The lub heart sound is due to the closing of atrioventricular valves.
Explanation:
Heart sounds are noises that are generated from beatings from heart and from blood that flows through it. The heart sound reflect the turbulence generated when the heart valves snap shut. Lub is the first heart sound also called S1 and it is caused by turbulence and closure of the atrioventricular valves(mitral and triscupid valves) at the start of systole. The second sound is the dub sound which caused by the closing of aortic and pulmonic valves at the end of systole.
So where then did all the carbon that living organisms are built of come from? It turns out that most of the carbon we use today came from a collision with another smallish planet about 4.4 billion years ago.
