Veins are the human blood vessels that commonly contain valves to help prevent backflow.
They carry blood toward the heart, often against gravity, especially from the legs. One-way venous valves help keep blood moving in the correct direction. Contraction of nearby skeletal muscles can compress veins and assist this return, a mechanism often called the muscle pump.
Most veins carry oxygen-poor blood, but the pulmonary veins are an important exception: they carry oxygen-rich blood from the lungs to the left atrium. Arteries generally carry blood away from the heart, while capillaries are microscopic exchange vessels linking arterioles and venules.
Valve function can be affected when veins become stretched or their valves fail to close effectively. This can contribute to varicose veins. The presence of valves is especially important in the limbs, where blood must travel upward toward the heart.