Ten Key Features of Heart Structure and Features

1. The heart is a specialized organ, and the only one in the body made of cardiac muscle.

2. Heart cells are called cardiomyocytes and make up muscle fibers that conduct electrical impulses. The function of the heart, which is to keep blood flowing throughout the body, is controlled by involuntary areas of the brain.

3. The heart has four chambers: two upper chambers (the atria) and two lower ones (the ventricles). The right atrium and right ventricle together make up the "right heart," and the left atrium and left ventricle make up the "left heart." A wall of muscle called the septum separates the two sides of the heart.

In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs, then returns as oxygenated blood to the left atrium of the heart via the pulmonary vein.

5. In the systemic circuit, oxygenated blood leaves the body via the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen. Deoxygenated blood returns via veins to the venae cave, re-entering the heart's right atrium.

6. The heart's outer wall consists of three layers. The outermost wall layer, or epicardium, is the inner wall of the pericardium. The middle layer, or myocardium, contains the muscle that contracts. The inner layer, or endocardium, is the lining that contacts the blood.

7. The tricuspid valve and the mitral valve make up the atrioventricular (AV) valves, which connect the atria and the ventricles. The pulmonary semi-lunar valve separates the left ventricle from the pulmonary artery, and the aortic valve separates the right ventricle from the aorta.

8. Electrical "pacemaker" cells cause the heart to contract, which happens in five stages. In the first stage (early diastole), the heart is relaxed. Then the atrium contracts (atrial systole) to push blood into the ventricle. Next, the ventricles start contracting without...