Three layers, inside to out
Pick a spot on the heart wall and cut straight through it, and you pass three layers. The innermost, lining every chamber and valve, is the endocardium — a smooth, slippery sheet so blood glides over it without clotting. The thick middle layer is the myocardium, the working muscle that does all the squeezing. The thin outer skin is the epicardium, which hugs the muscle's surface.
The myocardium is the star of the show. Its muscle cells are wired to contract together, and how forcefully they contract is what we call contractility. The endocardium matters because anything roughening it — say, an infection settling on a valve — gives clots and bacteria a foothold; inflammation of this lining is endocarditis.
The sac around the heart
The whole heart sits inside a tough, double-layered bag called the pericardium. Its inner layer is actually the epicardium we just met; its outer layer is a stiffer fibrous shell. Between the two layers is a thin film of fluid — only a teaspoon or so — that lets the beating heart glide without friction against its neighbours.
That sac does three quiet jobs: it lubricates, it anchors the heart in the chest, and its tough outer layer stops the heart from over-stretching when it fills. Most of the time you'd never know it was there — until something fills it with extra fluid.
When the sac fills up
If fluid builds up between the two pericardial layers, it's called a pericardial effusion. A slow, small effusion is often harmless because the fibrous outer layer can stretch a little over time. But because that outer shell is stiff, a rapid build-up has nowhere to go — it squeezes the heart from outside and prevents it filling. This dangerous state is cardiac tamponade.