The human heart is a muscular organ that circulates blood, delivering oxygen and nutrients throughout the body via a coordinated system of chambers, vessels, and valves. Understanding the parts of the heart as they appear in anatomical diagrams is the first step in appreciating how this system operates. This guide provides a straightforward overview to help identify these structures.
The Four Chambers of the Heart
The heart has four chambers organized into a right and left side, each with an upper and lower compartment. This structure separates deoxygenated blood returning from the body from oxygenated blood returning from the lungs.
The right atrium is the upper right chamber. It receives deoxygenated blood returning from all parts of the body except the lungs. It then passes this blood to the ventricle below it.
The left atrium, located in the upper left portion of the heart, collects oxygen-rich blood returning from the lungs. It holds this blood before it moves into the ventricle directly beneath it.
Below the right atrium is the right ventricle. This chamber receives deoxygenated blood from the right atrium and is responsible for pumping it to the lungs for oxygenation. Its muscular wall is thicker than that of the atria, as it must generate sufficient pressure to push blood through the pulmonary circuit.
The left ventricle is the most powerful of the four chambers. It receives oxygenated blood from the left atrium and pumps it out to the entire body. The left ventricle has the thickest and most muscular wall of all the chambers, a feature for identification in diagrams.
Major Blood Vessels of the Heart
Several large blood vessels connect to the heart’s chambers, serving as the main conduits for blood. These are prominent features on the heart’s exterior and are the entry and exit points for its circulatory pathways.
The superior and inferior vena cava are two of the largest veins in the body. They transport deoxygenated blood from the upper and lower regions of the body, respectively, into the right atrium. In anatomical illustrations, these are shown as large, blue-colored vessels feeding into the right atrium.
Exiting from the right ventricle is the pulmonary artery. This vessel carries deoxygenated blood away from the heart and directs it toward the lungs. It is unique among arteries because it transports blood that is poor in oxygen, and it splits into left and right branches to serve both lungs.
Returning blood from the lungs to the heart are the pulmonary veins. These vessels carry freshly oxygenated blood from the lungs directly into the left atrium. There are four pulmonary veins, two from each lung, that connect to the posterior, or back, surface of the left atrium.
The aorta is the largest artery in the body and emerges from the left ventricle. It distributes oxygenated blood to all other parts of the body through its extensive network of branching arteries. In diagrams, the aorta is identifiable by its arch shape, which rises from the top of the heart.
The Four Heart Valves
The heart has four valves that ensure blood flows in a single direction. These structures act as one-way gates, opening to allow blood to pass to the next chamber or vessel and closing to prevent backward flow.
Two of these valves are atrioventricular (AV) valves, situated between the atria and the ventricles. The tricuspid valve is located between the right atrium and the right ventricle. It consists of three small flaps, or leaflets, that open to let deoxygenated blood pass into the right ventricle.
The other AV valve is the mitral valve, also known as the bicuspid valve, located between the left atrium and the left ventricle. It has two leaflets and controls the flow of oxygenated blood from the left atrium into the left ventricle.
The other two valves are the semilunar (SL) valves, positioned at the exit of each ventricle. The pulmonary valve is located at the junction of the right ventricle and the pulmonary artery. It opens when the right ventricle contracts, allowing deoxygenated blood to be sent to the lungs.
The aortic valve is situated between the left ventricle and the aorta. When the left ventricle contracts, this valve opens to allow oxygen-rich blood to be ejected into the aorta and distributed throughout the body.
Layers of the Heart Wall
The heart wall is composed of three layers of tissue, each with a specific function. These layers contribute to everything from protecting the heart to generating the contractions needed for circulation.
The outermost layer is the pericardium, a protective, fluid-filled sac that encloses the heart. It has an outer fibrous layer that anchors the heart and an inner serous layer that lubricates it, reducing friction as it beats.
In the middle is the myocardium, which is the cardiac muscle tissue. This is the thickest of the three layers and is responsible for the contractions that pump blood.
The innermost layer, lining the heart’s chambers and covering its valves, is the endocardium. This thin, smooth membrane ensures that blood flows easily through the heart without sticking to the walls. Its surface is continuous with the lining of the blood vessels connected to the heart.