Slight vasodilation in an arteriole prompts a __. a. slight increase in resistance b. huge increase in resistance c. slight decrease in resistance d. huge decrease in resistance
The correct answer and explanation is:
The correct answer is: d. huge decrease in resistance
Explanation:
Vasodilation refers to the widening or relaxation of blood vessel walls, especially arterioles, which are small arteries leading into capillary beds. Even a slight vasodilation in an arteriole results in a huge decrease in resistance to blood flow. This is due to the nonlinear relationship between the vessel radius and resistance, described by Poiseuille’s law.
Poiseuille’s Law and Resistance:
Poiseuille’s law states that resistance RR to flow in a cylindrical vessel is inversely proportional to the fourth power of the radius rr: R∝1r4R \propto \frac{1}{r^4}
This means that if the radius of the arteriole increases even slightly, the resistance decreases dramatically because the radius is raised to the fourth power. For example:
- If the radius doubles, the resistance decreases by a factor of 24=162^4 = 16.
- If the radius increases by just 10%, the resistance decreases by about 34%.
Why does this matter?
Arterioles are the main site of variable resistance in the circulatory system and play a critical role in regulating blood pressure and flow to specific tissues. Because resistance in these vessels dramatically affects blood flow, slight vasodilation leads to a large decrease in resistance and thus a significant increase in blood flow downstream.
Physiological Implications:
- During exercise or increased metabolic demand, arterioles dilate in active muscles, decreasing resistance and allowing greater blood flow to meet oxygen and nutrient needs.
- Blood pressure regulation: Vasodilation lowers systemic vascular resistance, which can reduce arterial blood pressure.
- Pathology: Excessive vasodilation can lead to dangerously low blood pressure (hypotension), while inadequate vasodilation can cause high resistance and hypertension.
In summary, because resistance is inversely proportional to the radius to the fourth power, a slight vasodilation causes a huge decrease in resistance in an arteriole. This change has significant consequences for blood flow and cardiovascular regulation.