Heart tissue has three layers. The exterior layer is the thin epicardium. In the middle is the myocardium which is the heart muscle. Inside, the endocardium lines the heart. The heart is enclosed inside the pericardium, a fibrous sac with a smooth lining, with a small amount of fluid between the epicardium and the pericardium.
Blood returning to the heart from the tissues enters the right atrium.
This atrium moves the blood through the tricuspid valve into the right ventricle. The right ventricle's thicker, stronger walls pump the oxygen-poor blood through the pulmonic valve into the lungs. The blood travels through a series of smaller vessels to the capillaries, where the blood cells absorb oxygen and release carbon dioxide. The heart's pumping action moves the blood back to the heart to the left atrium, which gently pumps the oxygen-rich blood through the mitral valve into the left ventricle. The left ventricle's powerful pumping sends the blood into the aorta, the main artery that divides into smaller vessels to deliver blood to the body including the brain, organs, arms, and legs.