The endocardium is the inner layer of the wall of the heart. It lines the four chambers of this organ. It is a thin sheet consisting of collagen, elastic fibers, smooth-muscle cells, and endothelium. The endothelial layer of the endocardium faces the cavities of the heart and lie on the smooth muscle layer.
Strictly histologically speaking, the endocardium is the continuation of the inner coat of the blood vessels that arise from the heart; that is to say, the aorta, the pulmonary trunk, vena cava, and pulmonary artery. The endocardium is thicker in the atria than in the ventricles, and it is thickest in the left atrium, and it is thinner in the areas where it covers the papillary muscles, with the cordae tendineae and the trabeculae carneae.
In the thinnest areas of the atrial walls, where there are spaces in the myocardium, the endocardium comes into contact and it even fuses with the epicardium (the heart outer layer. In the regions of the fibrous rings and atrioventricular, aortic and pulmonary orifices, the endocardium forms folds.
