Question

what do the vessels carrying oxygen - poor blood back to the heart have that prevents blood from going backwards?
valves
thick musuclar walls
hemoglobin
sa nodes

Explanation:

Veins carry oxygen - poor blood back to the heart. They have valves that prevent back - flow of blood. Valves open to allow blood to move towards the heart and close to stop reverse flow. Thick muscular walls are more characteristic of arteries. Hemoglobin is an oxygen - carrying protein in red blood cells. SA nodes are part of the heart's electrical conduction system.

Answer:

valves