What is the function of vein valves?

• A. Prevent blood from moving back into the capillaries
• B. Increase blood flow toward the heart
• C. Dilate the veins
• D. Direct blood to the lungs

Answer: (A)

Vein valves act as one-way doors that keep blood moving toward the heart. They close to stop backward flow, especially when standing or with low venous pressure, so blood doesn’t pool or flow back toward the capillaries. When skeletal muscles contract, they squeeze the veins and push blood past the valve opening toward the heart, and the valve then snaps shut to prevent any reverse movement. This mechanism relies on pressure gradients rather than actively pushing blood forward. Valves don’t actively dilate veins, and they don’t direct blood to the lungs; those tasks involve smooth muscle tone and cardiac/pulmonary circulation, respectively.