functional residual capacity, and therefore can calculate the residual volume.

Question 39

Why is helium used to calculate residual volume in the lungs?

Answer 39

It can’t transfer across the alveolar-capillary membrane and therefore is contained within the lungs.

Question 40

How is a helium dilution test performed?

Answer 40

At the end of a normal tidal expiration, the patient is connected to a circuit connected to a container with a gas mixture of a known helium concentration and volume. The patient continues to breathe into the container until equilibrium occurs (4-7 minutes) and there is a new concentration of helium.

Question 41

How is residual volume calculated from the helium dilution test?

Answer 41

The known original concentration x the original volume = new concentration of helium x the new volume. So the new volume = original volume + functional residual capacity. The original concentration and volume and new volume are all known, so FRC can be calculated. Then the residual volume = FRC - ERV (measured in spirometry).

Question 42

What does the carbon monoxide transfer factor measure?

Answer 42

The rate of transfer of CO from the alveoli to the blood in ml per minute per kPa.

Question 43

What does the rate of CO transfer in CO transfer factor show?

Answer 43

The diffusion capacity of the lung, as the amount transferred will depend on how well gas diffusion takes place.

Question 44

How does the transfer factor test work?

Answer 44

CO is inhaled and due to its high affinity for Hb, very little remains in the plasma - assumed plasma ppCO is zero. This means the concentration gradient between alveolar ppCO and capillary ppCO is maintained. So the amount of CO transferred from alveoli to blood is only limited by diffusion capacity of the lung.

Question 45

How is the transfer factor test performed?

Answer 45

The patient performs a full expiration, followed by rapid maximum inspiration of a gas mixture with air, a tiny fraction of CO and a fraction of inert gas like helium. The breath is held for 10 seconds, then the patient exhales, and the gas is collected mid-expiration to gain an alveolar sample. The concentration of CO and inert gas is measured and the transfer factor calculated.

Question 46

What does the nitrogen washout test measure?

Answer 46

Serial (anatomical) dead space.

Question 47

How is a nitrogen washout test performed?

Answer 47

The patient takes a maximum inspiration of 100% oxygen, the oxygen that reaches the alveoli mixeswith alveolar air and the resulting mix has nitrogen in it. But air in the conducting airways will be filled with pure oxygen. The person exhales through a one way valve that measureds percentage of nitrogen in and volume of air expired.

Question 48

How are the results from a nitrogen washout test interpreted?

Answer 48

The nitrogen concentration initially exhaled is zero due to exhalation of dead space oxygen. As alveolar air moves out and mixes with dead space air, the nitrogen concentration climbs and then plateaus. A graph is drawn to determine the dead space by plotting nitrogen percentage against expired volume.