When you're depressed, doesn't your entire body feel like shrinking to a ball? All you want to do is curl up, be as close to the floor, to yourself, as possible. Heartstrings are strings that connect your heart to every nerve, every bit of flesh on your body. When your heart wrenches, it contracts and twists and these strings pull all the parts of your body inwards to it. As your heart shrinks and twists, your body is pulled inwards and all you can do is curl up, have your body contract towards your heart. knees to your chest, hands around yourself, because your heart craves some affection, reassurance.
When you cry, it's a result of your heart wrenching. A little like when you wring a wet towel, or a sponge. Your heart is painfully twisting itself, contracting itself, being squeezed by an invisible force called sadness, and this heart squeezes out a colourless liquid that flows out through your eyes.