Respiratory Physiology Lecture 3 Part 13: Resistance to Air Flow

Question 1

Flow = ??

Answer 1

Flow = ∆P/R

Question 2

Airway resistance to flow is _______

Answer 2

Airway resistance to flow is

generally small → small ∆P generates flow (~ 1 mmHg to move 0.5 L air)

Question 3

Resistive forces of airway resistance include?

Answer 3

1. Inertia of the respiratory system (Negligible)
2. Friction
3. Airflow patterns

Question 4

Factors of friction that cause airway resistance

Answer 4

• 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

Question 5

How do Airflow Patterns Influence the Resistance to Airflow?

Answer 5

• laminar flow
• transitional flow
• turbulent flow

Question 6

Which airflow pattern causes the least resistance?

Answer 6

laminar flow

Question 7

Laminar Flow

Answer 7

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

Question 8

Which airways typically have laminar flow?

Answer 8

characteristic to the small airways that are distal to terminal bronchioles

Question 9

Transitional airflow

Answer 9

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

Question 10

Which airways have transitional airflow?

Answer 10

airflow is transitional throughout most of the bronchial tree

Question 11

Turbulent flow

Answer 11

flow in which the fluid undergoes irregular fluctuations, or mixing,

• The effective resistance to airflow is the highest
• velocity is higher

Question 12

Which airways typically have turbulent airflow?

Answer 12

In the large airways (Trachea, larynx, pharynx), where the airway radius is large and linear air velocities may be extremely high

Question 13

Which airways have the highest resistance to airflow?

Answer 13

The smallest ones which are usually laminar flow

Question 14

Which flow pattern does poiseuilles law apply to?

Answer 14

laminar flow

Question 15

poiseuilles law

Answer 15

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)

Question 16

What is resistance to airflow sensitive to?

Answer 16

Resistance to airflow is highly sensitive to changes in airway radius

• (ie. A 10 % decrease in radius causes a 52 % increase in R, that is, a 34 % decrease in airflow)

Question 17

Total airway resistance in healthy subjects

Answer 17

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

Question 18

R is proportional to _____

Answer 18

1/r⁴ (inverse to r⁴)

Question 19

Each small airway has high _________

Answer 19

individual resistance

Question 20

Why do terminal bronchioles have lower resistance compared to the larger airways?

Answer 20

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

Question 21

series resistance compared to airflow

Answer 21

series has has higher resistance to airflow

Question 22

Which airways play a bigger role disease conditions?

Answer 22

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