What are these structures stop blood flowing backwards into the atria.

Valves stop blood flowing backwards

Explanation: Valve are flags of tissue that control the direction of the blood in our heart.,it prevent the backwards flow of blood. valves regulate blood from flowing back into the atria.The right and left atrio-ventricular valves prevent blood from flowing backwards into atria. When the ventricle is full,the valve prevent backwards flow of blood.

answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;

a=c and then d= c because of the band so you get ac/dc and you add them together you get sound waves