Heartache

I want to know what it is anatomically, physiologically, scientifically,
that causes the pain in our chests when we experience loss and sadness.
If what we feel is mental and therefor, in our heads, then why is this unbearable pain I feel located squarely in the center of my chest, where my heart dwells?
It is an ache that almost compels me to simply cut my heart clean out of my chest, just to make it stop aching. Never have I had less control of a situation I was in. Never have I been 100% at the mercy of someone else.
You open me up, my walls crumble away at just the slightest presence of you, so why, if what you don't want, is to hurt me, do you hurt me by running away to prevent hurting me? You can't hurt me if you stay. You can't hurt me if you make mistakes with me, the only thing that hurts is the loss of you.
Maybe if I had no heart I wouldn't feel this hurt, but that defies logic because the heart isn't where emotions live.
So what is this pain I feel in my chest?
To taste your kisses and feel your skin pressed against mine, heartbeat to heartbeat, to listen to the sound of your breathing, to smell you,
those things would wipe this hurt away.

Undo this. Unhurt me. I only have this one heart, please don't break it.