About Your Heart
Other than your brain, there is no organ in your body that is more amazing, more complex and more essential to life than your heart. The heart is a hollow, cone-shaped bundle of muscle that pumps blood all around your body. Its size varies depending on your age, weight, and height and on your heart's health; an average adult's heart is between the size of one and two fists and weighs about two-thirds of a pound (about 11 ounces).
This compact, efficient pump is attached to the arteries and veins that carry blood to and from your lungs, where the blood gets oxygenated, and then to and from all parts of your body, where it oxygenates and nourishes all your body's cells. The pumping action of the heart is controlled by an electrical system, which tells your heart when to beat and keeps blood flowing through your arteries and veins.
The heart of an average adult pumps about five to six quarts of blood every minute of every day (or more than that if you're exercising strenuously). Over the course of an average lifetime, that amounts to about a billion barrels of blood—which would fill three oil supertankers. To move all that blood around your body, an average adult's heart beats about 72 times a minute—or roughly 100,000 times every single day and more than 2.5 billion times over the course of an average lifetime. The pumping action of your heart is so powerful it can cause blood to spurt up to 30 feet.
Here's how the various parts of your heart work together to accomplish that monumental task.
You may have heard it said that your heart is located on the left side of your chest. In fact, it's located about in the center of your chest, just behind your sternum, or breastbone. However, it's tilted slightly to the left, so it does take up a little more room on the left side than the right side of your chest cavity.
Your heart has four chambers, or hollow spaces through which the blood flows: two lower chambers known as ventricles, and two upper chambers usually known as atriums, also sometimes called auricles. ("Ventricle" comes from a Latin word meaning "belly," a reference to the fact that it's the lower chamber on each side. And "atrium" comes from a Latin word meaning "entryway"—just like the atrium of a hotel; this is a reference to the fact that on each circuit through your body, your blood enters your heart via the atriums. Note that you may see the plural of atrium spelled either "atriums," following English rules, or "atria," following Latin rules.)
A wall of tissue called the septum divides the atrium and ventricle on your heart's left side from the atrium and ventricle on the right side ("septum" comes from a Latin word meaning "enclosure"). And note that whenever you see or hear a reference to your "left ventricle," say, or your "right auricle," it means your left or your right, not left or right from the perspective of someone who's facing you.
The muscular tissue that makes up your heart has three layers. The inner layer, called the endocardium, is a thin, smooth membrane that lines all four chambers and the surfaces of the valves between the chambers ("endo" comes from a Latin word meaning "within," and "cardium" comes from a Greek word meaning "heart"). The middle layer, the myocardium, is the thickest of the three layers; it's the workhorse of your heart, responsible for its pumping action ("myo" comes from a Latin word meaning "muscle"). And the smooth, transparent outer layer is called the epicardium ("epi" comes from a Latin word meaning "above").
This entire structure is then encased in a tough sac known as the pericardium ("peri" comes from a Latin word meaning "around"), which is filled with a small amount of lubricating fluid.
Blood vessels fall into two basic types: arteries, which transport blood out of your heart, and veins, which carry blood back your heart. (A handy way to remember which is which is that arteries carry blood away from your heart.) Healthy blood vessels are strong and elastic—they can expand and contract as blood is pumped through them.
There are several major blood vessels—often referred to as the "great vessels"—that connect your heart to the rest of your circulatory system.
Your body's largest artery, the aorta, is about three feet long and an inch in diameter—roughly the diameter of a garden hose. It has three parts: the ascending aorta, which extends upward from your heart's left ventricle to the aortic arch; the arch, which curves up and over the top of your heart; and the descending aorta, which extends from your aortic arch down through your chest and abdomen. The aorta is also classified into the thoracic aorta (the parts in your chest—the ascending aorta, the aortic arch, and the part of the descending aorta above your diaphragm) and the abdominal aorta (the part of the descending aorta below your diaphragm).
Your abdominal aorta then splits into two branches, called the iliac arteries, with one running down each leg.
The upper part of your aorta also has several branches off it. Branching off the aortic arch are the carotid, subclavian, and brachiocephalic arteries, which supply your head, neck, and arms with blood. And branching off the ascending aorta are the coronary arteries, which convey blood to your heart itself; the coronary arteries start out as two major vessels on the surface of the heart that then branch out further and penetrate the heart to supply it with oxygen and nutrients. When your heart's workload increases due to exertion or other stresses, so, too, does its need for oxygen. This makes your coronary arteries an especially important part of your circulatory system, because if they can't keep up with your heart's need for oxygen, then your heart can't keep up with the demands placed on it.
From all these major branches off the aorta, blood travels to every part of your body through increasingly smaller arteries, down to the tiniest vessels known as capillaries ("capillary" comes from a Latin word meaning "hair," but in fact the smallest capillaries are even finer than a human hair). Capillaries have very thin walls so that the oxygen and nutrients in your arterial blood can pass into the other cells of your body; at the same time, carbon dioxide and other waste products pass from the cells into the capillaries, so they can be returned to your lungs via your body's veins and exhaled. It is within the capillaries where your arterial system connects with your venous system.
Your body's largest veins, also attached to your heart, are the superior vena cava and the inferior vena cava ("vena cava" comes from Latin words meaning "hollow vein"); the plural form of the term is "vena cavae," following Latin rules. The vena cavae return oxygen-depleted blood back to your heart's right atrium—the superior vena cava from the upper part of your body, and the inferior vena cava from the lower part of your body. From your right atrium, the blood is then pumped into your right ventricle.
You have one other significant artery attached to your heart, the pulmonary artery; it exits your heart's right ventricle and then almost immediately splits into two branches, one headed to your left lung and one to your right lung. (The term "pulmonary" comes from a Latin word meaning "lung.") The pulmonary artery carries oxygen-depleted blood—which your vena cavae have returned to your heart—into your lungs, where it is replenished with oxygen and cleared of carbon dioxide. This process is known as gas exchange.
Finally, you also have two pulmonary veins, which carry the reoxygenated blood back to your heart's left atrium. From there, the blood is pumped into your left ventricle—where it begins its journey out through the aorta all over again.
A valve is several little flaps of tissue that open and close each time your heart beats, ensuring that the blood flows in the correct direction; the flaps are also referred to as leaflets or cusps ("cusp" comes from a Latin word meaning "point," since most of the valves have triangular-shaped flaps). Your heart has four valves:
- The aortic valve controls the flow of blood from your heart's left ventricle into your aorta. The aortic valve normally has three triangular-shaped flaps.
- The pulmonary valve controls the flow of blood from your right ventricle into your pulmonary artery, toward your lungs. It also normally has three triangular-shaped flaps.
- The mitral valve controls the flow of blood from your left atrium into your left ventricle. The mitral valve has just two flaps—known as the posterior leaflet (a C-shaped strip toward the back of your heart) and an anterior leaflet (a semicircle that fits into the open side of the C, toward the front of your heart). This valve's name comes from the fact that the shape of the flaps looks somewhat like a miter, the ceremonial headdress worn by a bishop. (The mitral valve is also sometimes referred to as the bicuspid valve—because it has just two cusps. This can cause confusion, however, because there is a common congenital defect, present from birth, called bicuspid aortic valve; this is because the aortic valve is supposed to have three cusps, so a bicuspid aortic valve is one with only two rather than the normal three flaps.)
- The tricuspid valve controls the flow of blood from your right atrium into your right ventricle. As you might guess from its name, it, too, normally has three triangular-shaped flaps.
The action of each of these valves is coordinated with your heart's pumping action. There are two primary phases to each beat of your heart—the diastole, when your heart relaxes and the atriums fill, and the systole, when your heart contracts and pumps ("diastole" comes from Greek words meaning "to place apart," and "systole" from Greek words meaning "to place pressure" or "to contract").
During the diastolic phase, the muscles of your heart relax while your atriums fill with blood—your right atrium with oxygen-depleted blood collected by the venous system from throughout your body, and your left atrium with reoxygenated blood from your lungs. Next comes what is known as atrial systole; during this phase, your atriums contract and the resulting pressure causes your tricuspid and mitral valves to open, allowing the blood to flow into your ventricles. As soon as the ventricles are full, your tricuspid and mitral valves close quickly, to keep the blood from flowing backward into the atriums. Finally comes ventricular systole; during this phase, your ventricles contract and the resulting pressure causes your aortic and pulmonary valves to open, allowing oxygen-rich blood to be pumped out to the body through the aorta and oxygen-depleted blood to be pumped out to the lungs through the pulmonary artery. At that point, the aortic and pulmonary valves close quickly, to keep the blood from flowing backward into the ventricles, and then the diastolic phase begins anew.
Your pulse and blood pressure are signs, or measurable indications, of these actions. Your pulse is a determination of how fast your heart beats; a normal resting pulse for a healthy adult averages about 72 beats per minute and can range between 60 and 100 beats per minute. And your blood pressure is a determination of the force with which your blood pushes against the walls of your blood vessels—measured in a unit known as millimeters of mercury, abbreviated as mmHg. Blood pressure is measured during both the systolic and the diastolic phases of a heartbeat; this is why your blood pressure is given as two numbers—systolic pressure (the higher number, during the pumping phase) over diastolic pressure (the lower number, during the relaxation phase). A normal resting blood pressure for a healthy adult should be below 120/80 mmHg.
What makes all these interrelated parts of your cardiovascular system run is your heart's electrical system, also known as the cardiac conduction system. The complicated and coordinated series of actions that make up a single heartbeat is controlled by electrical impulses that travel along bundles of fibers known as conduction pathways. There are three primary parts to this conduction system: the sinoatrial node (or SA node), the atrioventricular node (or AV node) and the His-Purkinje system.
The impulses originate in the SA node, which is located in your heart's right atrium; for this reason, the SA node is sometimes referred to as your body's "natural pacemaker." The signal from the SA node spreads across both atriums as soon as they've finished filling with blood—your right atrium from the vena cavae, and your left atrium from the pulmonary veins. The signal stimulates the muscle cells of both atriums, causing them to contract and pump the blood they contain onward into the ventricles. Your pulse is a measure of the number of signals the SA node sends out each minute.
The signal then travels on to the AV node, which is located near your tricuspid valve on your atrial septum—the wall of muscle that divides your left atrium from your right atrium. The AV node is a sort of way station for the signal; it slows as it passes through that junction and then, once the ventricles have filled with blood, is sent onward to perform its final function.
That occurs in the His-Purkinje system, a network of conduction fibers located in the walls of both ventricles. First, the signal passes through a structure called the bundle of His (sometimes also called the AV bundle). From there, the pathway divides into a left branch and a right branch—one going to each ventricle—made so-called Purkinje fibers, specialized conduction fibers that convey the electrical signal to the ventricles' muscle cells. The Purkinje fibers coordinate their action precisely so that the left ventricle contracts a split second before the right ventricle, pumping blood first into your aorta from the left ventricle and an instant later into your pulmonary artery from your right ventricle.
Then the walls of ventricles relax, the SA node sends out another signal, and the process begins all over again. These steps occur in a smooth, synchronized sequence that's repeated—in a normal, healthy heart—between 60 and 100 times every single minute (or even more often, if you're exerting yourself).
There are many things that can go awry at any stage of this complex process—and when that happens, the result is various forms of heart disease. Heart disease is the number-one cause of death in the United States; more than 600,000 Americans die each year of heart disease—almost one in four deaths nationwide.
But there are many things you can do to maintain a healthy heart, as well as to repair a diseased heart. Luckily, the human heart more often than not exemplifies the famous Timex watch slogan—"it takes a licking and keeps on ticking."
Here are links to some more facts about this intricate organ:
- What Is the Heart? (U.S. National Heart, Lung, and Blood Institute)
- How the Heart Works (U.S. National Heart, Lung, and Blood Institute)
- How the Heart Works (WebMD)
- "Answers by Heart" Fact Sheets (American Heart Association)
- Amazing Heart Facts (PBS-NOVA)
- Heart (National Geographic)
- Cardiovascular System (Inner Body/How)
- 36 Interesting Facts About the Human Heart (Random History)
- Heart Disease Facts and Statistics (U.S. Centers for Disease Control and Prevention)
- Your Guide to a Healthy Heart (U.S. National Heart, Lung, and Blood Institute)
- Heart Diseases (Medline Plus)
Page reviewed on: Jun 26, 2015
Page reviewed by: Jock McCullough, MD