What specific veins in the body are known not to contain valves?

The specific veins in the body known not to contain valves are the soleal sinuses and the inferior vena cava. The soleal sinuses are large venous reservoirs located in the soleus muscle of the calf, and they play a critical role in the venous return during periods of muscular contraction. The inferior vena cava, which is the largest vein in the body, also lacks valves. This is due to its function as a major conduit for deoxygenated blood returning to the heart from the lower body, where the pressure dynamics are unidirectional and do not require valves to prevent backflow.

In contrast, superficial veins and deep veins typically contain valves to help prevent the backflow of blood, relying on muscle contractions and changes in pressure to assist in venous return. External iliac veins, being part of the deep venous system, also contain valves for similar purposes. Understanding the unique anatomy and physiological roles of these veins highlights the importance of venous function and the mechanisms of blood return in the circulatory system.