FLOW RESISTANCE

A. Definition: R = ΔP / V'  (note: V' = Flow rate = dV/dt; ΔP = P_alv - P_mouth, but P_mouth is usually zero)

R = R_airway + R_{tissue viscosity}      (Note:  R_aw = R_airway)

B. Determines the rate of air movement for a given pressure change

V' = ΔP / R

C. Typical value in normal young adult of average size

2 cmH₂O / (Liter/sec) or less

D. Determinants of R

1. gas viscosity (generally constant, since air)

2. length of airways (constant)

3. airway diameter (main variable)

a. bronchial smooth muscle contraction

b. bronchial secretion: narrows lumen if accumulates faster than removed by cilia (or coughing)

c. autonomic nervous system, through its effects on (a) and (b) above:

d. volatile irritants: generally stimulate secretion; may also cause bronchoconstriction (note sensory endings in airways)

e. airway inflammation (bronchitis): stimulate secretion, can lead to tissue edema and swelling (brochiolitis)

f. tracheal and bronchial reflexes

g. temperature: e.g. inspiring cold air can cause smooth muscle contraction

h. agents acting directly on bronchial smooth muscle

1) constrictors: histamine, acetylcholine
2) dilators: epinephrine, atropine, CO₂ , Ventilin, albuterol

i. pressure across the airway wall (lumen pressure minus intrapleural pressure: tends to collapse airways if Ppl > 0

Note:  flow limiting segment in expiration: upon increasing expiratory effort, the increase in Ppl tends to reduce airway diameter, so that flow does not increase proportionally to effort

j. supporting tissue in airway wall: resists (i) above

k. lung volume: brochioles become smaller as lung volume decreases; not usually major factor

4. position in the bronchial tree (number in parallel) which influences flow velocity and flow resistance: major resistance is in the large and medium sized airways

5. Tissue viscosity also contributes to resistance to lung volume change, but the effect is not major (only about 20% of total resistance)