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The heart is an organ responsible for pumping blood through the blood vessels using rhythmic contractions of cardiac muscle.
Learning Objectives
- Describe the anatomy of the heart
Key Points
- The heart is a four-chambered muscular organ containing an involuntary conduction system that initiates rhythmic contractions to pump blood throughout the body.
- The heart has its own blood supply and is controlled by self-regulating nerve bundles called nodes.
- The SA and AV nodes send impulses through the Purkinje fibers that cause muscle contractions to the heart.
- The heart is composed of three layers: the epicardium (outer layer) which prevents excess expansion or movement of the heart, the myocardium (middle layer) which initiates contractions driving the cardiac cycle, and the endocardium (inner layer) that lines the cavities and valves.
- The heart contains two upper chambers called atria and two lower chambers called ventricles. The left and right sides of the heart are separated by the septum.
- Valves prevent the backflow of blood and separate the atria from the ventricle and the ventricle from the pulmonary artery or aorta.
Key Terms
- heart: A fist-sized muscular organ in the chest that pumps blood through the body using involuntary contractions of cardiac muscle.
- ventricle: One of two lower chambers of the heart that receives blood from the atria and pumps it out into pulmonary or systemic circulation, depending on side.
- myocardium: The middle of the three layers forming the wall of the heart, containing cardiac muscle tissue. Innervated by the Purkinje fibers.
The human heart is the pump for the circulatory system, and along with the circulatory system is considered to be an organ of the cardiovascular system. It consists of four chambers and pumps blood through both systemic and pulmonary circulation to enable gas exchange and tissue oxygenation. The heart is located in the thoracic cavity between the lungs and protected by the rib cage.
Structure of the Heart
The heart consists of four chambers separated into two sides. Each side contains an atria which receives blood into the heart and flows it into a ventricle, which pumps the blood out of the heart. The atria and ventricle on each side of the heart are linked together by valves that prevent backflow of blood. The wall that separates the left and right side of the heart is called the septum.
The left heart deals with systemic circulation, while the right heart deals with pulmonary circulation. The left side of the heart receives oxygenated blood from the pulmonary vein and pumps it into the aorta, while the right side of the heart receives deoxygenated blood from the vena cava and pumps it into the pulmonary vein. The pulmonary vein and aorta also have valves connecting them to their respective ventricle.
The heart has its own self-sustaining conduction system that sends nervous impulses to cardiac tissue. The sinoatrial (SA) and atrioventricular (AV) nodes are bundles of nerve fibers that form this conduction system. They are located in the left atrial wall of the heart and send nerve impulses to a large, highly specialized set of nerves called the Purkinje fibers, which in turn send those nerve impulses to the cardiac muscle tissue. These nodes can send impulses to the heart without central nervous system stimulation, but may be influenced by nervous stimulation to alter heart rate. The heart also has its own blood supply, the cardiac arteries that provide tissue oxygenation to the heart as the blood within the heart is not used for oxygenation by the heart.
Cardiac Histology
The heart is enclosed in a double-walled protective membrane called the pericardium, which is a mesothelium tissue of the thoracic cavity. The double membrane of pericardium contains pericardial fluid which nourishes the heart and prevents shock. This composite sac protects the heart, anchors it to surrounding structures, and prevents the heart from overfilling with blood.
The wall of the heart is composed of three layers of different tissues. The outer layer is called the epicardium, or visceral pericardium, since it is also the inner wall of the pericardium. The middle layer of the heart, the myocardium, and contains specialized cardiac muscle tissue responsible for contraction. Cardiac muscle tissue is distinct from skeletal or smooth muscle because it pumps involuntarily based on conduction from the AV and SA nodes. The inner layer is called the endocardium and is in contact with the blood that the heart pumps. It also merges with the inner lining of blood vessels and covers heart valves. Cardiac tissue is permanent tissue that does not heal or regenerate when damaged. As a result, is prone to scarring and enlargement due to mechanical stress and injury.
The Mammalian Heart: The position of valves ensures proper directional flow of blood through the cardiac interior. Note the difference in the thickness of the muscled walls of the atrium and the left and right ventricle.
