Yet another question you never asked, but now that I mention it, yeah, that's a good point
You know how, when you put a stethoscope to someone's chest, you hear a thump-thump? (Or lub-dub or da dum or whatever you wish to call it). Or if you haven't done it, you are familiar with what I'm talking about. Now if what we hear is the heart contracting, why are there two thumps and not one? Check your pulse at the wrist or neck - you only feel one thump per heart beat.
Well, the reason is that what you hear in the stethoscope is not the heart contracting. (You can't hear a muscle contracting, otherwise you'd be thumping all over the place when you eat, walk, etc.) It is the valves closing.
Let me explain a bit about the cardiac cycle: the top chambers of the heart are called the atria and the bottom chambers are called the ventricles. The job of the atria is to receive blood from the veins, while the job of the ventricles is to push blood out of the heart. The left atrium receives blood from the lungs and the right atrium receives blood from the rest of the body. The left ventricle sends 'new' (oxygenated) blood to the body (through the aorta), and the right ventricle sends 'used' (deoxygenated) blood to the lungs, via the pulmonary artery.
Between the atria and the ventricles are valves called the
atrio-ventricular valves (AV valves) - which are the mitral valve between the left atrium and the left ventricle, and the tricuspid valve between the right atrium and the right ventricle. Between the ventricles and their respective arteries there are valves called the aortic and the pulmonic valves.
In the cardiac cycle, the atria contract and push blood into the ventricles. A split second after the atria contract, the ventricles begin to contract. As the pressure in the ventricles increases, the AV valves close, to prevent blood from flowing backwards. The sound made by the AV valves closing is the first thump of the heart beat. The ventricles contract and push blood out through the arteries. The ventricles then relax, and the aortic and pulmonic valves shut down, to keep the blood from flowing back into the heart. The sound of these valves closing causes the second thump.
The atria then fill up again with blood flowing to them from veins, to begin the next cardiac cycle.