Mechanics of breathing

Question 1

What are the components of the mechanical respiratory system?

Answer 1

1. Chest wall (including rib cage and diaphragm)
2. Lungs (including alveoli and conducting airways)
3. Pleural space
4. Respiratory muscles

Question 2

What additional muscles are involved in forced inspiration?

Answer 2

Scalenes (elevate ribs 1 + 2)

Sternocleidomastoids (raises sternum)

Question 3

What muscles are involved in passive expiration?

Answer 3

None

Question 4

What muscles are involved in active expiration?

Answer 4

Abdominal muscles (contract pushing diaphragm upwards) 
Internal intercostals (move ribs downwards) 
Pectoral girdle muscles

Question 5

What muscles are involved in passive inspiration?

Answer 5

Intercostals (external + internal)

Diaphragm

Question 6

What is “work of breathing”?

Answer 6

Energy expended during respiration made up of 3 components:

1) Compliance work - energy required to expand the lungs against the lung and chest elastic forces
2) Tissue resistance work - energy required to overcome the viscosity of the lungs and chest wall structures
3) Airway resistance work - energy required to overcome airway resistance to movement of air into the lungs

Question 7

What is lung compliance?

Answer 7

The willingness of the lungs to expand

[volume change of the lung per unit of force applied]

Question 8

What is surface tension?

Answer 8

At a surface, attraction between molecules of a liquid are much stronger than attraction between liquid and gas molecules (due to hydrogen bonds between water molecules)
Measured in terms of force per unit length (N.m-1)

Question 9

What is Hooke’s Law?

Answer 9

The force needed to extend or compress a spring by some distance is proportional to that distance.

Question 10

What is Laplace’s Law?

Answer 10

P = 2T/r (where P = pressure, T = surface tension, r = radius)

So if 2 bubbles have the same surface tension, the smaller bubble will have a higher pressure.

Question 11

By what process does air travel from the atmosphere to the alveolar?

Answer 11

Bulk flow down pressure gradients (from positive atmospheric pressure to more negative peripheral airways created by lung expansion)

Question 12

What nerve innervates the diaphragm?

Answer 12

Phrenic (C3-5)

Question 13

What is the “bucket handle” effect?

Answer 13

Inspiration aided by external intercostal muscles moving the ribs upwards and outwards

Question 14

Which phases of quiet breathing require energy?

Answer 14

Inhalation = active process
Exhalation = passive (elastic recoil returns lungs to functional residual capacity)

Question 15

What is the functional residual capacity?

Answer 15

Lung volume at which opposing forces of the chest wall and the contractile lung are in equilibrium

Question 16

What anatomically permits the lungs to expand with the chest wall?

Answer 16

Pleura - parietal layer attached to chest wall and visceral layer attached to the lungs (with thin layer of pleural fluid between the two providing a seal between the layers)

Negative pressure in the pleural space becomes more negative as chest wall expands on inhalation meaning that the suction on the visceral layer and attached lung expands with the chest wall as the force of the lung’s elastic recoil is exceeded.

Question 17

What lung volume is represented when the forces inflating and deflating the lungs are equal?

Answer 17

Functional residual capacity

[Occurs at end of exhalation during quiet breathing]

Question 18

What prevents the lungs from collapsing away from the chest wall?

Answer 18

Negative pressure within the pleural space

Question 19

What causes a pneumothorax?

Answer 19

Pleural seal is broken (e.g. commonly when lung surface or chest wall is punctured) meaning pressure in the pleural space will rise, removing the suction effect and causing the lung to collapse away from the chest wall due to unopposed force of the elastic recoil of the lungs

Question 20

What signs on an X-Ray indicate a pneumothorax?

Answer 20

Deviated trachea
Displaced heart shadow
Black thorax (e.g. no evidence of bronchial tree)
Diaphragm depression

Question 21

What is elastance?

Answer 21

A measure of the disposition of the lungs to return to resting position due to their intrinsic elasticity

Question 22

What is the difference between compliance and elastance?

Answer 22

Compliance of the lungs refers to the willingness of the lungs to expand under pressure whilst elastance refers to the lungs ability to return to their resting position (due to elasticity)

Question 23

How does emphysema impact compliance?

Answer 23

Increased compliance as there is less tissue to stretch due to tissue damage

Question 24

What causes decreased compliance?

Answer 24

Fibrosis of the lungs

Question 25

What is the role of surfactant in the lungs?

Answer 25

Reduces surface tension, stabilises alveoli and increases compliance (particularly at low lung volumes)
Reduces likelihood of tissue fluid transudation
Antioxidant effect

Question 26

What is surfactant?

Answer 26

Secreted by epithelial cells (pneumocytes) lining the alveoli
Major ingredient of surfactant is phospholipid

Question 27

What is interdependence and its importance?

Answer 27

Refers to the tight packing of alveoli limiting tendency to collapse

Question 28

What is Poisuelle’s Law?

Answer 28

Determines resistance to flow in laminar flow
Resistance rises with the fourth power of the radius (e.g. if radius doubles, flow will increase by 2 to the power of 4 which is 16)

Question 29

Which parts of the airways have the least and most resistance?

Answer 29

Least resistance (collectively) = smallest, individual bronchioles due to number of airways 
Most resistance = medium sized bronchi

Question 30

What bedside test can be done to measure resistance?

Answer 30

Peak Expiratory Flow Rate

Question 31

What factors determine airway resistance?

Answer 31

ANS (parasympathetic via CN X = constriction of airways, sympathetic stimulation by catecholamines on beta2 receptors relaxes smooth muscle thus dilating airways) 
Lung volume (increased lung volume = increased airway radius) 
Turbulent vs. laminar flow (large airways more prone to turbulent flow)