Respiratory: Physiology - Mechanisms of breathing

Question 1

Two chief muscles of inspiration

Answer 1

1. Diaphragm
2. External intercostals

Question 2

Three accessory muscles of inspiration

Answer 2

1. Scalenes
2. Sternocleidomastoid
3. Alae nasi

Question 3

Five muscles of active expiration

Answer 3

1. Internal intercostals
2. Rectus abdominis
3. External obliques
4. Internal obliques
5. Transversus abdominis

Question 4

Nerve innervating diaphragm

Answer 4

Phrenic nerve (C3-5)

Question 5

What dimensions of the chest are increased by the action of the external intercostals?

Answer 5

Anterior-posterior (predominantly) and lateral via “bucket-handle” motion

Question 6

What is hysteresis with respect to the elastic properties of the lung?

Answer 6

Describes the phenomenon whereby lunge volume at any given pressure during deflation is greater than during inflation

Question 7

What is the transpulmonary pressure?

Answer 7

Pressure difference between inside and outside lung

Question 8

Compliance formula

Answer 8

Compliance = ΔV/ΔP

Question 9

What is specific compliance?

Answer 9

Compliance per unit volume of lung

Question 10

What is normal compliance?

Answer 10

200ml/cm H2O

Question 11

Five factors which reduce compliance

Answer 11

Pulmonary fibrosis
Alveolar oedema
Atelectasis
Increased surface tension (decreased surfactant)
Increased pulmonary venous pressure (engorged lung)

Question 12

Two factors which increase compliance

Answer 12

Age
Emphysema (loss of elastic recoil)

Question 13

What two factors determine lung elasticity and therefore affect compliance?

Answer 13

1. Elastic tissue (elastin, collagen)
2. Surface tension

Question 14

What is surface tension? What produces surface tension?

Answer 14

Force acting across an imaginary line 1cm long in surface of liquid
Arises because attraction is greater between liquid molecules than between liquid and gas molecules

Question 15

What is the relationship between surface tension and pressure? What law describes this?

Answer 15

Laplace’s law:
P = 4T/r where P = pressure, T = surface tension, r = radius

Question 16

How is the surface tension of alveolar lining fluid reduced to prevent collapse in normal lung function?

Answer 16

By surfactant (phospholipid) secreted by type II pneumocytes

Question 17

Three physiological advantages offered by surfactant

Answer 17

1. Reduced surface tension causes increased compliance (less recoil)
2. Improves alveolar stability: less collapse means less physiologic dead space
3. Keeps alveoli dry: decreased hydrostatic pressure outside capillaries decreases transudation

Question 18

What is interdependence?

Answer 18

Tendency of expansion of surrounding lung parenchyma to prevent alveolar collapse

Question 19

Three changes seen with absence of surfactant

Answer 19

1. Reduced compliance
2. Atelectasis
3. Alveolar oedema

Question 20

Why do basal regions of the lung ventilate more effectively than apical? How does this change at full expiration?

Answer 20

To counteract effects of gravity on weight of lung, intrapleural pressure is higher (less negative) at base
This means it has a smaller resting volume and expands more easily on inspiration

At residual volume (i.e. after full expiration), intrapleural pressure exceeds atmospheric pressure
Basal lung will therefore not ventilate until intrapleural pressure exceeds atmospheric, which may not occur if small inspiration taken after full expiration

Question 21

What is the Hagen-Poiseuille equation?

Answer 21

V = (Pπr^4)/8nl where V = flow rate, P = ΔP, r = radius, n = viscosity, l = length

Therefore R (resistance) = 8nl/πr^4

Question 22

What is the impact of halving the radius of a tube on the resistance?

Answer 22

Resistance increases 16x

Question 23

Describe the difference between laminar and turbulent flow

Answer 23

Laminar: flow in which stream lines are parallel to sides of a tube, and gas in centre of tube moves at twice the average velocity
Turbulent flow: fluid motion with changes in direction and velocity

Question 24

What is pressure proportional to in the setting of turbulent flow? What is more important in turbulent flow: viscosity or density?

Answer 24

Square of flow rate
Density more important in turbulent flow (increased density = increased turbulence)

Question 25

What is Reynold's number? How is it calculated and how is it interpreted?

Answer 25

Measure of whether flow will be laminar or turbulent Re = 2rvd/n where r = radius, v = velocity, d = density and n = viscosity Re > 2000 suggests turbulent flow

Question 26

Where in the bronchial tree does laminar flow occur?

Answer 26

Very small airways (terminal bronchioles)

Question 27

Describe the pattern of flow in most of the bronchial tree

Answer 27

Transitional

Question 28

Where in the bronchial tree may true turbulence occur?

Answer 28

In the trachea, particularly with increased velocity during exercise

Question 29

Describe the two mechanisms of measuring airway resistance

Answer 29

May be measured as the pressure difference between mouth and alveoli divided by the flow rate Can be measured either by body plethysmography using Boyle's law, or via measurement of intra-pleural pressure using oesophageal balloon

Question 30

Three reasons intra-pleural pressures falls during inspiration

Answer 30

1. Elastic recoil as lung expands 2. Reduction in alveolar pressure further reduces intra-pleural pressure 3. Tissue resistance

Question 31

Does alveolar pressure curve correspond with flow or volume?

Answer 31

Flow: would be identical if airway resistance remained constant

Question 32

Does intra-pleural pressure curve correspond with flow or volume?

Answer 32

Volume: would be identical if compliance remained constant

Question 33

What is the major site of airway resistance in the bronchial tree? What is the significance of this clinically?

Answer 33

Medium-sized bronchi, up to 7th generation Small airways disease may be present but not detectable on tests of pulmonary function

Question 34

What happens to airway resistance as lung volume falls and why?

Answer 34

Resistance increases as volume falls due to reduction in radial tension on bronchi (+/- small airway collapse as a result)

Question 35

What is the effect of the sympathetic and parasympathetic NS on bronchial smooth muscle tone and how is this mediated?

Answer 35

Sympathetic: bronchodilation via B2 receptors Parasympathetic: bronchoconstriction via muscarinic receptors

Question 36

What is the effect of decreased PCO2 on airway resistance?

Answer 36

Increases

Question 37

What is the effect of histamine on bronchial smooth muscle?

Answer 37

Acts as bronchconstrictor

Question 38

What is the effect of gas density on airway resistance? Give a clinical example

Answer 38

Increased density causes increased resistance As with diving (increased atmospheric pressure causes increased gas density)

Question 39

Describe dynamic airway collapse

Answer 39

Collapse of airways that occurs when intrapleural pressure exceeds alveolar pressure Occurs in normal lung: at certain point, increasing effort does not increase flow rate

Question 40

What is the effect of lung disease on effort-independent flow?

Answer 40

May occur at lower volumes Due to loss of elastic recoil and/or loss of radial traction

Question 41

Three factors which exaggerate dynamic airway collapse

Answer 41

1. Increased resistance (e.g. in obstructive airways disease) 2. Reduced elastic recoil 3. Decreased lung volume

Question 42

What is FEV1? What is the normal value? What change in FEV1 is seen in obstructive vs restrictive lung disease?

Answer 42

Forced expiratory volume: volume exhaled in first second of maximal expiration following maximal inspiration Normal value 4L Decreased in obstructive and restrictive lung disease but to greater extent in obstructive

Question 43

What is FVC? How does it compare to VC? What is the normal value? What change in FVC is seen in obstructive vs restrictive lung disease?

Answer 43

Forced vital capacity Slightly less than VC of slow expiration Normal value 5L Decreased in obstructive and restrictive lung disease

Question 44

What is FEV1/FVC? What is the normal value? What change in FEV1/FVC is seen in obstructive vs restrictive lung disease?

Answer 44

Ratio of FEV1 to FVC Normal value 80% Normal or increased in restrictive lung disease, reduced in obstructive lung disease

Question 45

What is PEF(25-75%)?

Answer 45

Forced expiratory flow rate over middle half of expiration

Question 46

What might cause inequality of ventilation in conducting vs respiratory zones?

Answer 46

Conducting: local differences in distensibility and resistance (influenced by RR: if there is a long time constant, inspiration may not be complete before expiration begins) Respiratory: diffusion limitation (e.g. if dilation increases distance over which diffusion must occur)

Question 47

What % of total resistance is represented by tissue resistance in healthy young adults?

Answer 47

~20%

Question 48

What is tissue resistance?

Answer 48

Pressure required to overcome viscous force of tissues sliding over one another during respiration

Question 49

What equation can be used to define work of breathing?

Answer 49

Work = pressure x volume

Question 50

What parameters increase viscous vs elastic work?

Answer 50

Increased RR causes increased viscous work (so those with obstructive lung disease tend to decrease their RR) Increased TV causes increased elastic work (so those with restrictive lung disease tend to decrease their TV)

Question 51

How is efficiency defined in terms of work of breathing? What is the normal efficiency % during (including during quiet breathing) and what can this be increased to with voluntary hyperventilation?

Answer 51

Efficiency % = work required to ventilate lung / total O2 cost x 100 Normal 5-10%, <5% quiet breathing, can be increased up to 30% with voluntary hyperventilation

Question 52

What is the difference in terms of the generation of driving pressure in positive pressure ventilation vs normal respiration?

Answer 52

Positive pressure ventilation generates driving pressure by increasing pressure at mouth rather than decreasing alveolar pressure