**Question:** Which of the following are known as the resistance vessels?

A. Arterioles
B. Capillaries
C. Veins
D. Postcapillary venules

**Core Concept:** Resistance vessels are small muscular arteries and their branches that play a crucial role in regulating blood flow and blood pressure. They act as the middle segment of the cardiovascular system between the large arterioles and the small capillaries.

**Why the Correct Answer is Right:** Arterioles are the correct answer as they are the first order branching of the arteries. They are responsible for controlling blood flow to the surrounding tissues and maintain blood pressure by constricting or dilating based on the body's need.

**Why Each Wrong Option is Incorrect:**

B. Capillaries: Capillaries are the smallest vessels in the circulatory system, where the exchange of oxygen, nutrients, and waste products occurs between the blood and the surrounding tissue cells. They are not involved in regulating blood flow or blood pressure, making them the incorrect choice.

C. Veins: Veins are the return vessels of the circulatory system, transporting deoxygenated blood from the body back to the heart. They are not involved in blood flow regulation, making this option incorrect.

D. Postcapillary venules: Postcapillary venules are the smallest veins in the capillary bed, where oxygen and nutrients are absorbed, and waste products are released into the surrounding tissue. They are not involved in regulating blood flow or blood pressure, making them the incorrect choice.

**Clinical Pearl:** Understanding the role of resistance vessels is essential for medical students and healthcare professionals as it helps in understanding the pathophysiology of various cardiovascular diseases, such as hypertension and hypotension, and the therapeutic strategies employed to treat them.