The Cardiovascular System

The cardiovascular system and the lymphatic system form what is collectively called the circulatory system. Together, these systems transport oxygen, nutrients, cell wastes, hormones, and many other substances to and from all cells in the body. The trillions of cells in the human body take up nutrients and excrete wastes every minute of every day. Although the pace of this exchange may increase with activity or slow with rest, it happens continuously. If it stops, so does life. Of the two systems, the cardiovascular system is the primary transport operator; the lymphatic system aids it in its function.

DESIGN: PARTS OF THE CARDIOVASCULAR SYSTEM

Cardiovascular comes from the Greek word cardia, meaning "heart," and the Latin vasculum, meaning "small vessel." The basic components of the cardiovascular system are the heart, the blood vessels, and the blood. The system can be compared to a large muscular pump (the heart) that sends a fluid (blood) through a series of large and small tubes (blood vessels). As blood circulates through the increasingly intricate system of vessels, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands. It delivers these to the cells, picking up carbon dioxide (formed when cells use sugars or fats to produce energy) and other wastes in return. The blood then takes these waste products to the lungs and kidneys, where they are excreted.

The heart

The heart is a hollow, cone-shaped muscular organ located behind and slightly to the left of the sternum or breastbone. Nestled between the lungs, the heart sits within a protective, bony cage formed by the sternum, ribs, and spine. The lower tip of the heart, called the apex, points toward the left hip and rests on the diaphragm (a membrane of muscle separating the chest cavity from the abdominal cavity). The upper portion of the heart, called the base, points toward the right shoulder and lies beneath the second rib. It is from the base that the major blood vessels of the body emerge.

The heart is about the size of a clenched fist. At birth, an infant's heart and fist are about the same size. As a human body develops, the heart and fist grow at about the same rate. In adults, an average heart weighs between 9 and 11 ounces (255 and 310 grams). It is slightly larger in males than in females.

The Cardiovascular System: Words to Know

Agglutination (ah-glue-ti-NA-shun):

Clumping of blood cells brought about by the mixing of blood types.

Alveoli (al-VEE-oh-lie):

Air sacs of the lungs.

Antibody (AN-ti-bod-ee):

Specialized substance produced by the body that can provide immunity against a specific antigen.

Antigen (AN-ti-jen):

Any substance that, when introduced to the body, is recognized as foreign and activates an immune response.

Aorta (ay-OR-ta):

Main artery of the body.

Arteriole (ar-TEER-e-ohl):

Small artery.

Artery (AR-te-ree):

Vessel that carries blood away from the heart.

Atria (AY-tree-a):

Upper chambers of the heart that receive blood from the veins.

Atrioventricular (AV) node (a-tree-oh-ven-TRICK-ular):

Node of specialized tissue lying near the bottom of the right atrium that fires an electrical impulse across the ventricles, causing them to contract.

Atrioventricular (AV) valves:

Valves located between the atria and ventricles.

Blood pressure:

Pressure or force the blood exerts against the inner walls of the blood vessels.

Capillary (CAP-i-lair-ee):

Minute blood vessel that connects arterioles with venules.

Cardiac cycle (CAR-dee-ack):

Series of events that occur in the heart during one complete heartbeat.

Cholesterol (ko-LESS-ter-ol):

Fatlike substance produced by the liver that is an essential part of cell membranes and body chemicals; when present in excess in the body, it can accumulate on the inside walls of arteries and block blood flow.

Diaphragm (DIE-ah-fram):

Membrane of muscle separating the chest cavity from the abdominal cavity.

Diastole (die-ASS-te-lee):

Period of relaxation and expansion of the heart when its chambers fill with blood.

Diffusion (dif-FEW-shun):

Movement of molecules from an area of greater concentration to an area of lesser concentration.

Endocardium (en-doe-CAR-dee-um):

Thin membrane lining the interior of the heart.

Epicardium (ep-i-CAR-dee-um):

Lubricating outer layer of the heart wall and part of the pericardium.

Erythrocyte (e-RITH-re-site):

Red blood cell.

Filtration (fill-TRAY-shun):

Movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure.

Hemoglobin (HEE-muh-glow-bin):

Iron-containing protein pigment in red blood cells that can combine with oxygen and carbon dioxide.

Hepatic portal circulation (heh-PAT-ick POR-tal):

System of blood vessels that transports blood from the digestive organs and the spleen through the liver before returning it to the heart.

Interstitial fluid (in-ter-STI-shul):

Fluid found in the spaces between cells.

Leukocyte (LUKE-oh-site):

White blood cell.

Megakaryocyte (meg-ah-CARE-ee-oh-site):

Large cell in the red bone marrow that breaks up into small fragments that become platelets.

Myocardium (my-oh-CAR-dee-um):

Cardiac muscle layer of the heart wall.

Osmosis (oz-MOE-sis):

Diffusion of water through a semipermeable membrane.

Pericardium (pair-i-CAR-dee-um):

Tough, fibrous, two-layered membrane sac that surrounds, protects, and anchors the heart.

Plasma (PLAZ-muh):

Fluid portion of blood.

Platelets (PLATE-lets):

Irregular cell fragments in blood that are involved in the process of blood clotting.

Pulmonary circulation (PULL-mo-nair-ee):

System of blood vessels that transports blood between the heart and lungs.

Purkinje fibers (purr-KIN-gee):

Specialized cardiac muscle fibers that conduct nerve impulses through the heart.

Red blood cells:

Most numerous blood cells in the blood, they carry oxygen bonded to the hemoglobin within them.

Semilunar valves (sem-eye-LOO-nar):

Valves located between the ventricles and the major arteries into which they pump blood.

Serous fluid (SIR-us):

Clear, watery, lubricating fluid produced by serous membranes, which line body cavities and cover internal organs.

Sinoatrial (SA) node (sigh-no-A-tree-al):

Node of specialized tissue lying in the upper area of the right atrium that fires an electrical impulse across the atria, causing them to contract.

Sinusoids (SIGH-nuh-soids):

Larger than normal capillaries whose walls are also more permeable, allowing proteins and blood cells to enter or leave easily.

Sphygmomanometer (sfig-moe-ma-NOM-i-tur):

Instrument used to measure blood pressure.

Systemic circulation (sis-TEM-ick):

System of blood vessels that transports blood between the heart and all parts of the body other than the lungs.

Systole (SIS-te-lee):

Rhythmic contraction of the heart.

Thrombocyte (THROM-bow-site):

Platelet.

Vein (VAIN):

Vessel that carries blood to the heart.

Vena cava (VEE-na KAY-va):

Either of two large veins that return blood to the right atrium of the heart.

Ventricles (VEN-tri-kuls):

Lower chambers of the heart that contract to pump blood into the arteries.

Venule (VEN-yool):

Small vein.

White blood cells:

Cells in blood that defend the body against viruses, bacteria, and other invading microorganisms.

The pericardium is a tough, fibrous membrane sac that surrounds, protects, and anchors the heart. It is composed of three layers. The thin inner layer tightly hugs the outer surface of the heart and is actually a part of the heart wall. The fibrous outer layer protects the heart and anchors it to surrounding structures such as the sternum and diaphragm. The inner portion of this outer layer is lined by another layer, which produces serous fluid. This watery lubricant between the inner and outer layers of the pericardium allows the layers to slide smoothly across each other, reducing friction when the heart beats.

The heart wall is made up of three layers: the epicardium, the myocardium, and the endocardium. The outer layer, the epicardium, is actually the thin inner layer of the pericardium. The middle layer, the myocardium, is a thick layer of cardiac muscle that contracts to force blood out of the heart. The inner layer, the endocardium, is a thin, glistening membrane that allows blood to flow smoothly through the chambers of the heart.

HEART CHAMBERS. The heart is divided into four chambers. A muscular septum or partition divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, the atria (singular atrium), are thin-walled. They are the receiving chambers of the heart. Blood flows into them from the body, which they then pump to the ventricles, the lower heart chambers. The ventricles are the discharging chambers of the heart. Their walls are thicker and contain more cardiac muscle than the walls of the atria. This enables the ventricles to contract and pump blood out of the heart to the lungs and the rest of the body.

As blood flows from one chamber to the next, one-way valves prevent the blood from flowing backward. The valves located between the atria and ventricles are called atrioventricular or AV valves. The left AV valve (between the left ventricle and left atrium) is the mitral or bicuspid valve. The right AV valve (between the right atrium and right ventricle) is the tricuspid valve. The valves located between the ventricles and the major arteries into which they pump blood are called semilunar valves. The pulmonary semilunar valve is located between the right ventricle and the pulmonary trunk. The aortic semilunar valve is located between the left ventricle and the aorta.

The valves open and close in response to pressure changes in the heart. Each set operates at a different time. The AV valves are open when the heart is relaxed and closed when the ventricles contract. The semilunar valves are closed when the heart is relaxed and forced open when the ventricles contract. The closing of the heart valves generates the "lub-dup" sounds that a physician hears through a stethoscope. The AV valves produce the "lub" sound; the semilunar valves produce the "dup" sound.

The heart is equipped with its own nervous system that controls its beating activity. This system, called the intrinsic conduction system, is located within the heart tissue. Nerve impulses sent out through the system cause parts of the heart to contract at various times. A small node of specialized muscle tissue located in the upper area of the right atrium is called the sinoatrial or SA node. Because it initiates the impulse, the SA node is known as the pacemaker. The system includes another node, the atrioventricular or AV node, located near the bottom of the right atrium just above the ventricles. The atrioventricular or AV bundle (also known as the bundle of His) is located in the upper portion of the septum between the ventricles. Two main branches leading from this bundle (called bundle branches) divide further into small fibers that spread out within the cardiac muscle of the ventricle walls. These are known as Purkinje fibers.

Blood vessels

The blood vessels form a closed transport system of tubes measuring about 60,000 miles (96,500 kilometers) in length—more than twice the distance

around the equator of Earth. The entire blood vessel system can be thought of as a series of connected roads and highways. Blood leaves the heart through large vessels (highways) that travel forth into the body. At various points, these large vessels divide to become smaller vessels (secondary roads). In turn, these vessels continue to divide into smaller and smaller vessels (one-lane roads). On its return trip, the blood travels through increasingly larger and larger vessels (one-lane roads merging into secondary roads merging into highways) before eventually reaching the heart.

BLOOD AS AN OCEAN IN THE BODY?

Up until only about 350 years ago, people believed blood in the body flowed back and forth like ocean tides. The ancient Greeks were the first to put forth this theory. They believed blood moved away from the heart, then ebbed back to it carrying impurities in the same vessels. This theory remained unchallenged for 1,400 years.

In 1628, English physician William Harvey (1578–1657) published a new concept of blood circulation. He maintained that there was a constant flow of blood through the arteries that returned to the heart through the veins. This formed a continuing circular flow of blood through the body.

Harvey's theory was immediately scorned, as it contradicted the basis of medical knowledge at the time. Some thirty years later, however, his idea was validated by the discovery of capillaries. Because of his pioneering work, Harvey is considered by many to be the father of modern medicine.

Arteries, capillaries, and veins are the main parts of this transport system. Arteries are the vessels that carry blood away from the heart. Large arteries leave the heart and then branch into smaller ones that reach out to various parts of the body. These divide even further into smaller vessels called arterioles. Within the tissues, arterioles divide into microscopic vessels called capillaries. The exchange of materials between the blood and the cells occurs through the walls of the capillaries. Before leaving the tissues, capillaries merge to form venules, which are small veins. As these vessels move closer to the heart, they merge to form larger and larger veins.

The main blood vessels differ in their structure. Although the walls of both arteries and veins are composed of three coats, they vary in thickness. Arteries have thicker inner and middle coats, which makes them more elastic. They can expand and contract easily when blood pumped from the heart surges through them. Veins, on the other hand, have thinner walls. This allows skeletal muscles surrounding them to contract and press against their flexible walls, squeezing the blood along as it returns to the heart. One-way valves in the walls of veins prevent backflow, keeping the blood flowing in one direction. The valves are most numerous in the legs, where blood must flow against the force of gravity on its way back to the heart. Unlike arteries or veins, the walls of capillaries are only one cell thick. In most capillaries, these singular cells are not joined together tightly. Because of this, oxygen, nutrients, and wastes are able to pass easily between the blood and the surrounding interstitial fluid, which fills the spaces between cells.

THE PULMONARY AND SYSTEMIC CIRCULATIONS. There are two main circulation circuits or routes in the body: the pulmonary circulation and the systemic circulation. Vessels involved in the pulmonary circulation transport blood between the heart and the lungs. Vessels in the systemic circulation transport blood to all other body parts.

The main artery of the systemic circulation is the aorta. In adults, the aorta is about the same size as a standard garden hose. It emerges upward out of the left ventricle for about an inch, then curves left over the heart (a portion called the aortic arch) before plunging downward to divide into branches that carry blood to the major parts of the body.

Branches of the aorta include the carotid arteries (which carry blood to the head), coronary arteries (which supply blood to the muscles of the heart), brachial arteries (which carry blood down the arms), and femoral arteries (which carry blood down the thighs).

The vena cava is the largest vein of the systemic circulation. It has two branches: the superior vena cava accepts blood drained from the head and arms; the inferior vena cava accepts blood drained from the lower body. Both sections (collectively called the venae cavae) empty into the right atrium.

Veins that drain into the venae cavae include the jugular veins (which drain the head), brachial and cephalic veins (which drain the arms), femoral veins (which drain the thighs), and iliac veins (which drain the pelvic or hip region).

The vessels involved in the pulmonary circulation carry blood to the lungs for gas exchange (carbon dioxide is unloaded and oxygen is picked

up), then return it to the heart. The main vessels are the pulmonary arteries and the pulmonary veins. The two pulmonary arteries branch off from the pulmonary trunk, which originates from the right ventricle. The right pulmonary artery goes to the right lung, the left pulmonary artery to the left lung. After gas exchange occurs in the lungs, the oxygenated (carrying oxygen) blood is transported back to the left atrium of the heart by four pulmonary veins.

Blood

Blood is the fluid pumped by the heart through the blood vessels to all parts of the body. It is connective tissue. As its name suggests, connective tissue connects body parts, providing support, storage, and protection. Found everywhere in the body, connective tissue is the most abundant type of the four types of tissues (the other three are epithelial, muscle, and nervous). Of all the tissues in the body, blood is unique—it is the only one that is fluid.

Blood has many functions in the body. It carries everything that must be transported from one place to another within the body: oxygen and nutrients to the cells, hormones (chemical messengers) to the tissues, and waste products to organs responsible for removing them from the body. It helps protect the body by clotting and by acting as a defense against foreign microorganisms. It also keeps the body at a constant temperature by taking heat away from cells.

Stickier and heavier than water, blood ranges in color from scarlet to dull red, depending on the amount of oxygen it is carrying (the brighter the color, the greater the amount of oxygen). Inside the body, blood has a temperature of about 100.4°F (38°C). It makes up approximately 8 percent of a person's body weight. A man of average weight has about 6 quarts (5.6 liters) of blood in his body; a woman of average weight has about 4.8 quarts (4.5 liters). Men tend to have more blood than women due to the presence of testosterone, the male sex hormone that also stimulates blood formation.

Blood is composed of both solid and liquid elements. Red blood cells, white blood cells, and platelets are the solid components that are suspended in plasma, a watery, straw-colored fluid. The living blood cells make up about 45 percent of the blood; the nonliving plasma makes up the remaining 55 percent.

PLASMA. Plasma is approximately 92 percent water. Over 100 different substances are dissolved in this fluid, including nutrients, respiratory gases, hormones, plasma proteins, salts, and various wastes. Of these dissolved substances, plasma proteins are the most abundant. These proteins, most of which are produced by the liver, serve a variety of functions. Fibrinogen is an important protein that aids in blood clotting. Albumins help to keep water in the bloodstream. Proteins called gamma globulins act as antibodies, which are substances produced by the body to help protect it against foreign substances.

The salts present in plasma include sodium, potassium, calcium, magnesium, chloride, and bicarbonate. They are involved in many important body functions, including muscle contraction, the transmission of nerve impulses, and the regulation of the body's pH (acid-base) balance.

RED BLOOD CELLS. Red blood cells, or erythrocytes, are the most prevalent of the three types of blood cells. They number about five million per cubic millimeter of blood (a cubic millimeter is an extremely small drop that is barely visible). Their main function is to transport oxygen from the lungs to all cells in the body. Red blood cells are tiny, flattened, disk-shaped structures with depressed centers: under a microscope they look like small doughnuts. Their size allows them to squeeze through the microscopic capillaries.

CHARLES DREW AND PLASMA STORAGE

The four main blood types—A, B, O, and AB—were discovered by medical researchers in the early twentieth century. This discovery greatly improved the effectiveness of blood transfusions. At the time, however, whole blood could only be kept for seven days before it perished. The problem of having the appropriate blood type readily available during emergencies still existed.

In the late 1930s, American surgeon Charles Drew (1904–1950) began to explore the possibility of using plasma as a substitute for whole blood in transfusions. Because plasma lacks red blood cells, it can be given to any patient, regardless of that patient's blood type. This property makes plasma ideal for use in emergencies.

By 1940, Drew had devised a method to process and preserve blood plasma through dehydration so that it could be shipped over great distances and stored for long periods of time. When it was needed, the dried, powderlike plasma was then reconstituted or reformed through the addition of water.

The use of plasma for transfusions proved especially useful during World War II (1939–45), when there was a desperate shortage of blood to treat the wounded. Because of his research, Drew is credited with saving countless numbers of lives.

BLOOD TYPES AND THEIR PERCENTAGE

In adults, red blood cells are formed in the red bone marrow of the ribs, vertebrae, sternum, and pelvis (marrow is the spongylike material that fills the cavities inside most bones). The primary element of red blood cells is a protein pigment called hemoglobin. Hemoglobin molecules account for one-third the weight of each red blood cell. At the center of each hemoglobin molecule is a single atom of iron, which gives red blood cells their color. In the lungs, the iron atoms combine with oxygen to create compounds called oxyhemoglobins. The main function of red blood cells is to transport this form of oxygen to the cells throughout the body. After the oxygen is transferred, hemoglobin combines with the carbon dioxide given off by the cells, and the red

blood cells carry it back to the lungs, where some of the carbon dioxide is exhaled.

Because red blood cells are constantly squeezing through tiny capillaries, their membranes receive much wear and tear. For this reason, each red blood cell lives only about four months. New red blood cells are constantly being produced in the bone marrow to replace old ones.

BLOOD TYPES. On their membranes, red blood cells carry proteins called antigens, or substances that the body recognizes as foreign. These inherited antigens determine to what blood group a person belongs: A, B, AB, or O. A person whose red blood cells carry the A antigen is type A. A person with the B antigen is type B. A person with both A and B antigens is type AB. A person with no antigens is type O.

Knowing a person's blood type is important for blood transfusions. A person with type A blood cannot receive type B blood because they carry antibodies to B antigens. B types carry antibodies to A antigens. AB types do not carry any antibodies to antigens, but O types carry antibodies to both A and B antigens. If a person is given the wrong type of blood, the blood cells clump together and can block small blood vessels. This reaction, called agglutination, can be fatal.

Another type of antigen carried on red blood cells is called the Rh antigen (so named because it was originally identified in Rh esus monkeys). Most Americans are Rh positive (Rh+), meaning they carry the Rh antigen. Rh negative (Rh−) people do not. Unlike the ABO blood group system, antibodies to the Rh antigen are not automatically found in the blood. The only problem that may arise with the Rh antigen and blood transfusions is when an Rh− individual is given Rh+ blood. In response, that person's body develops antibodies to the Rh+ antigen. Any further transfusions with Rh+ blood would then result in the previously formed antibodies attacking the donor blood.

WHITE BLOOD CELLS. White blood cells, or leukocytes, are far less numerous than red blood cells. Numbering between 4,000 and 11,000 per cubic millimeter of blood, they account for less than 1 percent of total blood volume. Despite their low numbers, white blood cells have a specialized function, serving as an important part of the body's immune system. They help defend the body against damage by bacteria, viruses, parasites, and tumor cells. Like red blood cells, white blood cells are formed in the red bone marrow (some white blood cells are produced in the lymphatic tissue as well). But whereas red blood cells are confined to the blood stream, white blood cells are not. They are able to squeeze through capillary walls on their way to an infected or damaged area of the body.

Rh ANTIGEN AND PREGNANCY

Rh factor is a special consideration during one type of pregnancy: when an Rh− woman carries an Rh+ baby. During delivery, a tear in the mother's placenta may allow a mother to be exposed to her baby's Rh+ red blood cells. (The placenta is a membrane lining the uterus through which nutrients and oxygen pass from mother to baby.) If this occurs, she then develops antibodies to the Rh+ antigen. During any subsequent pregnancy, if the woman is carrying another Rh+ baby, her anti-Rh antibodies may cross over into the baby's blood and destroy its red blood cells.

To prevent any of this from taking place, doctors will give the Rh− woman RhoGAM, an anti-Rh antibody, within seventy-two hours of her initial delivery. RhoGAM will destroy the Rh+ red blood cells that have entered her circulation before her immune system has had time to develop antibodies.

There are five kinds of white blood cells in the blood: neutrophils, eosinophils, basophils, monocytes, and lymphocytes. Each of the five plays a specific role in the body's defense system, being called into action to fight specific diseases. For example, during chronic (long-term) infections such as tuberculosis (an infectious disease of the lungs), monocytes increase in number. During asthma and allergy attacks, eosinophils increase in number.

White blood cells "know" where to go in the body by following certain chemicals. When tissue is infected or damaged, it releases chemicals into the surrounding area that "attract" the proper white blood cells to fight the infection or damage. This process is known as chemotaxis.

PLATELETS. Platelets, or thrombocytes, are not truly cells like red and white blood cells. They are small, disk-shaped fragments of extraordinarily large cells called megakaryocytes that are located in bone marrow. The megakaryocytes rupture, releasing fifty or more fragments that quickly form membranes to become platelets. Numbering about 300,000 per cubic millimeter of blood, platelets help to control bleeding in a complex process called homeostasis, or the stoppage of blood flow.

When an injury to a blood vessel causes bleeding, platelets begin the clotting process by sticking to the ruptured blood vessel. As they do so, they release chemicals that attract other platelets. Soon, a clump of platelets forms a temporary plug. After this, the platelets release serotonin (sir-o-TOE-nin), a chemical that causes the blood vessel to spasm and narrow, decreasing the amount of blood flowing to the site of the injury. While this is occurring, the injured tissue releases a substance that combines with calcium and other clotting factors in blood plasma to create prothrombin activator. This activator converts prothrombin (a substance produced by the liver that is present in plasma), to thrombin (an enzyme). Thrombin then joins with fibrinogen to create long, threadlike molecules called fibrin. Fibrin molecules establish a mesh that traps red blood cells and platelets, forming the basis for the clot.

WORKINGS: HOW THE CARDIOVASCULAR SYSTEM FUNCTIONS

In its continuous work, an average heart contracts more than 100,000 times a day to force blood through the thousands of miles of blood vessels to nourish each of the trillions of cells in the body. With each contraction, the heart forces about 2.5 ounces (74 milliliters) of blood into the bloodstream. At an average adult heart rate of 72 beats per minute, this equals about 1.4 gallons (5.3 liters) of blood every minute, 84 gallons (318 liters) every hour, and 2,016 gallons (7,631 liters) every day. With exercise, this amount may be increased by as much as five times.

Cardiac cycle

Cardiac cycle refers to the series of events that occur in the heart during one complete heartbeat. Each cardiac cycle takes about 0.8 second. During this brief moment, blood enters the heart, passes from chamber to chamber, then is pumped out to all areas of the body. Each cardiac cycle is divided into two phases. The two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The contraction phase, especially of the ventricles, is known as systole; the relaxation phase is known as diastole. The cardiac cycle consists of a systole and diastole of both the atria and ventricles.

BLOOD DOPING

In order to increase their endurance before competition, some athletes resort to a technique known as blood doping. The procedure involves withdrawing some of the athlete's red blood cells. After the blood is removed, the athlete's body responds by quickly producing more red blood cells to replace those withdrawn. Then, a few days before a competitive event, the withdrawn blood is infused back into the body.

The effect is to create a greater number of red blood cells and, in turn, a greater concentration of oxygen in the blood. Blood doping can increase an athlete's aerobic capacity by up to 10 percent.

However, blood doping is not only illegal but risky. It can impair blood flow as well as cause flulike symptoms. Instead of helping an athlete's performance, it can hinder it.

The process begins as deoxygenated (carrying very little oxygen) blood returns to the right atrium of the heart via the venae cavae. At the same time, oxygenated blood transported from the lungs by the four pulmonary veins empties into the left atrium. The AV valves open, and as blood flows into the atria it also flows passively into the ventricles. The semilunar valves, however, are closed to prevent blood from flowing out of the ventricles into the arteries. When the ventricles are about 70 percent full, the SA node sends out an impulse that spreads through the atria to the AV node. The atria contract, pumping out the remaining 30 percent of blood into the ventricles.

The AV node slows the impulse briefly, allowing the atria time to complete their contraction. The impulse then travels through the AV bundle, the bundle branches, and the Purkinje fibers to the apex of the heart. As the contraction of the ventricles is initiated at this spot, pressure begins building rapidly in the ventricles and the AV valves close (the "lub" sound heard through a stethoscope) to prevent blood from flowing back into the atria. When the pressure in the ventricles becomes higher than the pressure in the large arteries leaving the heart, the semilunar valves are forced open and blood is pumped out of the ventricles. Deoxygenated blood in the right ventricle is pumped to the lungs via the pulmonary arteries; oxygenated blood in the left ventricle is pumped to the rest of the body via the aorta.

While the ventricles are contracting (systole), the atria are at rest (diastole) and are filling with blood once again. When all the blood is pumped from the ventricles, the semilunar valves close (the "dup" sound heard through a stethoscope) to prevent the backflow of blood into the heart. For a moment, the ventricles are empty, closed chambers. When the pressure in the atria increases above that in the ventricles, the AV valves are forced open and blood begins to flow into the ventricles, starting a new cardiac cycle that will take less than one second to complete.

In short, during the cardiac cycle, the upper half of the heart (the atria) receives blood. The lower half (the ventricles) then pumps out the blood. The right side of the heart (right atrium and right ventricle) receives and pumps out deoxygenated blood; the left side (left atrium and left ventricle) receives and pumps out oxygenated blood.

Blood pressure

When the ventricles contract, they force or propel blood from the heart into the large, elastic arteries that expand as the blood is pushed through them. The pressure the blood exerts against the inner walls of the blood vessels is known as blood pressure. This pressure is necessary to keep the blood flowing to all areas of the body and then back to the heart.

Blood pressure is greatest in the large arteries closest to the heart. Because their walls are elastic, the arteries are able to recoil and keep most of the pressure on the blood as it flows away from the heart. As the blood courses through the system in less elastic vessels—arterioles into capillaries into venules into veins—blood pressure drops. When the blood finally returns to the right atrium via the venae cavae, the pressure behind it is almost zero.

Since the heart contracts and relaxes during a cardiac cycle, blood pressure rises and falls during each beat. It is higher during systole (left ventricle contracting) and lower during diastole (left ventricle relaxing).

Blood pressure is measured in millimeters of mercury (mmHg) with a sphygmomanometer (see box). A blood pressure reading is most often taken on the brachial artery in the arm. The systolic pressure is recorded first, followed by the diastolic pressure. Average young adults have a blood pressure reading of about 120 mmHg for systolic pressure and 80 mmHg for diastolic pressure (written as 120/80 and read as "one-twenty over eighty"). Depending on age, sex, weight, and other factors, normal blood pressure can range from 90 to 135 mmHg for the systolic pressure and 60 to 85 mmHg for the diastolic pressure. Blood pressure normally increases with age.

Regulating the heart rate

Under normal circumstances, the heart controls the rate at which it contracts or beats. But another body system—the nervous system—can and does affect heart rate to help the body adapt to different situations.

MEASURING BLOOD PRESSURE

Medical personnel measure a person's blood pressure using an instrument called a sphygmomanometer. This device consists of a rubber cuff, a hand bulb pump, and a pressure-reading mechanism.

The rubber cuff of the sphygmomanometer is wrapped snugly around a patient's arm just above the elbow. The individual taking the blood pressure then places a stethoscope (a hearing device) against the patient's brachial artery on the inside of the arm just above the elbow to listen for the pulsing of the blood.

The cuff is then inflated using the hand bulb pump until the blood flow into the arm is stopped and a pulse cannot be heard or felt. The pressure in the cuff is then released slowly. When a small amount of blood begins to spurt through the constricted artery, soft tapping sounds are heard through the stethoscope. The cuff pressure reading at which the first sound is heard is recorded as the systolic pressure.

As the pressure in the cuff is released further, the tapping sounds become louder, then soon soften. When the artery is no longer constricted and blood flows freely, the sounds disappear. The cuff pressure reading at the last sound heard is recorded as the diastolic pressure.

The medulla oblongata is a mass of nerve tissue at the top of the spinal cord and at the base of the brain that controls involuntary processes such as breathing and heart rate. Inside the medulla are two cardiac centers, the accelerator center and the inhibitory center. These centers send nerve impulses to the heart to regulate its beating.

The autonomic nervous system is a division of the nervous system that affects internal organs such as the heart, lungs, stomach, and liver. It functions involuntarily, meaning the processes it controls occur without conscious effort on the part of an individual. The autonomic nervous system is divided into two parts, the parasympathetic and sympathetic systems. The parasympathetic system is active primarily in normal, restful situations; the sympathetic system is most active during times of stress or when the body needs energy.

The accelerator center in the medulla sends impulses along sympathetic nerves to the heart to increase heart rate and the force of contraction. The inhibitory center sends impulses along parasympathetic nerves to the heart to decrease heart rate. The centers act in response to changes in blood pressure and the level of oxygen in the blood, often brought about by factors such as exercise, increased body temperature, and emotional stress. Such changes are detected by receptors located in the carotid arteries and the aortic arch.

Receptors in the carotid arteries detect a decrease in blood pressure; those in the aortic arch detect a decrease in the level of oxygen in the blood. Both send out impulses along sensory nerves to the accelerator center, which in turn sends impulses along nerves to the SA node of the heart to increase heart rate. When blood pressure or blood oxygen level has been restored to normal, the inhibitory center sends out impulses along nerves to the SA node to slow heart rate to a normal resting pace.

Exchanges between capillaries and general body tissues

Arteries, arterioles, venules, and veins: the only function of these vessels is to transport blood from or to the heart. The exchange of materials—oxygen, carbon dioxide, nutrients, and wastes—between the blood and interstitial fluid occurs through the capillaries. The movement of these materials is variously brought about by three processes: diffusion, filtration, and osmosis.

Diffusion is the movement of molecules from an area of greater concentration (existing in greater numbers) to an area of lesser concentration (existing in lesser numbers). Diffusion takes place because molecules have free energy, meaning they are always in motion. This is the case especially with molecules in a gas, which move quicker than those in a solid or liquid. Oxygen and carbon dioxide, the gases that pass between the capillaries and the interstitial fluid, move by diffusion. As blood courses through a capillary, the oxygen carried by the hemoglobin in red blood cells exists in a greater amount and thus moves into the surrounding interstitial fluid to be taken up by the cells. Conversely, carbon dioxide exists in a greater amount in the interstitial fluid and so moves into the capillary to be carried away. This exchange of gases between the blood and the interstitial fluid is called internal respiration.

Filtration is the movement of water and dissolved materials through a membrane from an area of higher pressure to an area of lower pressure. When blood enters capillaries, it has a pressure reading of about 33 mmHg; the pressure of the interstitial fluid is only about 2 mmHg. Thus, through filtration, plasma and nutrients such as amino acids, glucose, and vitamins are forced through the capillary walls into the surrounding interstitial fluid.

Osmosis is the diffusion of water through a semipermeable membrane (a membrane that allows some materials but not others to flow through it). It is the movement of water from an area where it is abundant to an area where it is scarce or less abundant. Directly related to this is osmotic pressure, which is the tendency of a solution to "pull" water into it. The strength of this pressure is determined by the amount of dissolved material, called solutes, in the solution. The greater the amount of solutes, the lower the amount of water in that solution. A solution containing a high amount of solutes has a high osmotic pressure, and water has a greater tendency to move into the solution.

At the venous end of capillaries, just before they merge to form venules, the osmotic pressure is greater in the capillaries than in the interstitial fluid. This is due to the presence of albumin and other large proteins that have

remained as solutes in the blood. Interstitial fluid has a low osmotic pressure and is thus "pulled" into the capillaries and carried away.

Capillary exchange in the lungs

After blood has flowed through the tissues of the body, exchanging oxygen and nutrients for carbon dioxide and wastes, it heads back to the heart. The deoxygenated blood empties into the right atrium via the venae cavae, then into the right ventricle. From here it is pumped into the pulmonary trunk, which then divides into the right and left pulmonary arteries. These arteries transport the deoxygenated blood to each lung.

In the lungs, the arteries branch out into successively smaller arteries and successively smaller arterioles. Finally, the smallest arterioles branch into capillaries. These pulmonary capillaries surround the alveoli, the air sacs of the lungs. The exchange of oxygen and carbon dioxide in the lungs, known as external respiration, takes place across the walls of the alveoli and nearby capillaries.

As in internal respiration, the exchange of gases in external respiration occurs according to the process of diffusion. Air in the alveoli has a high concentration of oxygen. The blood in the pulmonary capillaries has a high concentration of carbon dioxide. Following diffusion, oxygen in the alveoli moves into the capillaries while carbon dioxide in the capillaries moves into the alveoli.

Now oxygenated, blood flows from the capillaries into venules, which merge to form larger and larger veins. Finally, the blood exits each lung through two large pulmonary veins and is carried to the left atrium to be pumped back into the systemic circulation once again. The movement of blood from the lungs to the heart is a special occurrence in the body: it is the only time that veins carry oxygenated blood.

Hepatic portal circulation

Another unique circulation route is the hepatic portal circulation, a subdivision of the systemic circulation. Under this circulation pathway, blood from the digestive organs and the spleen flow through the liver before heading to the heart.

Capillaries that drain the stomach, small intestine, colon, pancreas, and spleen flow into two large veins, the superior mesenteric and the splenic. These two veins then unite to form the portal vein, which carries the blood into the liver.

Once in the liver, the portal vein branches to form capillaries called sinusoids. Sinusoids are larger than normal capillaries. Their walls are also more permeable, allowing proteins and blood cells to enter or leave easily. This is important since the blood entering the liver from the digestive organs contains large amounts of nutrients.

As the blood flows slowly through the sinusoids in the liver, some of these nutrients are removed from the blood and either stored in the liver for later use or changed into other materials the body needs. From the sinusoids, blood flows into the right and left hepatic veins, then into the inferior vena cava, and finally into the right atrium.

The complete flow of blood from the digestive organs to the heart is unusual. Normally, arteries flow into capillaries, which flow into veins. In the hepatic portal circulation, no arteries are involved. Here, capillaries merge to form veins, which branch into capillaries that merge again to form veins. This strange route is necessary so that blood may be altered by the liver. Nutrients may be stored or changed and possible poisons (such as alcohol and medicines) may be transformed into less harmful substances before the blood returns to the heart and the rest of circulation.

AILMENTS: WHAT CAN GO WRONG WITH THE CARDIOVASCULAR SYSTEM

Diseases that affect the heart and the cardiovascular system are among the most serious health problems facing Americans. In fact, cardiovascular or heart disease is the leading cause of death in the United States. The disease does not recognize gender, race, or age: it afflicts all people equally. According to statistics, almost 70 million people in the country suffer from some type of cardiovascular disease. Each year, more than one million of those people die.

The following are just a few of the many diseases and disorders that can impair the cardiovascular system or its parts.

Anemia

The word anemia literally means "lack of blood." It is a condition that results when the number of red blood cells or the amount of hemoglobin is reduced to a low level and the cells of the body do not receive all the oxygen they need to function and produce energy. Weakness, listlessness, drowsiness, headaches, soreness of the mouth, slight fever, and other discomforts are characteristics of anemia. Scientists have identified more than 400 types of anemia. Common forms of the condition may be brought about by rapid blood loss, the destruction or disease of the bone marrow, or an inadequate amount of iron or the vitamin B¹² in a person's diet.

CARDIOVASCULAR SYSTEM DISORDERS

Anemia (ah-NEE-me-yah): Diseased condition in which there is a deficiency of red blood cells or hemoglobin.

Arteriosclerosis (ar-tir-ee-o-skle-ROW-sis): Diseased condition in which the walls of arteries become thickened and hard, interfering with the circulation of blood.

Atherosclerosis (ath-a-row-skle-ROW-sis): Diseased condition in which fatty material accumulates on the interior walls of arteries, making them narrower.

Hemophilia (hee-muh-FILL-ee-ah): Inherited blood disease in which the blood lacks one or more of the clotting factors, making it difficult to stop bleeding.

Hypertension (hi-per-TEN-shun): High blood pressure.

Leukemia (loo-KEE-mee-ah): Type of cancer that affects the blood-forming tissues and organs, causing them to flood the bloodstream and lymphatic system with immature and abnormal white blood cells.

Sickle cell anemia (SICK-el cell ah-NEE-me-yah): Inherited blood disorder in which red blood cells are sickle-shaped instead of round because of defective hemoglobin molecules.

Atherosclerosis

Atherosclerosis is a type of arteriosclerosis, a general term for hardening of the arteries. Atherosclerosis is a condition in which fatty material and other substances accumulate on and in the walls of large arteries, impairing the flow of blood.

Cholesterol, a fatlike substance produced by the liver, is an essential part of cell membranes and body chemicals. Normally, the body produces all the cholesterol it needs. Eating foods high in saturated fats (found mostly in animal products such as egg yolks, fatty meats, and whole milk dairy products) can cause an increase in blood cholesterol levels. The excess cholesterol not taken up by the cells accumulates on the walls of arteries. There it combines with fatty materials, cellular waste products, calcium, and fibrin to form a waxy buildup known as plaque, which can either partially or totally obstruct blood flow.

Coronary heart disease (also known as coronary artery disease) arises when atherosclerosis occurs in the coronary (heart) arteries. When the blood flow in these arteries is restricted, the heart muscles do not receive the proper amount of blood and oxygen. Chest pain or pressure, called angina, may occur. If the blood flow is blocked, cardiac muscle cells begin to die and a heart attack may result.

If blood flow is blocked in any cerebral (brain) arteries, brain cells quickly begin to die and a stroke may result. Depending on what area of the brain has been affected, a stroke may cause memory loss, speech impairment, paralysis, coma, or death.

Atherosclerosis is a complex condition, and its exact cause is still unknown. However, scientific studies have shown that smoking, diabetes, a diet high in fats and low in fiber, and lack of exercise can all increase the risk of developing atherosclerosis.

Congenital heart disease

Congenital heart disease (sometimes called congenital heart defect) is any defect in the heart or its main blood vessels that is present at birth. Almost 1 out of every 100 infants are born with some sort of heart abnormality. At present, scientists have recognized thirty-five types of defects. Most of these abnormalities or defects obstruct or alter the flow of blood in the heart or the vessels near it. Defects include openings in the septum between the atria or the ventricles, the emergence of the aorta and pulmonary artery out of the same ventricle, the development of only one ventricle, or the formation of only one side of the heart. About half of those people with congenital heart disease require surgery to correct the problem.

THE FIRST HUMAN HEART TRANSPLANT

A medical milestone occurred on December 3, 1967, when South African surgeon Christiaan Neethling Barnard performed the first human heart transplant. The procedure, in which a fifty-five-year-old man received the heart of a young accident victim, took place at Groote Schuur Hospital in Cape Town, South Africa.

Barnard, born in 1922, had been focusing his attention on various kinds of open-heart surgery since the late 1950s. Soon after, he had begun to experiment with surgical transplantation. By the late 1960s, he was ready to perform a human heart transplant. All he needed were a proper patient and a donor.

That patient soon turned out to be Louis Washansky, a wholesale grocer who had suffered a series of heart attacks over the previous seven years. Washansky, admitted to the hospital in November 1967, was dying of a failing heart. Doctors estimated he had only weeks to live.

On December 2, twenty-five-year-old Denise Darvall was involved in a severe auto accident. When she was carried into the hospital's emergency room, her brain was dead, but her heart was still beating. Barnard asked her parents if they would donate her heart and they agreed.

The following day, assisted by a team of thirty associates, Barnard placed Darvall's heart into Washansky's chest. Within a few days, Washansky was well enough to sit up in bed. However, to prevent his body from rejecting the new organ, Washansky was given medication that suppressed his immune system. Although the medication worked, it also lowered his body's resistance to infection. Eighteen days after the surgery, Washansky died of pneumonia.

In January 1968, Barnard tried again, this time transplanting the heart of a twenty-four-year-old stroke victim into Philip Blaiberg, a fifty-eight-yearold retired dentist. Blaiberg lived for eighteen months after the surgery.

Today, with the development of more effective immunosuppressant drugs (those that hinder the workings of the immune system), transplant patients survive much longer. Three-quarters of heart-transplant patients currently survive five years after the operation.

Heart murmurs

Murmurs are abnormal, extra heart sounds made by the blood moving through the heart and its valves. Generally, blood flows smoothly and silently through the heart. The only sounds a physician normally hears through a stethoscope are the "lub-dup" sounds created by the closing of the heart valves.

Heart murmurs that are very faint, intermittent, and do not affect a person's health are called "innocent" heart murmurs. They may be caused by the failure of a heart valve to open or close completely. In the healthy hearts of children (or of the elderly), innocent heart murmurs may exist because the heart walls are relatively thin. As blood rushes through, the walls vibrate, creating extra sounds. Innocent heart murmurs in children usually disappear with age.

Murmurs that are caused by severe heart defects, however, are louder and continual. They can bring about chest pain, shortness of breath, dizziness, and, in extreme cases, death. Surgery is often required to correct severely damaged or diseased valves.

Hemophilia

Hemophilia is an inherited blood disease in which the blood lacks one or more of the clotting factors. Because of this, the blood is unable to form a clot, and even a small cut can result in prolonged bleeding and death. Commonly called "bleeder's disease," hemophilia principally affects males. When hemophiliacs (people afflicted with hemophilia) suffer a trauma and begin to bleed, they are given a transfusion of fresh plasma or an injection of the clotting factor they lack.

USING LIGHT TO REDUCE HEART-TRANSPLANT REJECTION

In December 1998, a team of doctors from the U.S. and Europe announced a possible breakthrough technique to reduce the risk of rejection in heart-transplant patients.

Currently, when patients are given new hearts, they are also given drugs, called immunosuppressants, to prevent their immune systems from developing antibodies that would attack the new organs. Unfortunately, these drugs also weaken their immune systems, allowing infections to develop.

Under the new technique, called photophoresis, light is used to destroy cells that prompt the body to reject the transplanted organ. First, blood is pumped from the patient's body. Then the white blood cells in that blood are treated with ultraviolet A light and methoxsalen (a chemical that makes the cells hypersensitive to that type of light). Afterward, the blood is returned to the patient's body.

Of those patients who received the light technique plus immunosuppressant drugs, 81 percent experienced only one episode of rejection. Of those patients receiving only the drugs, the rate was just 52 percent.

The new technique has not yet shown it can increase a patient's long-term chance of survival, but it has helped limit the amount of drugs needed to fight rejection.

Hypertension

Hypertension is high blood pressure. It is normal for blood pressure to be elevated for brief periods because of exercise, emotional stress, or a fever. Consistent arterial blood pressure measuring 140/90 or higher, however, is hypertension. The condition, the most common one affecting the cardiovascular system, is a serious one. Although it shows no symptoms, hypertension should be treated. If left unchecked, it can lead to atherosclerosis, heart attack, stroke, or kidney damage.

Hypertension most often strikes African Americans, middle-aged and elderly people, obese people, heavy alcohol drinkers, and people suffering from diabetes or kidney disease. Scientists do not know the cause for 90 to 95 percent of hypertension cases. However, studies have shown that reducing salt and fat intake, losing weight, quitting smoking, reducing alcohol consumption, and exercising regularly all combine to reduce blood pressure. Numerous drugs have also been developed to treat hypertension.

Leukemia

Leukemia (pronounced loo-KEE-mee-ah) is a type of cancer that affects the blood-forming tissues and organs, mainly the bone marrow, lymph nodes, and spleen. The disorder causes these blood-forming tissues and organs to flood the bloodstream and lymphatic system with immature and abnormal white blood cells. The overproduction of white blood cells causes a crowding-out of red blood cells and platelets.

Infections develop because these useless white blood cells have no infection-fighting ability. Anemia, easy bruising, and hemorrhaging (bleeding without clotting) also occur because of the lack of red blood cells and platelets. Leukemia is further marked by high fever and continual weakness.

Although ten times as many adults as children are stricken with the disease, leukemia is the number one disease killer of children. There are many types of leukemia, but no one cause of the disease is known. Scientists believe genetic abnormalities, exposure to toxic chemicals, and overexposure to X rays or other radioactive materials may play a part in the development of leukemia.

Chemotherapy—drug therapy to poison and destroy the abnormal cells—is effective against some types of leukemia, especially in children. Blood transfusions and bone marrow transplants have also proven effective in certain cases. With the best treatment, almost 75 percent of children suffering from leukemia survive.

Sickle cell anemia

Sickle cell anemia is an inherited blood disorder in which defective hemoglobin molecules tend to stick to one another in a red blood cell, forming strands of hemoglobin. Red blood cells that contain these strands become rigid, sticky, and crescent or sickle shaped.

These sickle-shaped cells do not last as long as normal red blood cells. They die quickly, leaving a shortage of red blood cells in the body. Anemia then develops. Sickle cells cause further problems by not fitting well through small blood vessels. They become trapped, forming a blockage that prevents normal blood flow. Tissues and organs deprived of oxygenated blood and nutrients begin to deteriorate. Acute pain develops. Over time, damage to the kidneys, lungs, liver, and central nervous and immune systems may be considerable. Severe complications may even lead to strokes and death. Those people who die from sickle cell anemia often do so before the age of thirty.

Sickle cell anemia primarily affects people with African, Mediterranean, Middle Eastern, and Indian ancestry. In the United States, the disease occurs

in about 1 out of every 500 African American births and 1 out of every 1,000 to 1,400 Hispanic American births.

There is no known cure for the disease. Painkillers, antibiotics (to fight infections), blood transfusions (to boost red blood cell count), and oxygen are treatments given to alleviate symptoms and decrease the chance of complication. Though risky, bone marrow transplants have proven effective for certain children who have been severely affected by sickle cell anemia.

TAKING CARE: KEEPING THE CARDIOVASCULAR SYSTEM HEALTHY

A healthy lifestyle is key to keeping the cardiovascular system healthy. Proper diet, regular exercise, maintaining a healthy weight, not smoking, moderate alcohol drinking, and reducing stress all lead to a healthy heart lifestyle.

A healthy diet can decrease the risk of atherosclerosis from developing, which can lead to heart disease, heart attacks, and strokes. The "Food Guide" Pyramid developed by the U.S. Departments of Agriculture and Health and Human Services provides easy-to-follow guidelines for such a diet. In general, foods that are low in fat (especially saturated fat), low in cholesterol, and high in fiber should be eaten. Fat should make up no more than 30 percent of a person's total daily calorie intake. Breads, cereals, pastas, fruits, and vegetables should form the bulk of a person's diet; meat, fish, nuts, and cheese and other dairy products should make up a lesser portion.

WHY DOES THAT HAPPEN?

Q: Why do I feel lightheaded or dizzy when I get up quickly after having lain down?

A: You feel lightheaded because of changes in two related forces: gravity and blood pressure. When you are lying down, gravity is pressing equally along the length of your body. Your heart is able to maintain a constant blood pressure throughout your body by beating at a constant rate.

When you arise suddenly, however, the pressure of gravity is greater upon the upper parts of your body, especially your head. This abrupt change in pressure causes blood in the vessels in your head to flow downward, bringing about a decrease in blood pressure in those vessels. Receptors in the carotid arteries immediately sense this drop in blood pressure and signal the accelerator center in the medulla. The accelerator center then signals the heart to increase the rate of contraction. After a few brief moments, the blood pressure in your brain is restored to normal and the feeling of lightheadedness fades.

Regular aerobic exercise can lower blood pressure, decrease weight, and keep blood vessels more flexible. The American College of Sports Medicine and the Centers for Disease Control and Prevention recommend that people engage in moderate to intense aerobic activity four or more times per week for at least thirty minutes at a time. Walking, jogging, cycling, swimming, and climbing stairs are just a few examples of aerobic activity that force the large muscles of the body to use oxygen more efficiently.

Maintaining a healthy weight can reduce blood pressure and lower a person's cholesterol level. Eating a healthy diet and exercising regularly are the principal factors in maintaining a proper body weight.

Smoking is the worst thing a person can do to their heart and lungs. It increases heart rate, constricts major arteries, raises blood pressure, contributes to the development of plaque, and can create irregular heartbeats. A person who smokes can help reverse the negative effects caused by smoking merely by quitting. Within five to ten years, that person will face the same risks of heart disease as a nonsmoker.

People who like to drink alcohol should do so in moderation. The American Heart Association defines moderate alcohol consumption as no more than one ounce of alcohol per day. This roughly equals one cocktail, one 8-ounce glass of wine, or two 12-ounce glasses of beer. Excessive drinking raises blood pressure, can cause abnormal heart rhythms, and can even poison the heart, leading to death. Cocaine, heroin, and all other illegal drugs can seriously damage the heart and should be avoided completely.

Studies have shown that stress can increase heart rate and raise blood pressure. Chronic (long-term) stress can cause plaque to develop in the arteries, increasing the risk of atherosclerosis, heart attacks, and strokes. Exercising, getting enough sleep, practicing relaxation techniques, and thinking positively are a few methods to help reduce stress and keep the cardiovascular system healthy.

FOR MORE INFORMATION

Books

Avraham, Regina, and C. Everett Koop. The Circulatory System. New York: Chelsea House, 1989.

Ballard, Carol. The Heart and Circulatory System. Austin, TX: Raintree/Steck-Vaughn, 1997.

Johansson, Philip. Heart Disease. Springfield, NJ: Enslow, 1998.

Parramon, Merce. How Our Blood Circulates. New York: Chelsea House, 1993.

Seymour, Simon. The Heart: Our Circulatory System. New York: Morrow, 1996.

Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. The Circulatory System. New York: Twenty-First Century Books, 1995.

WWW Sites

American Heart Association National Center
http://www.amhrt.org/ Homepage of the American Heart Association, featuring information on health matters relating to the heart: nutrition, exercise, stroke, diseases and their treatment, and scientific and professional publications.

The Heart: An Online Exploration
http://sln.fi.edu/biosci/biosci.html
Site prepared for the Franklin Institute Science Museum that includes information on topics such as development of the heart, circulation, diet, health, and disease. Also includes graphics and a short video clip of heart-bypass surgery.

The Heart and the Circulatory System
http://www.gene.com/ae/AE/AEC/cc/heart_background.html
Provides an extensive overview of the history of medical approaches to the heart and blood. Also provides information on the types of circulatory systems, a discussion of the anatomy of the heart, a heart glossary, and activities to explore the circulatory system.

NASA K–12 Internet: Shuttle Mir Biology Activity 9
http://quest.arc.nasa.gov/smore/teachers/act9.html
Part of NASA's Quest Project, this site (aimed at grade levels 5–8) provides teachers and students with a "circulatory system relay" activity that brings into focus the workings of the circulatory system. Background information for teachers also provided.

National Heart, Lungs, and Blood Institute Homepage
http://www.nhlbi.nih.gov/nhlbi/nhlbi.htm
Homepage of the NHLBI, part of the federal government's National Institutes of Health. Includes health- and research-related information on the cardiovascular system, the lungs, and blood. Also provides links to other health-related web sites.