What type of vein primarily relies on surrounding tissues for support due to the absence of valves?

The correct answer is that soleal sinuses primarily rely on surrounding tissues for support due to the absence of valves. Soleal sinuses are specialized veins located in the soleus muscle of the calf. Unlike many veins in the body that contain valves to assist in venous return and prevent backflow, soleal sinuses lack these structures.

Because they do not have valves, these sinuses depend on the surrounding muscle contraction and the support provided by the adjacent tissues to maintain venous return to the heart. The contraction of the gastrocnemius and soleus muscles helps to propel blood through the sinuses, utilizing the pump effect to facilitate circulation. This reliance on surrounding tissues underscores the unique anatomical and functional aspects of the soleal sinuses compared to other types of veins.