Cardiac Tissue

The structure of circulation

The form and function of the body’s master circulatory organ is in the heart wall itself, which is comprised of separate tissues arranged into three layers.

• The innermost endocardium is made from endothelial cells that provide the smooth, non-adherent surface needed for consistent blood collection, unimpeded blood flow and effective pumping. These same cells also line the heart's valves, along with the major blood vessels entering and exiting the heart.
• The middle myocardium is the heart wall’s thickest layer, composed of striated muscle tissue built from cardiomyocytes — specialized cardiac muscle cells that respond to nervous stimulation and contract in a tightly coordinated manner when a "depolarization wave" moves from the endocardial layer, through the myocardium and into the epicardial layer.
• The outermost epicardium is a thin layer of fat and elastic, connective tissue that helps keep the heart localized while providing an added layer of protection from trauma. The epicardium also contains coronary blood vessels that convey blood from coronary arteries to oxygenate tissues of the heart.

Even more responsive to electrical stimulation than the cardiomyocytes are the cells that comprise the heart’s Purkinje fibers: highly specialized tissue found throughout the heart's inner ventricular walls.

Purkinje fibers conduct action potentials (mediated by cationic flow across cell membranes) faster and more efficiently than any other cardiac tissue, making it possible for the heart to achieve the synchronized contractions that are critical for maintaining a consistent heart rhythm and ensuring healthy blood flow throughout the body.