Anatomy of the Heart

The human heart, a muscle about the size of a fist is made up of three layers of tissue. The outermost layer, the epicardium, is surrounded by the pericardium, a membrane that protects the heart and keeps it in place. In the middle is the myocardium, cardiac muscle cells that make up the majority of the heart's walls which are lined on the inside by the endocardium.

The heart is also organized into four chambers. The upper ones are known as atria. The right atrium receives blood from the coronary sinus, superior vena cava, and inferior vena cava. Whereas the left atrium receives blood from the pulmonary vein. The two lower chambers, the right and left ventricles pump blood.

In addition, there are four main valves, flats of connective tissue that ensure blood flows in one direction preventing backflow.

Specifically, atria ventricular separation occurs on the right side with the tricuspid valve and on the left via the mitral or bicuspid valve. The other two semilunar valves ensure that the blood leaves the heart. One exists in the pulmonary artery on the right side and is called the pulmonary valve. While the other is found on the left in the aorta and is similarly named the aortic valve.