👩‍⚕️🫀 CCRN Cardiac Review – What actually causes the S1 heart sound? If you’re grinding for CCRN (or just want to sharpen your cardiac game), this one’s for you. S1 — that “lub” — isn’t random noise. It’s the closure of the mitral and tricuspid valves when ventricular systole kicks off. It marks the start of systole and tells you the ventricles are about to push blood hard into systemic and pulmonary circulation. 💥 👉 If you’re prepping for CCRN, these are the kind of rapid-fire clinical pear