The heart has four chambers, two upper chambers called the atria, which are the two chambers that receive blood, and two lower chambers called the ventricles, which are the two chambers that send blood away from the heart. The atria open into the ventricles through two openings in the atrioventricular septum. This distinction between the chambers is also visible on the outer surface of the heart through the coronal sulcus.[19] At the top of the right atrium is an ear-shaped structure called the atrial appendage, auricle, or pinna, and another one at the top of the left atrium, called the atrial appendage.[20] The right atrium and ventricle are often referred to as the right heart, and similarly the left atrium and ventricle together are called the left heart.[8] The ventricles are separated from each other by the interventricular septum, which is projected onto the surface of the heart by the anterior longitudinal sulcus and the posterior longitudinal sulcus.[19]
The cardiac structure consists of dense connective tissue that gives the heart its structure. This tissue forms the atrioventricular septum that separates the atria from the ventricles. This tissue also forms the four fibrous rings that form the bases of the four heart valves.[21] The cardiac skeleton provides adequate insulation and is an important part of the heart’s electrical conductive system, as collagen fibers do not conduct electrical current. The chambers of the heart are also divided by the interatrial septum separating the atria from each other and the interventricular septum separating the ventricles from each other.[9] Since the ventricles need to generate more pressure to pump blood when they contract, the septum between the ventricles is thicker than the septum between the atria.[9]
