This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Flow is laminar in small vessels because 
Select and explain. 
A. Reynolds number is >2000, B. Total cross sectional area of small vessels is smaller, C. Diameter of smaller vessels is less, D. Effective velocity in small vessels is less
D: Effective velocity in small vessels is less
Answer: D: Effective velocity in small vessels is less 
Explanation: The most important determinant of flow of blood in small blood vessels is effective velocity. The velocity is inversely proportional to the cross-sectional area. In the case of small vessels, the cross-sectional area is large. So the effective velocity is low. When the velocity is low, flow is laminar.