The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day. Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. In order to understand how that happens, it is necessary to understand the anatomy and physiology of the heart.

Location of the Heart

The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Figure shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in Figure. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.

Your heart beats about 100,000 times a day, and every beat is a symphony. Each is a perfectly timed, carefully synchronized flow of electric current over a precise pathway on the heart's surface. This rhythmic flux literally wrings the blood out of the heart and pushes it through the 60,000 miles of arteries, capillaries, and veins that comprise your circulatory system. Arteries carry oxygen-rich blood away from the heart to the capillaries, where nutrients and oxygen flow out into the tissues. Veins collect the de-oxygenated blood from the capillaries and carry it back to the heart and lungs for replenishment.

Heart muscle cells are unique in the body. They pulsate without any external stimuli: a heart removed from a human body will continue to beat for hours. For every beat, your heart generates an electrical impulse that travels through the heart and causes each part of it to contract and relax in sequence.

The rate at which your heart beats is modulated by your autonomic nervous system, which controls functions in your body not under your conscious control. Baroreceptors, pressure-sensitive nerve endings found in the aortic arch, carotid arteries, walls of the auricles of the heart, and vena cava, are sensitive to changes in blood pressure. If your pressure is too low or too high, the baroreceptors send messages to your brain. It in turn sends signals to your heart to speed up or slow down, and to your blood vessels to constrict or dilate, as necessary, in order to increase or decrease blood pressure.

Anatomy of the Heart

Your heart has four hollow chambers. The atria, which are smaller and less muscular, are at the top, and the ventricles are at the bottom. The right atrium and ventricle pump oxygen-depleted blood to the lungs, and the left atrium and ventricle pump newly oxygenated blood to the body.

There are four valves within your heart: the mitral, tricuspid, aortic and pulmonary valves. The mitral valve and tricuspid valve lie between the atria and the ventricles. The aortic valve and pulmonary valve lie between the ventricles and the major blood vessels leaving the heart. As blood flows from each chamber of the heart, it passes through a valve. Your heart valves ensure that blood flows in only one direction through your heart.

Blood Pressure

Blood doesn't flow steadily and smoothly through your arteries, it surges each time the heart beats. That surge is what you feel as your pulse: the difference between the pressure exerted on the arterial walls when the heart beats and when it rests between beats. But although blood surges through the vessels, there is pressure on their interior walls all the time: your blood pressure.

Blood pressure is often stated as two numbers, systole and diastole. Systolic pressure is the first or top number, and it represents the pressure when the heart contracts. Diastolic pressure is the second or bottom number, and it represents the pressure when the heart rests between beats. Normal blood pressure for an adult is 120 mm Hg/80 mm Hg or less, while high blood pressure is considered to be 140 mm Hg/90 mm Hg or more. Hypertension is dangerous because it can lead to hardened arteries, or atherosclerosis, which decreases blood flow to the heart and other parts of the body. This can cause heart disease. Hypertension can also cause eye problems, including blindness, and lead to kidney disease and stroke. That's why it's so important to monitor your blood pressure, especially if you have high blood pressure.

The heart is an organ about the size of your fist that pumps blood through your body. It is made up of multiple layers of tissue.

Your heart is at the center of your circulatory system. This system is a network of blood vessels, such as arteries, veins, and capillaries, that carries blood to and from all areas of your body. Your blood carries the oxygen and nutrients that your organs need to work properly. Blood also carries carbon dioxide to your lungs so you can breathe it out. Inside your heart, valves keep blood flowing in the right direction.

Your heart’s electrical system controls the rate and rhythm of your heartbeat. A healthy heart supplies your body with the right amount of blood at the rate needed to work well. If disease or injury weakens your heart, your body’s organs will not receive enough blood to work normally. A problem with the electrical system—or the nervous or endocrine systems, which control your heart rate and blood pressure—can also make it harder for the heart to pump blood.

Anatomy of Your Heart

Your heart is in the center of your chest, near your lungs. It has four hollow heart chambers surrounded by muscle and other heart tissue. The chambers are separated by heart valves, which make sure that the blood keeps flowing in the right direction.

Heart Chambers

The two upper chambers of your heart are called atria, and the two lower chambers are called ventricles. Blood flows from the body and lungs to the atria and from the atria to the ventricles. The ventricles pump blood out of the heart to the lungs and other parts of the body. An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum.

Heart Tissue

The heart is made of three layers of tissue.

• Endocardium, the thin inner lining of the heart chambers that also forms the surface of the valves.
• Myocardium, the thick middle layer of muscle that allows your heart chambers to contract and relax to pump blood to your body.
• Pericardium, the sac that surrounds your heart. Made of thin layers of tissue, it holds the heart in place and protects it. A small amount of fluid between the layers helps reduce friction between the beating heart and surrounding tissues.

Some conditions can affect the heart's tissue. Examples include:

• Cardiomyopathy,in which the heart muscle becomes enlarged, thick, or rigid. As cardiomyopathy worsens, the heart becomes weaker and is less able to pump blood through the body and maintain a normal electrical rhythm.
• Heart inflammation, which is inflammation in one or more of the layers of tissue in the heart, including the pericardium, myocardium, or endocardium. This can lead to serious complications, including heart failure, cardiogenic shock, or irregular heart rhythm.

Blood Flow

Arteries and veins link your heart to the rest of the circulatory system. Veins bring blood to your heart. Arteries take blood away from your heart. Your heart valves help control the direction the blood flows.

Heart valves

Heart valves control the flow of blood so that it moves in the right direction. The valves prevent blood from flowing backward.

The heart has four valves.

• The tricuspid valve separates the right atrium and right ventricle.
• The mitral valve separates the left atrium and left ventricle.
• The pulmonary valve separates the right ventricle and the pulmonary artery.
• The aortic valve separates the left ventricle and aorta.

The valves open and shut in time with the pumping action of your heart's atria and ventricles. The opening and closing involves a set of flaps called cusps or leaflets. The cusps open to allow blood to flow out of a chamber and close to allow the chamber to refill with blood. Heart valve diseases can cause backflow or slow the flow of blood through the heart.

Adding Oxygen to Blood

Oxygen-poor blood from the body enters your heart through two large veins called the superior and inferior vena cava. The blood enters the heart's right atrium and is pumped to your right ventricle, which in turn pumps the blood to your lungs.

The pulmonary artery then carries the oxygen-poor blood from your heart to the lungs. Your lungs add oxygen to your blood. The oxygen-rich blood returns to your heart through the pulmonary veins.

The oxygen-rich blood from the lungs then enters the left atrium and is pumped to the left ventricle. The left ventricle generates the high pressure needed to pump the blood to your whole body through your blood vessels.

When blood leaves the heart to go to the rest of the body, it travels through a large artery called the aorta. A balloon-like bulge, called an aortic aneurysm, can sometimes occur in the aorta.

Supplying Oxygen to the Heart's Muscle

Like other muscles in the body, your heart needs blood to get oxygen and nutrients. Yourcoronary arteries supply blood to your heart. These arteries branch off from the aorta so that oxygen-rich blood is delivered to your heart as well as the rest of your body.

• The left coronary artery delivers blood to the left side of your heart, including your left atrium and ventricle and the septum between the ventricles.
• The circumflex artery branches off from the left coronary artery to supply blood to part of the left ventricle.
• The left anterior descending artery also branches from the left coronary artery and provides blood to parts of both the right and left ventricles.
• The right coronary artery provides blood to the right atrium and parts of both ventricles.
• The marginal arteries branch from the right coronary artery and provide blood to the surface of the right atrium.
• The posterior descending artery also branches from the right coronary artery and provides blood to the bottom of both ventricles.

Some conditions can affect normal blood flow through these heart arteries. Examples include:

• Angina
• Heart attack
• Ischemic heart disease

The coronary veins return oxygen-low blood from the heart's muscles back to the right atrium so it can be pumped to the lungs. They include:

• The anterior cardiac veins.
• The great cardiac vein.
• The middle cardiac vein.
• The small cardiac vein.

Your Heart's Electrical System

Your heartbeat is the contraction of your heart to pump blood to your lungs and the rest of your body. Your heart's electrical system determines how fast your heart beats.

Your heartbeat

The contraction of the atria and ventricles makes a heartbeat. When your heart beats, it makes a “lub-DUB” sound. You may have heard this if you listened with a stethoscope or with your ear on someone's chest.

• After your atria pump blood into the ventricles, the valves between the atria and ventricles close to prevent backflow. The “lub” is the sound of these valves closing.
• After your ventricles contract to pump blood away from the heart, the aortic and pulmonary valves close and make the “DUB” sound.

Electrical activity

Electrical signals cause muscles to contract. Your heart has a special electrical system called the cardiac conduction system. This system controls the rate and rhythm of the heartbeat.

With each heartbeat, an electrical signal travels from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood. The heartbeat process includes the following steps.

• The signal begins in a group of cells, called pacemaker cells, located in the sinoatrial (SA) node in the right atrium.
• The electrical signal travels through the atria, causing them to pump blood into the ventricles.
• The electrical signal then moves down to a group of pacemaker cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here the signal slows down slightly, allowing the ventricles time to finish filling with blood.
• The AV node fires another signal that travels along the walls of your ventricles, causing them to contract and pump blood out of your heart.
• The ventricles relax, and the heartbeat process starts all over again in the SA node.

Your heart’s conduction system. When special cells called pacemaker cells generate electrical signals inside your heart, the heart muscle cells, called myocytes, contract as a group. Some conditions affect the heart's electrical system. Examples include:

• Arrhythmia, or an irregular heart rhythm. Atrial fibrillation is one of the most common types of arrhythmia.
• Conduction disorders, in which electrical signals either do not generate properly, or they do not travel properly through the heart, or both.

Your heartbeat is the contraction of your heart to pump blood to your lungs and the rest of your body. Your heart's electrical system determines how fast your heart beats.

Your heartbeat

The contraction of the atria and ventricles makes a heartbeat. When your heart beats, it makes a “lub-DUB” sound. You may have heard this if you listened with a stethoscope or with your ear on someone's chest.

• After your atria pump blood into the ventricles, the valves between the atria and ventricles close to prevent backflow. The “lub” is the sound of these valves closing.
• After your ventricles contract to pump blood away from the heart, the aortic and pulmonary valves close and make the “DUB” sound.

What is my pulse, and how do I measure it?

Your pulse is the rate your heart beats. It is also called your heart rate. To find your pulse, gently place your index and middle fingers on the artery located on the inner wrist of either arm, below your thumb. You should feel a pulsing or tapping against your fingers.

Watch the second hand or set the timer on your stopwatch or phone, and count the number of beats you feel in 30 seconds. Double that number to find out your heart rate or pulse for one minute.

• At rest, your heart typically beats about 60 to 70 times per minute.
• When you exercise, your heart beats faster, and your heart rate speeds up to get more oxygen to your muscles.

Electrical activity

Electrical signals cause muscles to contract. Your heart has a special electrical system called the cardiac conduction system. This system controls the rate and rhythm of the heartbeat.

With each heartbeat, an electrical signal travels from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood. The heartbeat process includes the following steps.

• The signal begins in a group of cells, called pacemaker cells, located in the sinoatrial (SA) node in the right atrium.
• The electrical signal travels through the atria, causing them to pump blood into the ventricles.
• The electrical signal then moves down to a group of pacemaker cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here the signal slows down slightly, allowing the ventricles time to finish filling with blood.
• The AV node fires another signal that travels along the walls of your ventricles, causing them to contract and pump blood out of your heart.
• The ventricles relax, and the heartbeat process starts all over again in the SA node.

Some conditions affect the heart's electrical system. Examples include:

• Arrhythmia, or an irregular heart rhythm. Atrial fibrillation is one of the most common types of arrhythmia.
• Conduction disorders, in which electrical signals either do not generate properly, or they do not travel properly through the heart, or both.

The Heartbeat

Each heartbeat starts with an electrical impulse from the sinoatrial node, a small group of cells in your right atrium. The impulse is carried by the Purkinje fibers, located in the inner ventricular walls of the heart. The electrical signal causes your right and left atria to contract and fills the relaxed ventricles with blood. The electrical impulse in a normal, healthy heart follows a precise pathway across the heart. From the sinoatrial node, it travels to the atrioventricular node at the center of your heart and from there to your ventricles, causing them to contract and discharge blood throughout your body. The cells then recharge (repolarize) in preparation for the next heartbeat.

Electrocardiograms (EKGs)

An electrocardiogram, abbreviated as EKG or ECG, is a simple test that detects and records the electrical activity of your heart. EKGs are used to detect and locate the source of heart problems.

Each phase of a single heartbeat is the result of electrical activity that generates a distinctive electrical pattern on the EKG:

• Atrial contractions (both right and left) show up as the P wave.
• Ventricular contractions (both right and left) show as a series of 3 waves, Q-R-S, known as the QRS complex.
• The third and last wave is the T wave. This reflects the electrical activity produced when the ventricles are repolarizing.

EKGs can help reveal a number of heart problems, including:

• Heart attack
• Lack of blood flow
• A heart that does not pump forcefully enough
• A heart that is beating irregularly, too fast, or too slow
• Diseased heart valves
• Enlarged heart
• Birth defects of the heart

Heart Arrhythmias

Heart arrhythmias occur when the electrical impulses that coordinate the heartbeat don't function properly. For example, a scar from a heart attack may cause the electrical impulse to short circuit around it and veer from the normal electrical pathway. Arrhythmias may cause the heart to beat too quickly, too slowly, or irregularly.

Most arrhythmias are harmless and happen fairly frequently. You've probably experienced occasional irregular heartbeats when you felt your heart skip a beat, flutter, or race. But sometimes these irregular heartbeats can be problematic, even life threatening.

Types of Arrhythmia

Arrhythmias are classified by the speed of the heart rate they create and by where they originate.

• Tachycardia is a rapid heart rate (more than 100 beats per minute)
• Bradycardia is a slow heart rate (fewer than 60 beats per minute)

Tachycardias

Tachycardias in the atria:

• Atrial fibrillation- Fibrillation is the uncontrolled twitching of the heart muscle fibers. In atrial fibrillation, the heart beats so fast (up to 350-600 beats per minute) that it essentially quivers, rather than producing a single contraction. Atrial fibrillation can last from a few minutes to an hour or more. It is more common in older people and is seldom life threatening, but if it becomes chronic it can cause more serious problems, like stroke.
• Atrial flutter- Similar to atrial fibrillation, but the beats are more organized and the condition may respond to some forms of treatment.
• Supraventricular tachycardia (SVT)- This term refers to many forms of tachycardia that originate above the ventricles. SVTs cause a burst of rapid heartbeats (160-200 beats per minute) that can last from seconds to hours. They usually occur in young people and are not life threatening.

Tachycardias in the ventricles:

• Ventricular fibrillation (VF)- About 300,000 Americans die every year from sudden cardiac death, believed to be the result of VF. In it, the normal electrical impulse becomes chaotic and the heart quivers instead of pumping blood. Blood supply to vital organs, including the brain, is cut off, and often consciousness is lost within seconds. Chances of survival are improved if cardiopulmonary resuscitation (CPR) is administered until the heart can be shocked back into a normal rhythm with a defibrillator. Without medical intervention, death occurs in minutes. VF is usually connected with heart disease and is often triggered by a heart attack.
• Ventricular tachycardia (VT)- Fast, regular beating of the heart. VT is often caused by short circuits around scarred areas of the heart due to a previous heart attack or a damaged ventricle. Short episodes are usually harmless, but sustained VT is a medical emergency.

Bradycardias

A resting heart rate of below 60 beats per minute doesn't always signal a problem, if you're physically fit and have an efficient heart capable of pumping an adequate supply of blood to your body at that rate. But if you have a slow heart rate and your body isn't getting enough blood, you may have a bradycardia, such as:

• Sick sinus syndrome- This can be caused by an improperly functioning sinoatrial node, or an impulse block near the sinoatrial node.
• Conduction block- A block in or near the AV node or along the impulse pathways to your ventricles. This may be an early sign of heart problems.

Premature Heartbeats

A premature heartbeat is actually an extra heartbeat between two normal beats. They are often caused by stimulants such as caffeine, cold remedies, and some asthma medications. They are usually not of serious concern, but they can trigger a longer-lasting arrhythmia, especially in people with heart disease.

Location of the Heart

The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. image shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in image. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.

Position of the Heart in the Thorax

The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top, and tapers toward the base.

Everyday Connection

CPR
The position of the heart in the torso between the vertebrae and sternum (see image for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 (image), it is possible to manually compress the blood within the heart enough to push some of the blood within it into the pulmonary and systemic circuits. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you are unfamiliar with this song, a version is available on www.youtube.com. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional.

When performed by untrained or overzealous individuals, CPR can result in broken ribs or a broken sternum, and can inflict additional severe damage on the patient. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include practice of the compression technique on a mannequin.

CPR Technique

If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into the circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9.

The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex. A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300–350 grams (11 to 12 ounces). The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy. Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes. Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy. The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.

Chambers and Circulation through the Heart

The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.

There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.

The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.

The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions (image).

Dual System of the Human Blood Circulation

Blood flows from the right atrium to the right ventricle, where it is pumped into the pulmonary circuit. The blood in the pulmonary artery branches is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the pulmonary capillaries (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it into the systemic circuit. Following exchange in the systemic capillaries (oxygen and nutrients out of the capillaries and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.

The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body), as shown in Figure. Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe.

ART CONNECTION
The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood re-enters the heart through the left atrium. From the left ventricle, blood re-enters the systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides blood to the heart, is not shown.

Which of the following statements about the circulatory system is false?

1. Blood in the pulmonary vein is deoxygenated.
2. Blood in the inferior vena cava is deoxygenated.
3. Blood in the pulmonary artery is deoxygenated.
4. Blood in the aorta is oxygenated.

Structure of the Heart

The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure. In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.

Figure 40.11 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

ART CONNECTION
(a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

Which of the following statements about the heart is false?

1. The mitral valve separates the left ventricle from the left atrium.
2. Blood travels through the bicuspid valve to the left atrium.
3. Both the aortic and the pulmonary valves are semilunar valves.
4. The mitral valve is an atrioventricular valve.

The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in Figure. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.

The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack.

The Cardiac Cycle

The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body; this is followed by a relaxation phase (diastole), where the heart fills with blood, as illustrated in Figure. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic “lup” sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery). Closing of the semilunar valves produces a monosyllabic “dup” sound.

The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. Cardiomyocytes, shown in Figure, are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.

The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in Figure, are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG)—a recording of the electrical impulses of the cardiac muscle.

Arteries, Veins, and Capillaries

The blood from the heart is carried through the body by a complex network of blood vessels (Figure). Arteries take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body.

Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.

The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels (Figure). The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of the blood vessels, and vasodilation, widening of the blood vessels; this is important in the overall regulation of blood pressure.

Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.

Section Summary

The heart muscle pumps blood through three divisions of the circulatory system: coronary, pulmonary, and systemic. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle. The internal pacemaker starts at the sinoatrial node, which is located near the wall of the right atrium. Electrical charges pulse from the SA node causing the two atria to contract in unison; then the pulse reaches the atrioventricular node between the right atrium and right ventricle. A pause in the electric signal allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The blood from the heart is carried through the body by a complex network of blood vessels; arteries take blood away from the heart, and veins bring blood back to the heart.