From a medical point of view, a heart can indeed “break”, or in other words, it ceases to carry out its intended function due to a damage or defect. A heart attack or “cardiac arrest” occurs when a substantial amount of heart tissue has died. If enough tissue dies the person can go into heart failure where it stops pumping enough blood to keep the rest of the body alive. Then the heart is effectively “broken”.
While cardiac arrests are not uncommon, they are not caused by emotional stress; they are caused by much more mundane risk factors, such as smoking, high cholesterol, or having testicles (attached to your body, not in a jar). When somebody “dies of a broken heart”, it’s usually due to an uncommon disease known as Takotsubo cardiomyopathy, also known as “broken heart syndrome”, and possibly the inspiration for many fairy-tale tragedies.
“Takotsubo” is a Japanese word that roughly translates to “octopus pot”, which are clay pots used in traditional fishing practises. Takotsubo cardiomyopathy is so called because the main pumping part of the heart, the left ventricle, swells and causes the heart to adopt a shape similar to one of the pots.
The root cause of the condition is as yet unknown, but is triggered by emotional duress, whether positive or negative. So while this does include love-related disappointments, it also can be caused by stress from a natural disaster, loss of a family member, or even a wedding (the diametric opposite of heartbreak, usually).
“You might be wondering if a heart can physically break, not just wear down and die like a jalopy”
The good news is that Takotsubo cardiomyopathy is very rare, and people that do experience it will usually recover completely within months. 85% of patients only ever get one episode in their entire lives (but not because they immediately die), and the most at-risk group of developing this disease is post-menopausal women.
You might be wondering if a heart can physically break, not just wear down and die like a jalopy. As it turns out you can get a myocardial rupture, which is similar to Takotsubo cardiomyopathy, insofar as these occur as a direct consequence of weakened heart tissue.
Myocardial rupture occurs when an aneurysm forms and bursts, or after a heart attack. In an aneurysm, the heart muscle is weakened to the point where it can no longer endure the pressure, a tear may form, which subsequently breaks open and releases blood into the chest. If this tear occurs in the left ventricle (incidentally the part affected by Takotsubo cardiomyopathy) then death is nearly certain. It is worth noting that this is also an extremely rare occurrence.
Finally, to cover all bases, there is the issue about whether or not a heart can explode, either due to emotions or something else. Luckily, hearts do not explode (unless you count aforementioned myocardial rupture, which is more of a leak). The only circumstances under which a heart has exploded have been due to severe physical trauma from an outside source, such as being hit with a .50 cal bullet!