In human anatomy, blood vessels are classified primarily into arteries, veins, and capillaries, each with unique structural features adapted to their functions. Identifying these vessels involves understanding their wall thickness, lumen size, and presence or absence of valves.

Given the description:

• Blood vessel A has a thick wall, narrow lumen, and no valves.

• Blood vessel B has a thin wall, wide lumen, and valves.

The correct interpretation is:

(1) A is artery and B is vein.

Why Vessel A is an Artery

Arteries transport blood away from the heart under high pressure. To withstand this pressure, arteries have:

• Thick walls, containing multiple layers of smooth muscle and elastic fibers to maintain structural integrity and elasticity.

• Narrow lumens that help maintain blood pressure and facilitate rapid blood flow.

• No valves because the pressure generated by the heart’s pumping keeps blood moving forward continuously.

These features enable arteries to efficiently deliver oxygenated blood (except pulmonary arteries) to tissues throughout the body.

Why Vessel B is a Vein

Veins return blood to the heart, usually under lower pressure, making their structure quite different:

• Thin walls with less smooth muscle and elastic fibers since the blood pressure is low.

• Wide lumens to hold larger volumes of blood and to accommodate the slower blood flow.

• Presence of valves to prevent backflow of blood and assist in its movement toward the heart, particularly against gravity.

Veins carry primarily deoxygenated blood (except pulmonary veins) back to the heart.

Additional Structural and Functional Differences Between Arteries and Veins

Why Capillaries Are Different

Capillaries have different characteristics:

• Very thin walls (one cell thick) to enable exchange between blood and tissues.

• Extremely narrow lumen.

• No valves present.

• They function as the connection between arteries and veins and facilitate nutrient, gas, and waste exchange.

Since blood vessel A and B do not fit the characteristics of capillaries—especially regarding wall thickness and presence of valves—they cannot be capillaries in this context.

Functionality Based on Structure

• The thick walls of arteries allow them to endure and regulate high blood pressure generated by the heart. Their narrow lumen helps maintain that pressure.

• The thin walls and wide lumen of veins suit their role in accommodating large volumes of blood at lower pressure.

• Valves in veins are crucial for ensuring the one-way flow of blood back to the heart, combating the effects of gravity, especially in limbs.

Summary

• Blood vessel A, with thick walls, narrow lumen, and no valves, is an artery.

• Blood vessel B, with thin walls, wide lumen, and valves, is a vein.

• This corresponds to statement (1) being the correct option.