9.2 – Anatomy of the Heart
Location
The human heart is located within the thoracic cavity, between the lungs in the space known as the mediastinum. Figure 9.1 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 base of the heart is located at the level of the third costal cartilage. The inferior tip of the heart is called the apex. The apex lies just to the left of the sternum between the junction of the fourth and fifth ribs.
Concept Check 1
- On the diagram below (Figure 9.1), locate the mediastinum, the pericardial cavity, the base of the heart and the apex of the heart.
- Locate the largest vein in the body superior vena cava.
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 9.2). The pericardium consists of two distinct sub layers:
- The sturdy outer fibrous pericardium is made of tough, dense connective tissue that protects the heart and holds it in position.
- The inner serous pericardium is separated by the pericardial cavity and contains pericardial fluid. It consists of two layers:
- 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.
Concept Check 2
- Look at Figure 9.2 below, and name the layers of the heart wall and surrounding membranes, starting with the innermost layer.
- As shown on the diagram, suggest why the myocardium layer is thicker than the endocardium layer.
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 (singular: ventricle).
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 val