This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': "Poiseille's-Hagen law is related to \nSelect and explain. \nA. Airflow resistance, B. Rate of blood flow, C. Measurement of BP, D. None of the above", 'role': 'user'}, {'content': 'Answer: B \nExplanation: In the small blood vessels, essentially all the blood is near the wall, so extremely rapidly flowing central stream of blood doesn&;t exist. By integrating the velocities of all the concentric rings of flowing blood and multiplying them by the areas of the rings one can derive the formula known as poiseuille&;s law.In the equation, the rate of blood flow is directly propoional to 4th power of the radius of vessel.Ref: Guyton and Hall, Textbook of medical physiology, 2nd South Asia Edition, page no. 233', 'role': 'assistant'}]