Nursing Entrance Exam (NEX) Practice Exam 2025 - Free NEX Practice Questions and Study Guide

Veins contain valves that play a crucial role in maintaining unidirectional blood flow back to the heart. These valves are essential because they prevent the backward flow of blood, especially in areas of the body where blood must travel against gravity, such as in the legs. The presence of these valves helps to ensure that blood moves efficiently and reduces the risk of venous blood pooling.

In contrast, lymph ducts primarily function in the lymphatic system and also have valves, but they are not the primary structures associated with the circulation of blood. Arteries do not contain valves because they are designed to carry blood away from the heart under high pressure, and their muscular walls maintain the pathway of blood flow. Capillaries, being tiny vessels where exchange of gas and nutrients occurs, do not have valves since their primary function does not involve maintaining directed blood flow like veins do. Thus, the presence of valves is a defining feature of veins in the context of the question.