Which factor does NOT aid venous return?

• A. Skeletal muscle pumps
• B. Respiratory movements
• C. Valves in veins
• D. Activity of skeletal muscles

Answer: (D)

Venous return relies on pressure differences and one-way flow to move blood back to the heart. When skeletal muscles contract, they squeeze nearby veins and push blood upward—this is the skeletal muscle pump. Breathing changes also help: during inspiration the chest pressure drops and abdominal pressure rises, creating a pressure gradient that pulls blood toward the heart, which is the respiratory pump. Veins have valves that prevent backflow, so once blood is propelled toward the heart, the valves keep it moving in the correct direction. The phrase activity of skeletal muscles encompasses the very muscle contractions that drive the skeletal muscle pump, so this activity does aid venous return. Because all of these factors support venous return, none of the listed options truly does not aid it; the question as written is ambiguous since one would expect a factor that does not aid, but these mechanisms all help move blood back to the heart.