The heart is a pump that pumps blood to all parts of the body. Let us consider how this pump works.
For example, suppose there is a pump that pumps oil into a stove. When the pump pumps oil into the stove, the bottom lid first opens to let the oil in. As soon as the pump finishes pumping in the oil, the lid is closed. Next, when the oil is pumped out, another lid opens and the oil flows out the other end. This pump pumps the oil out while the two lids are opening and closing.
The heart works in the same way as the oil pump. The heart also has two lids that open and close as it pumps blood.
This lid must be closed in a great hurry to prevent the blood from flowing back in. The lid is tightened so tightly that it makes a cracking sound, and this sound is what is heard as a heartbeat. In other words, the pounding of the heart is the sound of the heart pump lid closing.