Because your heart must operate continuously to supply the rest of your body with blood, it works harder and requires a richer blood supply of its own than any other muscle in your body. It cannot extract oxygen and nutrients from the blood that moves through it, so it maintains its own dedicated circulatory system of arteries and veins. This coronary circulation begins with two coronary arteries that branch off of the aorta just above the aortic valve (on the left side). These arteries extend over the surface of the heart and branch into smaller vessels that pene- trate the heart muscles to provide oxygen. After the muscles of the heart have been nourished, the blood travels through coronary veins into the coronary sinus and then the right atrium. At this point, it ?ows in with the oxygen-depleted blood from the rest of the body.
The left coronary artery supplies blood to most of the powerful left ventricle. The circum?ex coronary artery is really a branch of the left coronary artery. It wraps around the back of the heart and has several smaller branches. The right coronary artery supplies part of the left ventricle and most of the right ventricle. Interestingly, the con?gura- tion and even the sizes of the coronary arteries differ signi?cantly from person to person.
The coronary arteries deliver oxygen-rich blood to the cardiac mus- cle cells according to the demand at the moment. If you are exerting yourself physically, your heart beats faster and more vigorously, and your coronary arteries expand to allow greater blood ?ow.
Category: Guide to Preventing and Treating Heart Disease
Essential Information You and Your Family Need to Know about Having a Healthy Heart
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Coronary Circulation
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The Heart’s Electrical Conduction System
The electrical activity that stimulates and paces the heartbeat is critical. In order to deliver an appropriate blood supply to body tissues, the heart must beat at an adequate rate, and the timing and sequence of muscular contractions must be precisely coordinated.
Your heart’s natural pacemaker is the sinoatrial (SA) node, a microscopic group of specialized electrical cells located at the top of the right atrium. Each heartbeat originates in the SA node when it ?res off an electrical impulse. This impulse travels via specialized pathways to the cells in the muscle tissues of the heart wall. The impulse ?rst stim- ulates the upper chambers, the atria, to contract and squeeze blood out into the ventricles.
Then the impulse moves to another area of electrical cells called the atrioventricular (AV) node, located over the ventricles. This node acts as a relay station, allowing for a brief interval during which the atria empty completely before releasing the impulse along branching pathways that travel to the two ventricles to stimulate ventricular contraction. The ventricles similarly contract and empty, and blood is pumped into the pulmonary artery and the aorta.
The SA node speeds up when your body needs more blood. It also slows down during rest or in response to some medications. The mes- sage to increase or decrease the rate of impulses is controlled by the autonomic nervous system—the part of the nervous system that con- trols unconscious, automatic body functions including heart rate, blood pressure, and breathing. Autonomic nervous system activity regulates the release of the hormones epinephrine and norepinephrine, which act as accelerators for the heart’s electrical impulses during times of stress or exercise.
Your heart’s electrical activity can be followed and recorded on paper as an electrocardiogram (ECG, see pages 122–125). The initial impulse from the SA node is seen as a wave on the ECG, followed by a more static interval. The ECG recording shows spikes as the impulse travels from the AV node through the ventricular pathways and is again fol- lowed by a static interval that is a segment of recovery. -
The Heartbeat
The continuous function of your heart is probably easiest to understand if you break it down into a single unit of pumping action, the heartbeat. A healthy heart beats between 50 and 75 times per minute, so a single heartbeat may occur in less than a second. It involves two distinct phases, the systole and the diastole. The systole is the pumping phase and the diastole is the resting phase.
The systole actually occurs in two sequential pumping actions: the atrial systole and the ventricular systole. The lub-dub that is heard through a stethoscope is the sound of the heart valves closing during the heartbeat’s pumping cycle. The ?rst heart sound, the “lub,” coincides with the closing of the mitral and tricuspid valves. The second heart sound, the “dub,” occurs with the closing of the aortic and pulmonary valves.A single heartbeat moves a quantity of blood through the heart in two phases: a resting, or dilating (diastolic), phase and a pumping, or squeezing (systolic), phase. During the diastolic phase, the heart relaxes and fills, as oxygen-depleted blood flows into the right atrium from the body, and oxygen-rich blood flows from the lungs into the left atrium. The ventricles fill partially. Then, during the systolic phase (right), an electrical impulse causes the heart to contract. First, the atria contract and completely fill the ventricles with blood. Then the ven- tricles contract, pumping blood out of the heart.
The diastole, the ?rst and longer resting phase, occurs as blood col- lects in the two (right and left) atria. In the right atrium, depleted blood enters from the body, and in the left atrium, oxygen-rich blood ?ows in from the lungs.
Systole begins when an electrical signal from the heart’s pacemaker cells stimulates the atria to contract and empty. The tricuspid and mitral valves open and blood ?ows into the two ventricles.
When the ventricles are full, the electrical impulse passes into an area just above the ventricles and triggers the ventricular systole, the third and ?nal step. All four valves are in action: the tricuspid and mitral valves close to prevent back?ow from the ventricles to the atria, and the pulmonary and aortic valves are pushed open as blood surges out. On the right side of the heart, oxygen-poor blood travels from the right ventricle into the pulmonary artery on its way to the lungs to acquire oxygen. On the left side of the heart, oxygen-enriched blood ?ows from the left ventricle through the aorta and into the general and coronary circulation.
After the blood has left the ventricles, they relax, and the pulmonary and aortic valves close. As the ventricles relax, the pressure in the ven- tricles lowers, allowing the tricuspid and mitral valves to open, and the cycle begins again.
Throughout this cycle, the two adjacent pumps move exactly the same amounts of blood; the volume of blood that enters and leaves the right chambers is the same as the volume that passes through the left chambers. Any change in the amount of blood entering the right side of your heart—in response to exertion, stress, or temperature changes, for example—causes a corresponding change in the amount of blood passing through the left side. Your brain is constantly monitoring the conditions that might require a change in blood supply and adjusting your heart’s function accordingly.
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The Heart Valves
The blood ?ow through the heart needs to be one-way and carefully regulated. Four one-way valves between the chambers ensure that the blood moves through the heart and lungs in sequence and never dams up or back-?ows. All the heart valves are constructed of overlapping ?aps (lea?ets or cusps) that open and close to control blood ?ow. The valves differ by structure and function.
The pulmonary and aortic valves between a ventricle and the great artery are called the semilunar valves because of their crescent-shaped lea?ets. The tricuspid and mitral valves between the right and left atria and a ventricle are also called atrioventricular valves. The lea?ets of the two atrioventricular valves are attached to the ventricular walls by ?brous cords. When the ventricle contracts and the valve closes, the cords secure the lea?ets in place so they are not blown backward by the force of the contraction.
The tricuspid valve, on the right side of the heart, is named for its three leaflets, or cusps. Returning, oxygen-depleted blood flows through this valve into the right ventricle.
As the blood ?ows out of the right ventricle and into the pulmonary circulation, it passes through the pulmonary valve. The pulmonary valve’s three lea?ets open as the right ventricle contracts and close again as it relaxes.
As the oxygen-enriched blood passes back into the left atrium, it passes through the mitral valve (named for its shape, which resembles a type of bishop’s hat called a miter). This valve has just two highly mobile cusps that can close rapidly when the powerful left ventricle contracts. This valve is attached by cords to muscles within the ventricle.
Finally, as the blood ?ows out of the left ventricle and into the aorta, the three-part aortic valve opens against the walls of the aorta. When the blood has passed into the aorta, the valve falls shut. -
The Heart Chambers
The heart is constructed of four chambers: the right atrium and the right ventricle, and the left atrium and the left ventricle. These four chambers function as two side-by-side pumps, each of which sends blood through a completely different system of circulation. The right side of the heart pumps blood through the less forceful pulmonary circulation of the lungs, where oxygen-depleted blood is replenished in lung tissue with oxygen from the air we breathe. The left side of the heart pumps blood into the rest of your blood vessels, allowing nourishing, oxygen-rich blood to leave the heart and travel throughout the rest of the body.
The atria receive blood from the body on the right side and from the lungs on the left side. The two ventricles are the pumping chambers that expel the blood. The two pumps operate in synchronized fashion, the two atria and then the two ventricles contracting and relaxing simultaneously. The two sides of the heart are separated by a thick mus- cular wall called the septum, which prevents blood from passing directly from one side of the heart to the other. To understand the mechanics of pulmonary (lung) circulation, you can trace the path of about half a cup of blood—the amount pumped in a given heartbeat through the right side of the heart. Blood enters the right atrium of the heart through two large veins: the superior vena cava, which collects blood from your head and upper body, and the inferior vena cava, which collects blood from your legs and abdomen. At this point, the red blood cells in the blood return- ing to the right atrium have delivered oxygen and nutrients to other body tissues. The depleted blood has a low oxygen content. The right atrium contracts and sends the blood through a one-way valve into the right ventri- cle, which in turn contracts and pushes the blood out through the pulmonary artery into the lungs. As the blood circulates through the lungs, it unloads carbon dioxide, a waste prod- uct of cellular function that it has carried from body tissues. The red blood cells then pick up fresh oxygen, and the blood is enriched, or oxygenated.
Similarly, on the upper left side of the heart, circulation to the rest of your body starts with the left atrium. Bright red, oxygen-rich blood enters the chamber via the pulmonary vein, from the lungs. The walls of the left atrium contract and push the blood through a one-way valve into the left ventricle. Then the left atrium relaxes while the powerful left ventricle contracts with the considerable force required to propel the blood into the aorta—the major artery at the top of the heart that directs the blood throughout the body. The left ventricle is the main pump and the strongest muscle tissue in the heart.
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The Structure and Function of the Heart
In the heart, blood makes a ?gure-eight passage. It enters on the right side through two major veins, moves through the two right chambers, loops back through the lungs to pick up oxygen, then passes back into the left chambers and out through the aorta. The blood ?ow is pro- pelled through the heart and body as the heart’s muscles contract. Flow is directed by the opening and closing of one-way heart valves between the chambers of the heart and the great vessels, or veins and arteries.
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Your Heart and Circulatory System
Your heart is a vital organ, tirelessly pumping blood throughout your body whether you are resting, carrying out your daily activities, or exercising strenuously. It is the muscular powerhouse at the center of your circulatory system, and its healthy function keeps you alive, deliver-
ing nourishing blood to every cell of every tissue in your body.
The heart and the circulatory system together make up your cardio- vascular system, which accomplishes the complex function of distribut- ing oxygen and other nutrients to body cells, as well as carrying away carbon dioxide and other waste products of cellular function for elimi- nation. Your heart provides the pumping action and force to push your blood ?rst through the lungs to take on oxygen, and then out into the circulatory system. Your circulatory system ferries the blood out to body tissues via arteries, then back to the heart through veins. More than 60,000 miles of blood vessels are involved in this vast network.
The heart itself is about the size of a ?st and weighs less than a pound. To “put your hand over your heart,” you place it just to the left of your sternum, or breast bone, which is located in the center of your chest. You can usually feel your heart’s regular beat, because the right side of the roughly cone-shaped organ tilts closest to your chest wall at this point. Behind the heart are the lungs. These organs are well pro- tected within the bony structure of your chest cavity, with the spinal column and ribs behind them.