- Veins are blood vessels that carry blood back to the heart.
- Unlike arteries, veins operate under lower pressure, which increases the risk of blood flowing backward due to gravity, especially in the legs.
- To prevent this, veins contain one-way valves that ensure blood flows in only one direction—toward the heart.
- These valves are essential for maintaining proper circulation and preventing issues such as venous insufficiency or varicose veins.

- In contrast, arteries carry blood away from the heart at higher pressure and thus do not require valves.
- Capillaries are tiny blood vessels where gas and nutrient exchange occur and are too small to contain valves.
- Venules are small veins that collect blood from capillaries and funnel it into larger veins; while venules may contain valves, they are much fewer compared to the larger veins.

Key points:
- Veins have valves to prevent backflow of blood.
- Valves maintain unidirectional blood flow toward the heart.
- Arteries, capillaries, and venules do not primarily rely on valves like veins do.

Reference: Gray's Anatomy, Volume 2, Chapter 12, Page 678