Structure of the Heart
by Dr. Gary Farr on 27 May 2002

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The heart is a muscular organ located just to the left of the breast bone (sternum). It is about the size of your fist, and this amazing muscle pumps 4300 gallons of blood a day. The heart has four chambers:

Valves. There are four valves in the heart that help to direct blood flow. As they open and close, the valves produce sounds that can be heard with a stethoscope. The heart sounds can often tell your doctor about your hearts function.

Considering how much work it has to do, the heart is surprisingly small. It weighs about 11 ounces (310 grams). Located in the middle of the chest behind and to the left of the breastbone, between the lungs, the heart rests in a moistened chamber called the pericardial cavity which is surrounded by the ribcage. The diaphragm, a tough layer of muscle, lies below. As a result, the heart is well protected.

Although the heart is thought of in the shape of a Valentine, your heart is actually shaped more like an upside-down pear.

The human heart is primarily a shell. There are four cavities, or open spaces, inside the heart that fill with blood. Two of these cavities are called atria. The other two are called ventricles. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of your chest. The left ventricle contracts most forcefully, so you can best feel your heart pumping on the left side of your chest.

The heart is made up of four chambers: the right (RA) and left (LA) atria, which are the two upper chambers that receive blood, and the right (RV) and left (LV) ventricles, which are the pumping chambers. A wall, called the septum, separates the right and left sides of the heart. A valve connects each atrium to the ventricle below it. The mitral valve connects the left atrium with the left ventricle. The tricuspid valve connects the right atrium with the right ventricle. After oxygenated blood has circulated through the body, it is returned to the heart through a network of veins that culminate in the superior vena cava and inferior vena cava, which empty into the right atrium. This blood passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve and into the pulmonary arterios (PA), which transport it to the lungs. Here, carbon dioxide and other wastes are exchanged for a fresh supply of oxygen; the newly oxygenated blood is returned to the heart through the pulmonary veins into the left atrium. It passes through the mitral valve into the left ventricle, the heart's major pumping chamber, which forces it through the aortic valve into the aorta (Ao), the body's largest artery, which feeds it into smaller arteries to again circulate through the body.

The top of the heart connects to a few large blood vessels. The largest of these is the aorta, or main artery, which carries nutrient-rich blood away from the heart. Another important vessel is the pulmonary artery which connects the heart with the lungs as part of the pulmonary circulation system. The two largest veins that carry blood into the heart are the superior vena cava and the inferior vena cava. They are called "vena cava" because they are the "heart's veins." The superior is located near the top of the heart. The inferior is located beneath the superior.

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. This is just one phase of the overall circulatory system.

The veins bring waste-rich blood back to the heart, entering the right atrium throughout two large veins called vena cavae. The right atrium fills with the waste-rich blood and then contracts, pushing the blood through a one-way valve into the right ventricle. The right ventricle fills and then contracts, pushing the blood into the pulmonary artery which leads to the lungs. In the lung capillaries, the exchange of carbon dioxide and oxygen takes place. The fresh, oxygen-rich blood enters the pulmonary veins and then returns to the heart, re-entering through the left atrium. The oxygen-rich blood then passes through a one-way valve into the left ventricle where it will exit the heart through the main artery, called the aorta. The left ventricle's contraction forces the blood into the aorta and the blood begins its journey throughout the body.
The one-way valves are important for preventing any backward flow of blood. The circulatory system is a network of one-way streets. If blood started flowing the wrong way, the blood gases (oxygen and carbon dioxide) might mix, causing a serious threat to your body.
You can use a stethoscope to hear pulmonary circulation. The two sounds you hear, "lub" and "dub," are the ventricles contracting and the valves closing.

The heart's structure makes it an efficient, never-ceasing pump. From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. Nerves connected to the heart regulate the speed with which the muscle contracts. When you run, your heart pumps more quickly. When you sleep, your heart pumps more slowly.

To {heart_monitor} monitor the heart, scientists can use x-ray or scanning technology to get a picture. To really explore the heart, scientists have to perform surgery. {heart_surgery} Heart surgery is very risky because the heart's pumping action is so critical for survival. If the heart stops pumping, the body cannot survive. Before beginning heart surgery, doctors connect the patient to a machine that pumps the blood for the heart. Only then is it safe for the doctor to stop the heart in order to operate.

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