What do many limb veins contain?

1. Venous pressure is low, so backflow is a risk. 2. A structure enforces one way flow. 3. Many veins, particularly those of the limbs, contain valves that assist unidirectional flow toward the heart. 4. So the answer is valves. _Source: OpenStax 'Anatomy and Physiology 2e', Ch 20 'The Cardiovascular System: Blood Vessels and Circulation'_