Structure of the Heart

The heart is the muscle that lies between the lungs, and wrapped in a double layer of membrane called the pericardium. The heart has four chambers and is divided through the middle into two halves by a septum (a partition), with each half separated again by a septum. These divisions create the left atrium and left ventricle, and the right atrium and the right ventricle. The septum creates a thick muscular wall between the chambers, ensuring each chamber is isolated from its neighbors. Connected to the heart are some of the main blood vessels, arteries and veins, that make up the blood circulatory system.

The ventricle on the right side of your heart pumps blood from the heart to your lungs. When you breathe air in, oxygen passes from your lungs through blood vessels where it's added to your blood. Carbon dioxide, a waste product, is passed from your blood through blood vessels to your lungs and is removed from your body when you breathe air out.

The atrium on the left side of your heart receives oxygen-rich blood from the lungs. The pumping action of your left ventricle sends this oxygen-rich blood through the aorta to the rest of your body.

The Right Side Of The Heart

The superior and inferior vena cavae are to the left of the muscle as you look at the picture. These veins are the largest veins in your body. They carry used (oxygen-poor) blood to the right atrium of your heart. "Used" blood has had its oxygen removed and used by your body's organs and tissues. The superior vena cava carries used blood from the upper parts of your body, including your head, chest, arms, and neck. The inferior vena cava carries used blood from the lower parts of your body.

The used blood from the vena cavae flows into your heart's right atrium and then on to the right ventricle. From the right ventricle, the used blood is pumped through the pulmonary (PULL-mun-ary) arteries to your lungs. Here, through many small, thin blood vessels called capillaries, your blood picks up oxygen needed by all the areas of your body.

The oxygen-rich blood passes from your lungs back to your heart through the pulmonary veins).

The Left Side Of The Heart

Oxygen-rich blood from your lungs passes through the pulmonary veins. It enters the left atrium and is pumped into the left ventricle. From the left ventricle, your blood is pumped to the rest of your body through the aorta.

Structure of the human heart
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The Septum

The right and left sides of your heart are divided by an internal wall of tissue called the septum. The area of the septum that divides the two upper chambers (atria) of your heart is called the atrial or interatrial septum. The area of the septum that divides the two lower chambers (ventricles) of your heart is called the ventricular or interventricular septum.

Heart Chambers

The picture shows the inside of your heart and how it's divided into four chambers. The two upper chambers of your heart are called atria. The atria receive and collect blood. The two lower chambers of your heart are called ventricles. The ventricles pump blood out of your heart into the circulatory system to other parts of your body.

Heart Valves

Strategically positioned between each of the atria and their respective ventricles are powerful valves. The picture shows your heart's four valves: the aortic (ay-OR-tik) valve, the tricuspid (tri-CUSS-pid) valve, the pulmonary valve, and the mitral (MI-trul) valve. The aortic and pulmonary valves lie at the exit to each of the ventricles. These valves must operate together in a rhythmic, ordered routine to regulate blood flow. The four valves are all designed to allow one-way flow only.

Blood Flow

For the heart to function properly, your blood flows in only one direction. Your heart's valves make this possible. Both of your heart's ventricles has an "in" (inlet) valve from the atria and an "out" (outlet) valve leading to your arteries. Healthy valves open and close in very exact coordination with the pumping action of your heart's atria and ventricles. Each valve has a set of flaps called leaflets or cusps, which seal or open the valves. This allows pumped blood to pass through the chambers and into your arteries without backing up or flowing backward.


  1. U.S. Department 'of Health & Human Services, National Institues of Health
  2. Gordon Cheers. Anatomica's Body Atlas.