Which blood vessels in humans contain structures that allow blood to flow in only one direction? (A) Arteries (B) Capillaries (C) Lymph vessels (D) Veins - HSC - SSCE Biology - Question 1 - 2008 - Paper 1

Veins contain structures known as valves that allow blood to flow in only one direction, preventing backflow. These valves are essential for the proper functioning of the venous system, especially in the limbs, where blood must travel against the force of gravity.

In contrast, arteries carry oxygenated blood away from the heart and do not have these values. Capillaries are the smallest blood vessels, where exchanges occur but are not involved in directional flow regulation. Lymph vessels also contain valves to ensure unidirectional flow of lymph fluid, but they are not classified as blood vessels.

Given these considerations, the correct answer is (D) Veins.