The cardiovascular system includes the heart and the blood vessels and is responsible for the transport of blood throughout the body.
The main components of the cardiovascular system are the heart, arteries, arterioles, capillaries, venules, and veins. Adults have approximately 60,000 miles (96,000 km) of blood vessels. By moving blood throughout this network of vessels, the cardiovascular system supplies all cells of the body with oxygen and nutrients and removes carbon dioxide and other waste products.
The heart is the focal point of the cardiovascular system. It supplies the driving force for the movement of blood. The heart functions as a pump, actively forcing blood out of its chambers and passively relaxing to allow the next quantity of blood to enter. On refilling, the blood does not get actively sucked into the heart, but moves into the chambers due to the underlying pressure of the cardiovascular system as a whole.
The heart is cone-shaped, pointing down and to the left, and is located left of center of the chest between the lungs. The organ is made of three types of tissue: the myocardium (middle layer), the epicardium (outer layer), and the endocardium (thin inner layer). A fluid-filled sac called the pericardium surrounds the heart, helping to reduce friction during contraction. When the myocardium applies force on the blood by contracting, the cells of the tissue become short and thick. The contraction phase of the myocardium is called systole. This is followed by relaxation of the cells, where they become thinner and longer. The relaxation phase is called diastole.
The heart functions as a double pump, with both the right and left heart having a structure to receive blood and a structure to pump the blood. The blood-receiving structures are called the atria and the blood-pumping structures are called the ventricles. During a heartbeat, the two atria contract together, moving the blood from the atria to the ventricles. Then, while the two atria relax and refill, the two ventricles contract, moving the blood out of the heart. This system means that blood leaves the heart in pulsed waves.
The right atrium and ventricle pump blood from the heart to the lungs using a subset of the blood vessels called the pulmonary circulation system. The blood travels to the lungs where it gives off waste carbon dioxide and receives oxygen, then returns to the left side of the heart to be pumped to the rest of the tissues and organs of the body. The blood vessels that carry blood to the body are called the systemic circulation system. In a healthy heart, blood does not pass directly between the left and right sides of the heart. The two atria are separated by a wall known as the atrial septum, and the wall separating the two ventricles is known as the ventricular septum.
Valves within the heart ensure that the blood travels in the right direction. On the right side of the heart, the tricupsid valve allows blood to travel only from the right atrium to the right ventricle. The mitral valve performs the same function on the left side of the heart. As the blood leaves the right ventricle to go to the lungs, the pulmonary valve controls the direction of the blood flow, while the aortic valve functions between the right ventricle and the aorta, the largest artery.
During diastole, when the ventricles relax, the mitral and tricuspid valves open, allowing blood to flow into the ventricles. At the same time, the aortic and pulmonary valves are closed to prevent reentry of the blood that had been pumped from the heart. During systole, when the ventricles contract, the mitral and tricupsid valves close to prevent backflow, and the aortic and pulmonary valves open to allow the blood to leave the heart. There are no valves at the atrial inputs, part of what ensures consistent blood inflow into the ventricles.
The heart works on an electrical conduction system, as the cells contract in response to electrical signals. All cells of the heart can contract spontaneously, with the beginning of the heartbeat dependent on the cells with the most rapid innate rate. These cells are located in the sinoatrial (SA) node of the heart, sometimes called the heart's natural pacemaker. The electrical signal moves from the SA node to the atrium, in a cluster of conducting cells called the atrioventricular (AV) node. The slowing of the signal at this point allows the atria to contract slightly before the ventricles, giving the ventricles more time to fill before they contract. The signal passes on to the electrical network of the ventricles, called the His-Purkinje system, which causes the ventricles to contract. The electrical workings of the heart can be visualized using an electrocardiography unit.
Overall, heart rate is controlled by signals from the autonomous nervous system to the SA node. The autonomic nervous system automatically controls the heart rate as well as many other functions of the body including breathing, blood pressure, and excretion. The system is extremely flexible and can double the heart rate in as fast as three to five seconds.
The arteries and arterioles
Blood leaving the heart from either the left or right ventricle enter a network of vessels called the arteries. Arteries are highly elastic vessels, having flexible fibers in their structure and a relatively thick layer of smooth muscle. Larger arteries have three layers—the inner (intima), the middle (media), and the outer (adventitia). Blood flows through the central opening, known as the lumen, which is lined with endothelial cells. The layers of the blood vessels interact to exert major control over blood pressure and where the blood flows. The adventitia contains the nervous control and blood vessels for the arteries, the media contains smooth muscles, and the endothelial layer of the intima is important for sensing environmental changes.
The aorta, the largest artery, branches directly off the left ventricle, and is especially elastic because of the addition of cardiac muscle cells in the area where it branches off the heart. The elastic qualities of arteries are important so that they can expand to receive the blood volume under high pressure, and contract to continue forcing the blood into the rest of the circulatory system. The elasticity of the arteries is a significant component of the blood pressure during diastole, when the ventricles of the heart relax.
From the left ventricle the coronary arteries, which supply blood to the heart itself, emerge from the aorta. Then the aorta makes a large U-turn in the chest, eventually becoming the abdominal artery. Major branches to the head (carotid arteries), arms (axillary arteries), and legs (femoral arteries) come off this one vessel. The flow of blood in the arteries is pulsile, increasing and decreasing with each heartbeat, about 70 times per minute. The flow of blood in the branch arteries accounts for the pulse that can be felt in the wrists and neck.
The other major artery, the pulmonary artery, carries blood from the right ventricle to the lungs. Although the systemic arteries carry oxygenated blood, the arteries of the pulmonary system carry deoxygenated blood to the lungs. A vessel is called an artery because it carries blood away from the heart, not because the blood it carries contains oxygen.
As arteries move away from the heart, they branch into smaller vessels called arterioles. Arterioles are structurally similar to arteries and play an important role in directing blood to the parts of the body needing it most, such as muscles under stress.
The veins and the venules
The major veins of the body are collectively called the venae cavae. The superior vena cava takes in blood from the arms through the axillary veins, from the head through the jugular veins, and from the heart through the coronary veins. The inferior vena cava collects the blood from the legs from the femoral veins and from the abdomen from the hepatic, portal, and renal veins, among others. Both the superior and inferior venae cavae empty into the right atrium.
The pulmonary vein brings blood oxygenated in the lungs back to the left atrium, so it can be pumped to cells throughout the body. As with arteries, veins are not so named because the vessel carries deoxygenated blood, but by their role in bringing blood back to the heart.
Veins have the same three structural layers as arteries but the layers contain less elastic tissue and muscle components, making the walls thinner and six to ten times more expandable. The blood pressure in veins is lower than in the arteries, so to keep the blood flowing to the heart there are one-way valves that prevent backflow. Additionally, the action of the muscles in the legs help to return the blood to the heart, a mechanism called the venous pump.
As veins move farther from the heart they branch into smaller structures known as venules. The venules end in very thin blood vessels known as the capillaries.
The arteries and the veins are connected by the vessel web of the capillaries. The lumen of these vessels is very small, to the extent that blood cells must line up single file to pass through the thinnest of them. Capillary walls are also very thin, allowing the passage of gases and nutrients between the blood cells and the cells of the body.
The exact role of the capillaries varies depending on the part of the body in which they are located. The capillaries of the pulmonary circulation are found in the air sacs of the lungs, called alveoli, and it is there that the exchange of oxygen into the blood and carbon dioxide out of the blood occurs. In the kidneys, the capillaries in the organ's tubules are the point where waste products are taken out of the blood to be excreted in the urine. The capillaries of the intestine are the location where nutrients from digested food are absorbed into the bloodstream. Capillaries serving the muscles bring in oxygen and nutrients and take away carbon dioxide and waste products.
For reference, at any particular point in time, about 9% of the body's blood is located in the pulmonary circulation and about 7% is in the heart's circulation. The remaining 84% is located in the systemic circulation, with 64% in the veins, 13% in the arteries, and 7% in the arterioles and capillaries. The greater percentage in the veins is due to the less elastic nature of the vessels and the tendency of the blood to pool there.
As the pulmonary circulation has a relatively smaller network of vessels when compared to the systemic circulation, the right side of the heart does not have to work as hard as the left side to move the blood. Accordingly, the left side of the heart is larger and more muscular. The passive-filling nature of the heart keeps the unequal balance in blood volume between the pulmonary and systemic circulation. Without active filling, the physical differences between the systemic and pulmonary capillaries such as relative size of the vessel bed and relative elasticity determine the blood distribution. If the heart was a different kind of pump, cardiac characteristics, such as rate or stroke volume (amount of blood pumped by one contraction of the left ventricle), would govern the relative volumes.
One way to visualize the function of the cardiovascular system is to follow the movement of one blood cell throughout the body. The path can begin at the left ventricle, where an oxygenated blood cell is pumped out by contraction of the myocardium, through the aortic valve into the aorta. The cell follows the curve into the abdominal artery and into the axillary artery into the arm. The artery subdivides into smaller and smaller branches, small enough to be called arterioles. Blood is needed at a muscle in the arm, so the arterioles are open to keep a large quantity of blood flowing in that direction. The blood cell continues through smaller vessels until it is in a capillary bed next to a muscle cell.
There the cell gives up its oxygen cargo, takes up carbon dioxide waste produced by the muscle, and begins the journey back to the heart. Travelling through the capillaries to the venules and then into the axillary vein, the cell goes into the superior vena cava and into the right atrium. The right atrium contracts, and the cell moves through the tricuspid valve into the right ventricle. On the next systole, the cell rushes out of the right ventricle, through the pulmonary valve into the pulmonary artery to the lungs. The branches of vessels grow smaller and smaller, until the cell is in the capillaries of the alveoli where it releases the carbon dioxide to the lung space to be exhaled, and picks up another load of oxygen.
Travelling back to the heart through the veins of the pulmonary circulation system, the cell enters the left atrium through the pulmonary vein. When the atrium contracts, the cell goes through the mitral valve into the left ventricle, having made one cycle through the cardiovascular system. In this way, the cardiovascular system supplies all the cells of the body with oxygen and nutrients and carries away carbon dioxide and other wastes.
Role in human health
It is difficult to overestimate the role the cardiovascular system plays in human health, with literally every cell of every tissue dependent on its function for survival. The cardiovascular system is the way the body transports things to and from the body's cells. Oxygen, nutrients, and hormones are carried from the point these substances are made or brought into the body to the cells for their use. Cellular wastes are transported from the cells to the lungs, kidneys, or liver to be broken down or removed from the body. The circulatory system is also one of the transport systems (along with the lymph) for the immune cells responsible for protecting the body from disease.
Changes in the functioning of the circulatory system have far reaching effects. A defect of the circulatory system, heart disease, is the number one cause of death in humans. Some of the common names and medical terms for the symptoms of a malfunctioning cardiovascular system include
- chest pain (angina pectoris)
- shortness of breath (dyspnea)
- general tiredness (fatigue)
- swelling (edema)
- loss of consciousness (syncope)
- light-headedness (presyncope)
- palpitations (arrhythmia or extrasystoles)
- limb pain or tiredness (claudication)
- abnormal skin color (pallor, cyanosis, erythemia, necrosis)
- sores on skin (ulceration)
- collapse (shock )
- sudden changes in vision, strength, coordination, speech, or sensation
Common diseases and disorders
Diagnosing cardiovascular disease can be complicated because often more than one cardiovascular problem exists at the same time in the same person. Symptoms of one problem can mask symptoms of another. Sometimes the multiple problems have a common cause or one cardiovascular problem can be causing another. This can make diagnosis and treatment a difficult task.
High blood pressure
The most common cardiovascular disease is high blood pressure (hypertension ), affecting one in four Americans (one in three black Americans). Blood pressure is measured in millimeters of mercury, based on how high the pressure in the arteries can raise a column of mercury above baseline using a blood pressure cuff. With a generally accepted normal of systolic to diastolic of 120/80, the disease is categorized into three stages. The systolic measurement, the diastolic measurement, or both can be elevated with hypertension.
Stage 1 disease is present with systolic measurements of 5.12-5.47 in (130-139 mm) Hg, stage 2 with 6.29-7.04 in (160-179 mm) Hg, and stage 3 with 7.08 in (180 mm) Hg or higher. For diastolic measurements, stage 1 occurs from 3.54-3.89 in (90 to 99 mm) Hg, stage 2 with 3.93-4.29 in (100 to 109 mm) Hg, and stage 3 with measurements above 4.33 in (110 mm) Hg. Treatment decisions for hypertension take into account not only the measured blood pressure, but also the presence of other cardiovascular disease, hereditary risk factors, evidence of damage to internal organs, and lifestyle (stress, diet, exercise ).
Primary hypertension is associated with a persistent increase in resistance of blood flow in the arterioles, the smaller branches off the arteries. The precise cause is unknown.
Some specific diseases of the heart include cardiomyopathy, congenital heart disease, heart valve defects, myocardial infarction (heart attack), problems of the pericardium, and arrhythmias. If any of these diseases cause the heart to lose its ability to pump blood effectively, the patient is said to have heart failure. Because poor pumping ability often results in an accumulation of fluid in the tissues and lungs, it is often called congestive heart failure.
Cardiomyopathy is a disease of the heart muscle with multiple causes and is the number one reason people undergo heart transplants. Categorized by the type of muscle damage, there are three general types of cardiomyopathy: dilated, hypertrophic, and restrictive. Dilated cardiomyopathy refers to the enlargement of the heart that is a response to the overall myocardial weakness. Many problems can cause dilated cardiomyopathy, including viral infections, excessive alcohol intake, and myocarditis (inflammation of the heart).
Hypertrophic cardiomyopathy is an abnormal overgrowth of the heart muscle. An inherited disease, the overgrown muscle blocks the movement of blood both into and out of the heart. The most common cause triggering hypertrophic cardiomyopathy is hypertension. Restrictive cardiomyopathy is due to a stiffening of the heart muscle that prevents it from fully relaxing during diastole. This problem is a symptom of other diseases such as hemochromatosis (a defect in iron use by the body) or amyloidosis (overproduction of antibodies by the bone marrow that cannot be broken down).
Congenital heart disease is a collective term used to describe defects of the heart present at birth. Defects can be relatively mild and asymptomatic to severe and life-threatening. Some more common problems are abnormally formed blood vessels that block blood flow, malformed heart valves, incorrect connections between arteries, veins, and the heart, or defects in the atrial or ventricular septa. The most common congenital heart defect is a combination of four problems called the tetralogy of Fallot. With this problem the ventricular septum is incomplete, there is an obstruction to blood flow beneath the pulmonary artery, the aorta is shifted rightward, and the right ventricular wall is thickened.
Any of the hearts valves can obstruct blood flow if they are too stiff (stenosis) or do not close properly and allow blood to leak (regurgitation). Valve problems can cause congestive heart failure or heart enlargement, which can lead to angina or heart arrhythmias. Causes of valve disease include congenital defects, calcium deposits, and infections, such as endocarditis (a bacterial infection of the endocardium, the lining of the heart). Severe valve problems can be treated by removal of the diseased valve and replacement with an artificial valve.
A myocardial infarction (heart attack) is death of heart tissue due to the sudden lack of blood flow from the coronary arteries. Doctors believe the most common cause of the blockage is a blood clot that formed at a rupture of an atherosclerotic plaque that has broken loose. The results of the heart attack are dependent on the amount of heart tissue that is damaged. With less than 10% of the heart affected, there is a reduction in the ability of the heart to pump blood, but a normal lifestyle can often be maintained. At 25%, enlargement of the heart and heart failure is a common result. If 40% or more of the heart is damaged, shock or death usually occurs.
Pericarditis is inflammation of the pericardium, usually caused by a viral infection. Although this disease can cause sharp, piercing chest pain, it is usually self-limiting and ordinarily does not lead to further problems. Pericardial effusion is a collection of fluid around the heart in the pericardial sac. If the fluid amount is great enough, it can reduce the heart's ability to expand and receive blood, reducing its efficiency. This condition is known as cardiac tamponade. A final condition of the pericardium is pericardial constriction, an abnormal inflexibility of the pericardial membrane. Some types of pericarditis often result in this problem. If the inflexible membrane causes heart failure, it can be removed surgically.
Arrhythmias are abnormal heartbeats. Very broadly, arrhythmias can be classified into four different types: conduction system abnormalities, abnormally slow, abnormally fast, and irregular. Conduction system abnormalities are seen using electrocardiography units and do not directly cause an outwardly altered heartbeat. An example is some heart blocks, where the electrical signal adopts alternative paths in the heart to avoid nonconductive tissue.
Slow heartbeat (brachycardia) is the most common cause for the implantation of a pacemaker and can be caused by problems with the autonomic nervous system, the SA node, or the conduction system. Abnormally fast heartbeats (tachychardia) can be atrial flutter, the presence of an extra, abnormal pathway for electrical conduction in the heart, or ventricular tachychardia (V tach). Some common irregular heartbeats include extra beats (extrasystoles) and atrial fibrillation, where the atria stop having effective contractions and beat chaotically at several hundred times per minute.
Some diseases of the arteries include atherosclerosis, arterial thrombosis, aneurysm, and arteritis. The most common cause of heart attacks, coronary artery disease is the blockage of one or more of the vessels that supply blood to the heart. The arteries can be obstructed by a blood clot (thrombosis), atherosclerosis, or a coronary spasm. These problems can be treated with drugs that dissolve the clot or surgical procedures that remove or circumvent the blockages, such as coronary angioplasty or bypass surgery.
Atherosclerosis is caused by the degradation of the lining of the arteries (endothelium) and the resultant plaque, a build-up of platelets, cholesterol, and other substances such as calcium that forms at the site. Atherosclerosis occurs to some extent in everyone and can occur in any of the body's arteries. Depending on the location, the disease can lead to other cardiovascular problems such as heart attack, leg pain, stroke, and aneurysm. Arterial thrombosis is another way that arteries can be blocked, but in this case an abnormal blood clot, called an embolus, is responsible. This condition presents with very similar symptoms to atherosclerosis. If it occurs in a coronary artery, it can cause heart attacks.
An aneurysm is an abnormally widened area of an artery. A common site for this problem is in the abdominal aorta and it is usually caused by atherosclerosis. Aneurysms can be surgically treated if detected before rupture. A final disease of the arteries is arteritis, an inflammation of the arteries. This problem is usually a part of another general disease, such as Takayasu's disease, temporal arteritis, Buerger's disease, and polyarteritis nodosa.
Some diseases of the veins include venous thrombosis, thrombophlebitis, pulmonary embolism, and varicose veins. Blockages in the veins are not usually caused by atherosclerosis, but by blood clots or venous thrombi. Venous thrombosis and the resulting inflammation, thrombophlebitis, can occur in superficial veins, usually a relatively minor problem, or in deep veins, a more serious condition where the threat of the clot breaking off and traveling to the heart or lungs is present.
These conditions are generally treated with blood-thinning drugs. If the clot does travel and get lodged in the lungs the condition is called a pulmonary embolism. This is a serious problem that often requires hospitalization. If blood-thinning drugs do not resolve the problem, surgical removal of the clot can be necessary.
Varicose veins refer to a condition where the veins become abnormally dilated and most commonly appear as soft bluish bulges in the legs. Caused by elevated pressure in the veins and the resulting damage to the valves within the vessels, varicose veins, unless severe, are a cosmetic problem. They can be treated with surgery, injections (sclerotherapy), or lasers.
Adventitia— The outer layer of the arteries containing nerves and blood vessels.
Diastole— Phase of the heartbeat where the ventricles relax and fill with blood.
Endocardium— The thin, innermost layer of the heart, which can be infected with endocarditis.
Epicardium— The outermost layer of the heart.
Intima— The innermost layer of the arteries containing a layer of endothelial cells that can be damaged with atherosclerosis.
Media— The middle layer of arteries containing the smooth muscle.
Myocardium— The middle, working layer of the heart containing the heart muscle cells.
Pulsile— Movement in waves, a characteristic of the blood when leaving the heart and in the circulation.
Regurgitation— A defect of the heart valves that interferes with its ability to close completely, allowing blood to leak in the direction opposite of circulation.
Septum— A physical divider between chambers, found between the atria and the ventricles.
Stenosis— A stiffening of the heart valves, which narrows its opening and can interfere with function.
Systole— Phase of the heartbeat where the ventricles contract and force blood from the heart.
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The cardiovascular system is the best known of the heart-centered processes in the body. Its actual functions are sometimes confused with other cardiac systems, and thus may be misunderstood. While sometimes characterized as including all of the organs involved in the entire relationship between the heart and the body, the cardiovascular system is the circulatory system, composed of the heart and the network of blood vessels that it anchors. The cardiovascular is the body's distributor of oxygen and nutrients, as well as the mechanism for waste transport.
Consistent with its primary function, the efficient circulation of blood, the cardiovascular system is interconnected with two other heart-centered systems: the cardiopulmonary system, which controls the relationship between the heart and the lungs, and the cardiorespiratory system, the interrelationship between the heart and the general breathing mechanisms in the body, including the exchange of oxygen and carbon dioxide that occurs within the lungs.
The cardiovascular system is a complex and extensive network. The circulatory process begins with a pump action in the heart muscle, known familiarly as the heart beat. Each beat is a two-part action, the timing of which is regulated by the heart component known as the SA node, whose function is in turn tied to brain signals. The first part of each beat is the longer diastole, and the second is the shorter systole. Blood pressure in the circulatory system is calculated as a function of the two components of the pulse and the resistance of the arterial wall.
Each beat sends a quantity of nutrient-rich, oxygenated blood into the channels known as arteries. The arteries are relatively thick walled and highly flexible cylinders, encased in a ring of muscle. As the pumping action of the heart creates pressure in the artery through the flow of blood, the arterial walls are constructed to contract and thus slow the rate of blood as it travels through the artery.
The arteries ultimately narrow into arterioles. These become the tiny capillaries, which are the system's exchange point for actual transfer of oxygen and nutrients to individual muscle and organ cells, and the corresponding receipt of waste carbon dioxide. The carbon dioxide is then transferred into small veins known as venules. The venules lead to larger veins; as the vein is not constructed with any muscle to regulate propulsion of blood through it, the blood travels more slowly on its return to the heart. Near the heart, the blood enters the pulmonary artery, located on the right side of the heart, which directs the blood to the lungs to be recharged with oxygen. The blood is then pumped back into the cardiovascular system from the left side of the heart.
The fluid components of blood is called plasma and is comprised of more than 90% water. The erythrocytes, or red blood cells, are the organisms in the blood that carry the chemical hemoglobin, and are thus able to transport oxygen. Red blood cells are manufactured at a rate of two million per second from the bodily stores of bone marrow. Fluid replacement during exercise has the important effect of maintaining proper blood volume, which permits the efficient transport of oxygen.
Approximately 25% of the body's blood is filtered through the kidneys, the organs that purify the blood as it is directed through the cardiovascular system. Some fluid waste products and toxins are extracted by the kidneys and secreted into the bladder as urine, which is passed from the body.
The cardiovascular system is generally the most important of the heart-centered physical systems to athletic performance. While athletes can often significantly improve muscular strength and endurance in every form of athletic activity through rigorous training, the level of improvement in cardiac output will dictate the ultimate level of the athlete's success. Cardiac output is defined as the amount of blood that the heart can pump per minute. The greater the cardiac output, the greater the number of red blood cells available to transport oxygen to working muscles, essential to the generation of muscle energy.
Diet and physical activity are the crucial factors to general cardiovascular health. The heart, like any muscle, requires the stimulation and muscle building of exercise to maintain heart health. The walls of the heart will grow as a result of exercise. Diets that are not a healthy mix of carbohydrates, proteins, and fats (usually consumed in the general ratio of (60-65% carbohydrates, 15-20% proteins, and 25% fats) typically lead to excess weight, which puts a strain on heart function. Diet, especially if it is high in fats, or the athlete smokes cigarettes, can cause a buildup of plaque in the arteries. This causes both a narrowing of the channel, known as stenosis, or the hardening and thickening of the artery, the condition known as arteriosclerosis. The unhealthy artery also presents the risk that the plaque material may break off and cause a clotting of the vessel, which blocks the flow of blood to the heart. This condition is known as a stroke, and it is often fatal. If heart function loss occurs from the stroke, the result may be damage to vital organs such as the brain.