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 6.1 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, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs.

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. 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.
• Separated by the pericardial cavity and containing pericardial fluid the inner serous pericardium 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.

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.

Anatomy Labeling Activity

1. Circulation through the heart: Blood is pumped by the heart in order to provide oxygen and nutrients to every cell in the body.
2. The heart as an organ (coronary blood supply): The heart is an organ, made of cells and tissues which require their own blood supply.
3. The heart’s electrical conduction system: The heart is able to independently generate and transmit instructions to the myocardium, in order to make it contract and pump the blood.

Circulation Through the Heart: The Heart as a Pump

The heart pumps blood to two distinct but linked circulatory systems called the pulmonary and systemic circuits. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and drops off carbon dioxide. The systemic circuit transports freshly oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.

1. Blood that is carrying carbon dioxide and waste products from the body tissues is returned to the right atrium via the superior vena cava and the inferior vena cava.
2. From the right atrium, the deoxygenated blood moves through the tricuspid valve into the right ventricle.
3. The right ventricle pumps deoxygenated blood through the pulmonary valve into the pulmonary trunk, which splits into the right and left pulmonary arteries, leading toward the lungs.  These arteries branch many times before reaching the pulmonary capillaries, where gas exchange occurs: carbon dioxide exits the blood and oxygen enters. The pulmonary arteries are the only arteries in the postnatal body that carry deoxygenated blood. Did you notice that they are often colored blue on diagrams of the heart?
4. Freshly oxygenated blood returns from the lungs to the left atrium via the pulmonary veins. These veins are the only veins in the body that carry highly oxygenated blood, and are often colored red on heart images.
5. From the left atrium, the blood moves through the mitral valve into the left ventricle.
6. The left ventricle pumps blood through the aortic valve, into the aorta, delivering blood to all parts of the body.