Why do veins have a large diameter?

Veins have a large diameter to accommodate the lower pressure of blood returning to the heart from the body's extremities. The wider lumen allows for a greater volume of blood to flow back efficiently, minimizing resistance. Additionally, the large diameter helps prevent blood pooling and ensures proper circulation, especially in the limbs where gravity can impede venous return. This structure, combined with valves, aids in maintaining unidirectional flow towards the heart.