A two-walled sac hugs my heart (yours, too).
The pericardium, aptly named:
Peri, or around; Cardion, or heart.
The outer layer – fibrous, rigid – bears the brunt of the work. Holds my heart steady to avoid its swings, tethers it in the expanse of my chest cavity. Keeps it from beating right out.
This layer is tough, just enough for safety purposes. Like the hard peel around an orange, it shields my heart. From infections, so prone to developing in nearby lungs. From my heart itself, which could overfill with blood and too big. My heart may be eager, but this fibrous layer knows everything has its limits.
Inside that hard exterior is an inner layer, cushioned and kind. After all, to do its work, my heart cannot have friction building up with nearby tissues. Even the strongest parts need a bit of softness.
Like anywhere, things can go wrong here, in the pericardium. Fluid can build, cysts can grow, layers can swell. Nothing is perfect. Nothing is guaranteed.
But still my heart pulses. And still my pericardium guards every beat.