The human heart is a four-chambered muscular organ, shaped and sized roughly like a man's closed fist with two-thirds of the mass to the left of midline.
The heart is enclosed in a pericardial sac that is lined with the parietal layers of a serous membrane. The visceral layer of the serous membrane forms the epicardium.
Layers of the Heart Wall
Three layers of tissue form the heart wall. The outer layer of the heart wall is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium.
Chambers of the Heart
The internal cavity of the heart is divided into four chambers:
- Right atrium
- Right ventricle
- Left atrium
- Left ventricle
The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate.
The right atrium receives deoxygenated blood from systemic veins; the left atrium receives oxygenated blood from the pulmonary veins.
Valves of the Heart
Pumps need a set of valves to keep the fluid flowing in one direction and the heart is no exception. The heart has two types of valves that keep the blood flowing in the correct direction. The valves between the atria and ventricles are called atrioventricular valves [also called cuspid valves], while those at the bases of the large vessels leaving the ventricles are called semilunar valves.
The right atrioventricular valve is the tricuspid valve. The left atrioventricular valve is the bicuspid, or mitral, valve. The valve between the right ventricle and pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and the aorta is the aortic semilunar valve.
When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.
Pathway of Blood through the Heart
While it is convenient to describe the flow of blood through the right side of the heart and then through the left side, it is important to realize that both atria and ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left, working simultaneously. Blood flows from the right atrium to the right ventricle, and then is pumped to the lungs to receive oxygen. From the lungs, the blood flows to the left atrium, then to the left ventricle. From there it is pumped to the systemic circulation.
Blood Supply to the Myocardium
The myocardium of the heart wall is a working muscle that needs a continuous supply of oxygen and nutrients to function efficiently. For this reason, cardiac muscle has an extensive network of blood vessels to bring oxygen to the contracting cells and to remove waste products.
The right and left coronary arteries, branches of the ascending aorta, supply blood to the walls of the myocardium. After blood passes through the capillaries in the myocardium, it enters a system of cardiac [coronary] veins. Most of the cardiac veins drain into the coronary sinus, which opens into the right atrium.
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Heart ComponentsWhat Are the Heart's Main Components?
Heart Chambers, Valves, Vessels, Wall and Conduction System
The heart is made up of four chambers. The upper two chambers are called atria [singular: atrium] and the lower two are known as ventricles [singular: ventricle].
Muscular walls, called septa or septum, divide the heart into two sides.
On the right side of the heart, the right atrium and ventricle work to pump oxygen-poor blood to the lungs.
On the left side, the left atrium and ventricle combine to pump oxygenated blood to the body.
Heart Valves
There are four valves within the heart:
- The tricuspid valve is between the right atrium and the right ventricle.
- The pulmonary valve is between the right ventricle and the pulmonary artery.
- The mitral valve is between the left atrium and the left ventricle.
- The aortic valve is located between the left ventricle and the aorta.
These valves open when blood passes through them and then close to keep the blood from flowing in the wrong direction.
Major Vessels
The four chambers of the heart are attached to major veins or arteries that either bring blood into or carry blood away from the heart.
The atria are the receiving stations of the heart. The right atrium receives its supply of oxygen-poor blood from the two largest veins in the body, the superior and inferior vena cava.
The left atrium receives blood that has been oxygenated in the lungs from the pulmonary veins. Both atria then pump their supply of blood into the ventricles.
The ventricles are the shipping stations of the heart. The right ventricle pumps oxygen-poor blood into the lungs through the pulmonary artery while the left ventricle pumps oxygenated blood to the body through the aorta, the largest artery in the body.
Heart Wall
The heart wall consists of three layers: the endocardium, myocardium and epicardium.
The endocardium is the thin membrane that lines the interior of the heart.
The myocardium is the middle layer of the heart. It is the heart muscle and is the thickest layer of the heart.
The epicardium is a thin layer on the surface of the heart in which the coronary arteries lie.
The pericardium is a thin sac the heart sits in, often filled with a small amount of fluid, which separates the heart from the other structures in the chest such as the lungs.
Conduction System
The conduction system is the heart's own built-in pacemaker. This special tissue sets the heart rate and allows the upper and lower chambers to communicate with each other so they can function in a coordinated fashion.
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Last Updated 04/2020
Reviewed by Marji Bretz, MSN, RN, CCRN, Education Specialist II