Final answer:
The subject of this question is Biology, specifically related to the circulatory system and blood flow. Small changes in vessel radius can cause dramatic changes in blood flow due to the relationship described by Poiseuille's law.
Step-by-step explanation:
The subject of this question is Biology, specifically related to the circulatory system and blood flow. In Biology, the concept of blood flow and vessel radius is important to understand how changes in vessel size can have a significant impact on the flow of blood in the body. The equation mentioned in the question, Poiseuille's law, explains that blood flow is directly proportional to the fourth power of vessel radius, which means small changes in vessel radius can result in dramatic changes in blood flow.