Valves in blood vessels prevent backflow in veins

Steps:

• Identify vessel types: Capillaries are thin exchange sites, arteries carry blood from heart under pressure, veins return blood to heart.
• Recall valve function: Valves are semilunar flaps that stop blood reversing direction, mainly in peripheral veins against gravity.
• Check specific vessels: Renal artery is an artery (no valves needed due to pressure); capillaries lack them for free flow; renal vein, as an abdominal vein, has no valves due to steady pressure gradients.
• Conclude: None of these contain valves.

Why B is correct:

• Veins generally have valves, but abdominal veins like the renal vein lack them because blood flow relies on pressure differences without gravity opposition, per circulatory anatomy.

Why the others are wrong:

• A: Incorrectly assigns valves to capillaries, which need open flow for diffusion.
• C: Wrongly places valves only in renal vein, ignoring that abdominal veins typically lack them.
• D: Erroneously adds valves to renal artery, which maintains unidirectional flow via arterial pressure alone.

Final answer: B