Respiratory Physiology Lecture 3 Part 13: Resistance to Air Flow Flashcards
Flow = ??
Flow = ∆P/R
Airway resistance to flow is _______
Airway resistance to flow is
generally small → small ∆P generates flow (~ 1 mmHg to move 0.5 L air)
Resistive forces of airway resistance include?
- Inertia of the respiratory system (Negligible)
- Friction
- Airflow patterns
Factors of friction that cause airway resistance
- Lung tissue past itself during expansion in alveoi
- Lung & chest wall tissue surfaces gliding past each other
- (Intrapleural fluid significantly reduces friction)
- Frictional resistance to the flow of air through the airways represents 80% of total airway resistance
How do Airflow Patterns Influence the Resistance to Airflow?
- laminar flow
- transitional flow
- turbulent flow
Which airflow pattern causes the least resistance?
laminar flow
Laminar Flow
state of flow in which air moves through a tube in parallel layers, with no disruption between the layers, and the central layers flowing with the greatest velocity.
- The subject invests relatively little energy in airflow RESISTANCE
- Airflow resistance is even more sensitive to changes in radius when flow is NOT laminar
Which airways typically have laminar flow?
characteristic to the small airways that are distal to terminal bronchioles
Transitional airflow
Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges
- It takes extra energy to produce vortices → the resistance increases
Which airways have transitional airflow?
airflow is transitional throughout most of the bronchial tree
Turbulent flow
flow in which the fluid undergoes irregular fluctuations, or mixing,
- The effective resistance to airflow is the highest
- velocity is higher
Which airways typically have turbulent airflow?
In the large airways (Trachea, larynx, pharynx), where the airway radius is large and linear air velocities may be extremely high
Which airways have the highest resistance to airflow?
The smallest ones which are usually laminar flow
Which flow pattern does poiseuilles law apply to?
laminar flow
poiseuilles law
For laminar flow (Gases/liquids) Poiseuille’s law state that airway resistance is proportional to the viscosity of the gas (η) and the length of the tube (l), but is inversely proportional to the fourth power of airway radius (r)
Total airway resistance in healthy subjects
From the total airway resistance in healthy subjects, that is 1.5 cm H2O/(L/s) [R varies between 0.6 and 2.3]
- 0.6 is in the upper airway passages (Pharynx/larynx)
- 0.6 in the large airways
- Only 0.3 in the small airways
Why do terminal bronchioles have lower resistance compared to the larger airways?
the approximately 65,000 terminal bronchioles aligned in parallel have a much lower aggregated resistance compared to the only few large airways because for airways arranged in parallel the resistance at each specific generation when there is respiratory airflow will be given by the inverse of each specific resistance. So respiratory airflow at terminal bronchioles is the sum of them instead of taken as individual.
1 = 1 + 1 + 1 +… Rin parallel R1 R2 R3
series resistance compared to airflow
series has has higher resistance to airflow
Which airways play a bigger role disease conditions?
In disease conditions, the small airways often play a far greater role in determining airflow resistance than the large airways, because airflow can change here significantly and they are easily occluded by:
- Smooth muscle contraction in their walls will increase resistance
- Edema occurring in the walls of the alveoli and bronchioles because pressure of fluid in small airways can reduce space for air to move in
- Mucus collecting in the lumens of bronchioles reduces alveolar space at level of bronchioles
