Lub-Dub

In medical school, complete restructuring of what you always assumed fact happens quite a lot. You formulate a hypothesis on why something works the way it does when first introduced to a mechanism--only to realize it does not work that way at all when graduate school reintroduces you to it.

For example, our exam two Fridays ago, covered the cardiovascular system. We've already gone over the material cursorily in physiology, anatomy, and embryo last year (I say cursorily only because I don't remember doing particularly well on any of the material covering the heart). But even stretching beyond what I learned in my first year at medical school, I've been learning about the human heart for how long? Since fifth, maybe sixth, grade? That's thirteen years of knowing how the heart works, more or less. A lengthy exposure to the material plus the admission that the sound of a beating heart was the first thing I ever heard, either resounding from my own heart or from my mother's--and you'd think I would know why the heart sounds the way it does. 

But I just realized today that I was completely wrong in my thinking. The lub-dub sound your heart makes about 80,000 times a day is not caused by cardiac muscle rebounding from contractions. It's caused by your valves snapping shut after they're done ejecting blood. You'd think that would sound more like a clap. Nope. Lub-dub, lub-dub. I blame the giant heart at the Museum of Science and Industry downtown for this faulty information. I recall being distinctly unnerved by the sound of a beating heart emanating from an unseen corner of the multi-floored entryway atrium, and upon arriving at the source of such a terrifying sound--a giant modeled heart a child could walk through--there were no flapping heart valves that coincided with the lub-dub. I think there were lights around it stimulating electricity moving from node to node, so I assumed that the electricity, and thus muscle contractions, had to be the source of the heartbeat sound.

So valves make heart sounds. Got it.

Oh wait, Bates (Guide to Physical Exam and History Taking 10th ed.), you have something you want to add?

An extensive literature deals with the exact causes of heart sounds. Possible explanations include actual closure of valve leaflets, tensing of related structures, leaflet positions and pressure gradients at the time of atrial and ventricular systole, and the effects of columns of blood. The explanations given here are oversimplified but retain clinical usefulness.

Wait, so maybe I was right? Hooray! But more importantly: It amuses me to no end that we still don't know the exact causes of things that seem really fundamental. While it is true I am in the process of learning more factual information than what 99% of the global population could ever learn*, what's even more true is that I'm also being given more questions than anyone should have to think about.

*emphasis on "factual information". While I'll acknowledge that I am smart, I have not dealt with enough of the 7 billion odd humans on this planet to ascertain if I'm worthy of declaring I'm more intelligent than most of them.