Being valveless, the dural venous sinuses allow which type of blood flow?

• A. Bidirectional blood flow
• B. Unidirectional blood flow
• C. Arterial-only flow
• D. No blood flow

Answer: (A)

The key idea is that these dural venous sinuses are valveless channels, so their flow isn’t locked into one direction. Blood moves through them according to pressure gradients between the cerebral veins feeding the sinuses, the sinuses themselves, and the systemic venous system (the internal jugular veins). If the pressure beyond the sinuses is higher, flow can move toward the jugular veins; if the pressure on the brain side rises relative to the downstream veins, the direction can shift. In normal physiology, blood tends to drain from the brain into the sinuses and onward to the jugular system, but without valves, the direction can adapt to changing pressures, allowing bidirectional flow when needed.