______are similar structurally to arteries but have a flap-like valves inside to prevent back flow of blood. A. capillaries B. veins C. arterioles D. venules
The correct answer and explanation is:
Correct Answer: B. veins
Veins are blood vessels that carry blood toward the heart. Structurally, they are similar to arteries in that they have three layers: an inner endothelium, a middle layer of smooth muscle, and an outer layer of connective tissue. However, veins have thinner walls than arteries because the blood they carry is under lower pressure.
A unique feature of veins is the presence of flap-like valves inside their lumen. These valves are formed by folds in the inner lining (endothelium) of the vein. Their primary function is to prevent the backflow of blood and ensure it moves in one direction — toward the heart. This is especially important in the limbs, where blood must travel against gravity to return to the heart. When muscles contract, they squeeze the veins and push blood forward; the valves close behind the blood to stop it from flowing backward.
Capillaries, on the other hand, are microscopic blood vessels with only a single layer of endothelial cells. They allow for the exchange of gases, nutrients, and wastes between the blood and surrounding tissues. They do not have valves and are structurally very different from arteries or veins.
Arterioles are small branches of arteries that lead to capillaries. They have muscular walls and help regulate blood pressure and flow into the capillary networks. Like arteries, they do not contain valves.
Venules are small vessels that collect blood from capillaries and transport it to veins. They are part of the transition from capillary beds to larger venous vessels and generally do not contain valves.
Therefore, veins are the correct answer because they resemble arteries in structure but contain valves that prevent backflow of blood, a critical function in maintaining efficient circulation.