Mechanics II


Respiratory Lecture 10: Mechanics of Breathing II

 

Topics to be covered:

 

  1. Dynamic properties of ventilation
    1. Elastic recoil of lungs and chest wall
    2. Resistance to airflow
    3. The three patterns of airflow through tubes
      1. Laminar flow
      2. Turbulent flow
      3. Transitional flow
  2. Factors determining airway resistance
    1. Viscosity
    2. Lung volume
    3. State of contraction of bronchial smooth muscle
  3. Pressure changes during restful breathing
  4. Dynamic compression of airways
  5. Causes of uneven ventilation
  6. Work of breathing

 

Main objective:

To learn how the fluid properties of air and the structure of the conducting airways affect airflow in the lungs.

 

Assigned reading: West, Chapter 7, pp 93-120.

 

Problems: Questions 1-14 on pp 118-120;

Problem Set 10


Mechanics of Breathing II

 

A. Dynamic Properties of Ventilation

 

* 80% of airway resistance (AWR) is produced by medium sized bronchi, 20% of AWR is produced by small airways with a diameter <2mm.

Pressure/Flow Relationship for Laminar Flow is given by:

Poiseuille’s Law: V = Pπr4/8ηl

η = viscosity ; l = length of tube or vessel

Reynold’s number (Re) = 2rvd/η

r = radius

v = velocity

d = density

η = viscosity

If Re >2000, air flow is turbulent

 

Flow of air (V) = ∆P/R

∆P = pressure difference between alveoli (or “equal pressure point” in airways during forced expiration) and mouth

∆P down an airway depends on Flow Rate (V) and airflow pattern.

Since R is chiefly AWR, AWR = (Palv - Pmouth) / V

Pressure drop for Laminar Flow, P∝V

Pressure drop for Turbulent Flow, P∝V2

 

The overall driving pressure is determined by both V and V2:

P = K1V + K2V2

K1 and K2 are constants that take into consideration all the resistances associated with flow.

 

B. Factors Determining Airway Resistance (AWR)

 

NE (α1, β1, weak β2)

E (α1, α2, β1, β2)

Isoproterenol (β agonist)

Pressure Changes during Restful Breathing (pp. 105-107 of West)

x. Pressure needed to overcome lung elastic recoil; y. Pressure needed to overcome lung elastic recoil plus AWR

Figure 5. Intrapleural pressure during the breathing cycle. The pressure difference between -5 (start of inspiration) and the broken line (ABC) represents the pressure needed to overcome lung elastic recoil. The additional pressure needed to overcome AWR (and some tissue resistance) is depicted by the hatched area. The actual intrapleural pressure follows the solid line (AB’C). NOTE: the pressure difference between B and B’ is the alveolar pressure at that instant.

D. Dynamic Compression of Large Airways (Figs. 6 and 7)

 

Figure 6. Flow-volume curves obtained by recording flow rate against volume during a forced expiration from maximum inspiration. The figure shows absolute lung volumes, although these cannot be measured from single expiration. A) A normal flow-volume pattern. B) Comparison of typical obstructive and restrictive diseases to the normal flow-volume curve.

Figure 7. Compression of larger airways during forced expiration. When transmural pressure (pressure difference across the airway) becomes negative (inside – outside), airways will collapse. As the forced expiration continues, the equal pressure point will move inwards (towards the lung) because you are exhaling air and the pressure falls throughout the airways.

 

F. Work of Breathing**