Heart is the part of our circulatory system, it is very important and a major part of the organ in our body system. 
It is a pumping organ, because it helps our blood to travel all over in our body system to keep as alive and do heavy activities like playing outdoors such as volleyball.
Fun Facts:
Our heart contains 3-5Liters in our blood circulatory system, so that's a lot of blood to carry in ourselves and do activities everyday. Also sleeping in 8-10hrs helps our heart to relax and pump easily.