How many chambers does the human heart have?

Four - two atria that receive blood and two ventricles that pump it out - and the arrangement exists to keep two separate circuits from mixing. The right side sends blood to the lungs, the left sends it round the body, and because the body is a much longer circuit the left ventricle is enormously thicker than the right.

The evolutionary sequence is legible in living animals. Fish have two chambers and one circuit; amphibians have three, with a single ventricle in which oxygenated and deoxygenated blood partly mix; most reptiles have a partly divided ventricle; birds, mammals and crocodiles have four and complete separation. The advantage of separating is pressure: the lungs are delicate and need low pressure, the body needs high, and one pump cannot supply both well.

The valves between the chambers are what you hear. The lub is the atrioventricular valves closing as the ventricles contract, the dub the aortic and pulmonary valves closing as they relax - and a murmur is turbulence where a valve is not sealing or not opening properly. The whole thing runs on its own electrical rhythm from the sinoatrial node, which is why a heart continues beating outside the body and why cardiac cells in a dish will spontaneously pulse and fall into step.