Laminar flow in small airways – Physiology MCQ

Laminar flow in small airways is due to?
A. Small diameter
B. Small area of cross section
C. Low velocity of flow
D. Reynolds’s number more than 2000

Correct answer : C. Low velocity of flow

• The probability of turbulence is determined by Reynold’s number.
• When it is more than 3000, the flow is almost always turbulent.
• When it is less than 2000, the flow is usually non turbulent (laminar).
• Reynold’s number is directly proportional to:
  • Density of fluid
  • Diameter of vessel
  • Velocity of flow
• It is inversely proportional to:
  • Viscosity of fluid
• So in this case, options A (small diameter) and C (low velocity of flow) are correct.
• But velocity of flow is a more important factor and is hence the answer of choice.