The human heart is located within the thoracic cavity, between the lungs in the space known as the mediastinum. Figure 8.1 shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The great vessels, which carry blood to and from the heart, are attached to the superior surface of the heart, which is called the base. The inferior tip of the heart is called the apex.
Membranes and Layers of the Heart Walls
The heart and the roots of the great vessels are surrounded by a membrane known as the pericardium or pericardial sac (Figure 8.2). The pericardium consists of two distinct sublayers:
- the outer parietal pericardium, which is fused to the fibrous pericardium
- the inner visceral pericardium, or epicardium, which is fused to the heart and forms the outer layer of the heart wall
The walls of the heart consist of three layers:
- the outer epicardium, which is another name for the visceral pericardium mentioned above
- the thick, middle myocardium, which is made of muscle tissue and gives the heart its ability to contract
- the inner endocardium, which lines the heart chambers and is the main component of the heart valves
Internal Structures of the Heart
The heart consists of four chambers:
- The upper chambers are the right and left atria (singular: atrium).
- The lower chambers are the right and left ventricles.
The interventricular septum is a muscular wall that separates the right and left ventricles. The interatrial septum separates the right and left atria. The atrium and ventricle on each side of the heart are separated by an atrioventricular (AV) valve:
- The right AV valve, or tricuspid valve, separates the right atrium and right ventricle.
- The left AV valve, or bicuspid valve, separates the left ventricle and the left atrium. This valve is also called the mitral valve.
There are also two semilunar valves:
- The pulmonary valve separates the right ventricle from the pulmonary trunk.
- The aortic valve separates the left ventricle from the aorta.
Figure 8.1 image description: This diagram shows the location of the heart in the thorax (sagittal and anterior views). The sagittal view labels read (from top, clockwise): first rib, aortic arch, thoracic arch, esophagus, inferior vena cava, diaphragm, thymus, trachea. The anterior view labels read (from top, clockwise): mediastinum, arch of aorta, pulmonary trunk, left auricle, left lung, left ventricle, pericardial cavity, apex of heart, edge of parietal pericardium, diaphragm, edge of parietal pleura, ribs, right ventricle, right atrium, right auricle, right lung, superior vena cava. [Return to Figure 8.1].
Figure 8.2 image description: This image shows a magnified view of the structure of the heart wall. Labels read (from top, clockwise): pericardial cavity, fibrous pericardium, parietal layer of serous pericardium, epicardium (visceral layer of serous pericardium), myocardium, endocardium. [Return to Figure 8.2].