0825- mechanics of breathing- CG

Question 1

Describe the characteristics of airflow in the airways

Answer 1

Flow over vocal cords is the biggest (turbulent), and decays later to small values in alveolar airways (laminar).
Transitional flow overall in bronchi- turbulent at bifurcation points, laminar in airways
Laminar flow in alveoli

Question 2

What are the determinants and components of Raw

Answer 2

R(visc) = 40% (dynamic flow dependent) (stands for viscoelastic?)
• Laminar and turbulent conditions (80%)
• Tissue resistance (“friction” between elastic fibres; 20%)
• Inertia (very little)

R(elastic) = 60% (static, volume dependent)

In COPD, resistance goes up due to reduced radii

Question 3

What is Raw modulated

Answer 3

Modulation of Raw- neurally modulated

ANS controlled smooth muscle tone
- PNS: bronchial constriction and increased mucus production
- SNS: B2 action- smooth muscle relaxation, reduced mucus
Alveolar Pco2
- Increase: local airway dilation
- Decrease: constriction

Radius also affected by inspiration/expiration
Raw is much higher during expiration compared to inspiration –>air
Expiration- alveoli have positive pressure, compress airways = reduced radius
Inspiration- negative pressure, expand airways large radius

Transmural pressure (affecting bronchioles) also contribute 
Forced inspiration- postive transmural prssure keeps small airways open
Forcecd expiration- due to positve intrapleural pressure tranmural pressure can become negative, collapse airways

Question 4

How is lung compliance measured? (static and dynamic)

Answer 4

Static: no flow, volume fixed
Dynamic: both flow and volume change

Cstatic measured with spirometer and valve (measure volume change after breathing out- usually linear within range of TV)

Question 5

What are the different compliances that may be significant for breathing?

Answer 5

Static compliance
C(T) (total breathing system)
C(L)- lung compliance
C(cw)- chest wall compliance
Within TV, C(L) = C(cw) = 2C(T)
1/C(T) = 1/C(cw) + 1/C(L)

Pathology: C(l) too small = fibrosis. Too large = emphysema
Scoliois: C(cw) too small

Dynamic- compliance changes between inspiration and expiration (hysteresis
Effort sets width of hysteresis)
Static compliance is the average
Also changes at the beginning and end (lower compliance at the beginning)

Question 6

What factors determine C(L)

Answer 6

1. Laplace’s law= smaller alveoli have higher recoil pressure –> blow up bigger alveoli IF NOT (interdependent in parallel, not series, AND lined with surfactant
   P(recoil) = 2T/r
   1. Surface tension is the largest determinant of C(L)

Minimise surface tension = higher compliance

Question 7

Describe surfactants and explain what they do

Answer 7

Surfactant (surface-active agent)- form micelles, keeps alveoli open and dry (lipids)
• Reduces surface H2O and hence surface tension: it is an attractive force of surface molecules that tends to minimise surface area.
• Combination of dipalmitoyl- phosphatidylcholine and apoproteins secreted by alveolar type II cells
• Can easily be destroyed with O2.
• Produced shortly before birth; problem in premature babies (respiratory distress syndrome).
– Steroid priming for 2-3 d can initiate surfactant expression.

Question 8

Describe what occurs to the surfactants during respiration

Answer 8

Dynamic system:
– During I, as alveolar surface increases and [surfactant] decreases, surfactant from micelles is recruited to surface.- compliance increases
– During E, alveolar surface decreases, [surfactant] is higher and surfactant then re- forms micelles- compliance decreases

• Rapidly expanding alv. → [surfactant]↓ → CA↓ → ventilation↓. 
• Slowly expanding alv. → [surfactant]↑ → CA↑ → ventilation↑. 
Or rather, initial expansion difficult (due to Laplace's law and miscelles are buried underneath the surface. As work is put in, however, the wall distends, and the micelles come through the surface, which also reduces surface tension. Hence compliance is low at first, and then becomes higher (ventilation also increases)