Circulatory System

Circulation in vascular plants

Circulation in invertebrates

Human circulatory system

The lymphatic system

Resources

Animals possess a circulatory system to deliver food, oxygen, and other needed substances to all cells and to take away waste products. Materials are transferred between individual cells and their internal environment through the cell membrane by diffusion, osmosis, and active transport. During diffusion and osmosis, molecules move from a higher concentration to a lower concentration. During active transport, carrier molecules push or pull substances across the cell membrane, using adenosine triphosphate (ATP) for energy. Unicellular organisms depend on passive and active transport to exchange materials with their watery environment. More complex multicellular forms of life rely on transport systems that move material-containing liquids throughout the body in specialized tubes. In vascular plants, tubes transport food and water. Some invertebrates rely on a closed system of tubes, while others have an open system. Humans and other higher vertebrates have a closed system of circulation.

Circulation in vascular plants

Water and dissolved minerals enter a plant’s roots from the soil by means of diffusion and osmosis. These substances then travel upward in the plant in xylem vessels. The transpiration theory ascribes this

ascending flow to a pull from above, caused by transpiration, the evaporation of water from leaves. The long water column stays intact due to the strong cohesion between water molecules. Carbohydrates, produced in leaves by photosynthesis, travel downward in plants in specialized tissue, phloem. This involves active transport of sugars into phloem cells and water pressure to force substances from cell to cell.

Circulation in invertebrates

Animal circulation depends on the contraction of a pump—usually a heart that pumps blood in one direction through vessels along a circulatory path. In a closed path, the network of vessels is continuous. Alternately, an open path has vessels that empty into open spaces in the body. The closed system in the earthworm uses five pairs of muscular hearts (the aortic arches), to pump blood. Located near the anterior or head end of the animal, the aortic arches contract and force blood into the ventral blood vessel that

runs from head to tail. Blood then returns back to the hearts in the dorsal blood vessel. Small ring vessels in each segment connect dorsal and ventral blood vessels. As blood circulates throughout the body, it delivers nutrients and oxygen to cells and picks up carbon dioxide and other wastes.

Most arthropods and some advanced mollusks such as squid and octopuses have an open circulatory system. In the grasshopper, a large blood vessel runs along the top of the body, and enlarges at the posterior or tail end to form a tubelike heart. Openings in the heart (ostia) have valves that permit only the entry of blood into the heart. The heart contracts, forcing blood forward in the blood vessel and out into the head region. Outside the heart, the blood goes into spaces that surround the insect’s internal organs. The blood delivers food and other materials to cells and picks up wastes. Animals with open circulatory systems depend on the respiratory system to transport oxygen and carbon dioxide. The blood moves slowly from the head to the tail end of the animal. At the posterior, the blood reenters the heart through the openings. Contraction of muscles helps speed up the blood flow.

Human circulatory system

The human circulatory system is termed the cardiovascular system, from the Greek word kardia, 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 work done by the cardiovascular system is astounding. Each year, the heart pumps more than 1, 848 gal (7, 000 l) of blood through a closed system of about 62, 100 mi (100, 000 km) of blood vessels. This is more than twice the distance around the equator of Earth. As blood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands, and delivers these to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these to the lungs and kidneys, where they are excreted. Substances pass out of blood vessels to the cells through the interstitial or tissue fluid which surrounds cells.

The human heart

The adult human heart is a hollow cone-shaped muscular organ located in the center of the chest cavity. The lower tip of the heart tilts toward the left. The heart is about the size of a clenched fist and weighs approximately 10.5 oz (300 g). The heart beats more than 100, 000 times a day and close to 2.5 billion times in the average lifetime. A triple-layered sac, the pericardium, surrounds, protects, and anchors the heart. A liquid pericardial fluid located in the space between two of the layers, reduces friction when the heart moves.

The heart is divided into four chambers. A partition or septum divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, atria (singular atrium), are thin-walled. They receive blood entering the heart, and pump it to the ventricles, the lower heart chambers. The walls of the ventricles are thicker and contain more cardiac muscle than the walls of the atria, enabling the ventricles to pump blood out to the lungs and the rest of the body.

The left and right sides of the heart function as two separate pumps. The left side of the heart pumps blood through the lungs, and the right side of the heart pumps blood through the rest of the body. The right atrium receives oxygen-poor blood from the body from a major vein, the vena cava, and delivers it to the right ventricle. The right ventricle, in turn, pumps the blood to the lungs via the pulmonary artery. The left atrium receives the oxygenrich blood from the lungs from the pulmonary veins, and delivers it to the left ventricle. The left ventricle then pumps it into the aorta, a major artery that leads to all parts of the body. The wall of the left ventricle is thicker than the wall of the right ventricle, making it a more powerful pump able to push blood through its longer trip around the body.

One-way valves in the heart keep blood flowing in the right direction and prevent backflow. The valves open and close in response to pressure changes in the heart. Atrioventricular (AV) valves are located between the atria and ventricles. Semilunar (SL) valves lie between the ventricles and the major arteries into which they pump blood. The “lub-dup” sounds that the physician hears through the stethoscope occur when the heart valves close. The AV valves produce the “lub” sound upon closing, while the SL valves cause the “dup” sound. People with a heart murmur have a defective heart valve that allows the backflow of blood.

The rate and rhythm of the heartbeat are carefully regulated. Doctors know that the heart continues to beat even when disconnected from the nervous system. This is evident during heart transplants when donor hearts keep beating outside the body. The explanation lies in a small mass of contractile cells, the sino-atrial (SA) node or pacemaker, located in the wall of the right atrium. The SA node sends out electrical impulses that set up a wave of contraction that spreads across the atria. The wave reaches the atrioventricular (AV) node, another small mass of contractile cells. The AV node is located in the septum between the left and right ventricle. The AV node, in turn, transmits impulses to all parts of the ventricles. The bundle of His, specialized fibers, conducts the impulses from the AV node to the ventricles. The impulses stimulate the ventricles to contract. An electrocardiogram, ECG or EKG, is a record of the electric impulses from the pacemaker that direct each heartbeat. The SA node and conduction system provide the primary heart controls. In patients with disorganized electrical activity in the heart, surgeons implant an artificial pacemaker that serves to regulate the heart rhythm. In addition to self-regulation by the heart, the autonomic nervous system and hormones also affect its rate.

The heart cycle refers to the events associated with a single heartbeat. The cycle involves systole, the contraction phase, and diastole, the relaxation phase. In the heart, the two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The heart cycle consists of a systole and diastole of both the atria and ventricles. At the end of a heartbeat all four chambers rest. The typical resting heart rate is about 75 beats per minute, with each cardiac cycle taking about 0.8 seconds.

Heart disease is the number one cause of death among people living in the industrial world. In coronary heart disease (CHD), a clot or stoppage occurs in a blood vessel of the heart. Deprived of oxygen, the surrounding tissue becomes damaged. Education about prevention of CHD helps to reduce its occur-rence. We have learned to lessen the risk of heart attacks by eating less fat, preventing obesity, exercising regularly, and by not smoking. Medications, medical devices and techniques also help patients with heart disease. One of these, the heart-lung machine, is used during open-heart and bypass surgery. This device pumps the patient’s blood out of the body, and returns it after having added oxygen and removed carbon dioxide. For patients with CHD, physicians sometimes use coronary artery bypass grafting. This is a surgical technique in which a blood vessel from another part of the body is grafted into the heart. The relocated vessel provides a new route for blood to travel as it bypasses the clogged coronary artery. In addition, cardiologists can also help CHD with angioplasty. Here, the surgeon inflates a balloon inside the aorta. This opens the vessel and improves the blood flow. For diagnosing heart-disease, the echocardiogram is used in conjunction with the ECG. This device uses high frequency sound waves to take pictures of the heart.

Blood vessels

The blood vessels of the body make up a closed system of tubes that carry blood from the heart to tissues all over the body and then back to the heart. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries connect small arteries (arterioles) and small veins (venules). Large arteries leave the heart and branch into smaller ones that reach out to various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, the arterioles branch into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which are small veins. The venules merge to form larger and larger veins that eventually return blood to the heart. The two main circulation routes in the body are the pulmonary circulation, to and from the lungs, and the systemic circulation, to and from all parts of the body. Subdivisions of the systemic system include the coronary circulation, for the heart, the cerebral circulation, for the brain, and the renal circulation, for the kidneys. In addition, the hepatic portal circulation passes blood directly from the digestive tract to the liver.

The walls of arteries, veins, and capillaries differ in structure. In all three, the vessel wall surrounds a hollow center through which the blood flows. The walls of both arteries and veins are composed of three coats. The inner coat is lined with a simple squamous endothelium, a single flat layer of cells. The thick middle coat is composed of smooth muscle that can change the size of the vessel when it contracts or relaxes, and of stretchable fibers that provide elasticity. The outer coat is composed of elastic fibers and collagen. The difference between veins and arteries lies in the thickness of the wall of the vessel. The inner and middle coats of veins are very thin compared to arteries. The thick walls of arteries make them elastic and capable of contracting. The repeated expansion and recoil of arteries when the heart beats creates the pulse. We can feel the pulse in arteries near the body surface, such as the radial artery in the wrist.

The walls of veins are more flexible than artery walls and they change shape when muscles press against them. Blood returning to the heart in veins is under low pressure often flowing against gravity. One-way valves in the walks of veins keep blood flowing in one direction. Skeletal muscles also help blood return to the heart by squeezing the veins as they contract. Varicose veins develop when veins lose their elasticity and become stretched. Faulty valves allow blood to sink back thereby pushing the vein wall outward.

The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allow the exchange of materials between cells and the blood. Their extensive branching provides a sufficient surface area to pick up and deliver substances to all cells in the body.

Blood pressure is the pressure of blood against the wall of a blood vessel. Blood pressure originates when the ventricles contract during the heartbeat. In a healthy young adult male, blood pressure in the aorta during systole is about 120 mm Hg, and approximately 80 mm Hg during diastole. The sphygmomanometer is an instrument that measures blood pressure. A combination of nervous carbon and hormones help regulate blood pressure around a normal range in the body. In addition, there are local controls that direct blood to tissues according to their need. For example, during exercise, reduced oxygen and increased carbon dioxide stimulate blood flow to the muscles.

Two disorders that involve blood vessels are hypertension and atherosclerosis. Hypertension, or high blood pressure, is the most common circulatory disease. For about 90% of hypertension sufferers, the blood pressure stays high without any known physical cause. Limiting salt and alcohol intake, stopping smoking, losing weight, increasing exercise, and managing stress help reduce blood pressure. Medications also help control hypertension. In atherosclerosis, the walls of arteries thicken and lose their elasticity. Fatty material such as cholesterol accumulates on the artery wall forming plaque that obstructs blood flow. The plaque can form a clot that breaks off, travels in the blood, and can block a smaller vessel. For example, a stroke occurs when a clot obstructs an artery or capillary in the brain. Treatment for atherosclerosis includes medication, surgery, a low-fat, high-fiber diet, and exercise. The type of cholesterol carried in the blood indicates the risk of atherosclerosis. Low density lipoproteins (LDLs) deposit cholesterol on arteries, while high density lipoproteins (HDLs) remove it.

Blood

Blood is liquid connective tissue. It transports oxygen from the lungs and delivers it to cells. It picks up carbon dioxide from the cells and brings it to the lungs. It carries nutrients from the digestive system and hormones from the endocrine glands to the cells. It takes heat and waste products away from cells. The blood helps regulate the body’s base-acid balance (pH), temperature, and water content. It protects the body by clotting and by fighting disease through the immune system.

When we study the structure of blood, we find that it is heavier and stickier than water, has a temperature in the body of about 100.4°F (38°C), and a pH of about 7.4. Blood makes up approximately 8% of the total body weight. A male of average weight has about 1.5 gal (5 L) of blood in his body, while a female has about 1.2 gal (4 L). Blood is composed of a liquid portion (the plasma), and blood cells.

Plasma is composed of about 91.5% water, which acts as a solvent, heat conductor, and suspending medium for the blood cells. The rest of the plasma includes plasma proteins produced by the liver, such as albumins, that help maintain water balance, globulins, that help fight disease, and fibrinogen, that aids in blood clotting. The plasma carries nutrients, hormones, enzymes, cellular waste products, some oxygen, and carbon dioxide. Inorganic salts, also carried in the plasma, help maintain osmotic pressure. Plasma leaks out of the capillaries to form the interstitial fluid (tissue fluid) that surrounds the body cells and keeps them moist, and supplied with nutrients.

The cells in the blood are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). More than 99% of all the blood cells are erythrocytes, or red blood cells. Red blood cells look like flexible biconcave discs about 8 nm in diameter that are capable of squeezing through narrow capillaries. Erythrocytes lack a nucleus and therefore are unable to reproduce. Antigens, specialized proteins on the surface of erythrocytes, determine the ABO and Rh blood types. Erythrocytes contain hemoglobin, a red pigment that carries oxygen, and each red cell has about 280 million hemoglobin molecules. An iron ion in hemoglobin combines reversibly with one oxygen molecule, enabling it to pick up, carry and drop off oxygen. Erythrocytes are formed in red bone marrow, and live about 120 days. When they are worn out, the liver and spleen destroy them and recycle their breakdown products. Anemia is a blood disorder characterized by too few red blood cells.

Leukocytes are white blood cells. They are larger than red blood cells, contain a nucleus, and do not have hemoglobin. Leukocytes fight disease organisms by destroying them or by producing antibodies. Lymphocytes are a type of leukocyte that bring about immune reactions involving antibodies. Monocytes are large leukocytes that ingest bacteria and get rid of dead matter. Most leukocytes are able to squeeze through the capillary walls and migrate to an infected part of the body. Formed in the white/yellow bone marrow, a leukocyte’s life ranges from hours to years depending on how it functions during an infection. In leukemia, a malignancy of bone marrow tissue, abnormal leukocytes are produced in an uncontrolled manner. They crowd out the bone marrow cells, interrupt normal blood cell production, and cause internal bleeding. Treatment for acute leukemia includes blood transfusions, anticancer drugs, and, in some cases, radiation

Key Terms

Artery —Vessel that transports blood away from the heart.

Atherosclerosis —Abnormal narrowing of the arteries of the body that generally originates from the buildup of fatty plaque on the artery wall.

Atrium (plural: atria) —One of two upper chambers of the heart. They are receiving units that hold blood to be pumped into the lower chambers, the ventricles.

Capillary —The smallest blood vessel; it connects artery to vein.

Phloem —Plant tissue that transports food.

Pulmonary route —Circuit that transports blood to and from the lungs.

Systemic route —Circuit that transports blood to and from all parts of the body except the lungs.

Vein —Vessel that transports blood to the heart.

Ventricles —The two lower chambers of the heart; also the main pumping chambers.

Xylem —Plant tissue that transports water and minerals upward from the roots.

Thrombocytes or platelets bring about clotting of the blood. Clotting stops the bleeding when the circulatory system is damaged. When tissues are injured, platelets disintegrate and release the substance thromboplastin. Working with calcium ions and two plasma proteins, fibrinogen and prothrombin, thromboplastin converts prothrombin to thrombin. Thrombin then changes soluble fibrinogen into insoluble fibrin. Finally, fibrin forms a clot. Hemophilia, a hereditary blood disease in which the patient lacks a clotting factor, occurs mainly in males. Hemophiliacs hemorrhage continuously after injury. They can be treated by transfusion of either fresh plasma or a concentrate of the deficient clotting factor.

The lymphatic system

and the circulatory system The lymphatic system is an open transport system that works in conjunction with the circulatory system. Lymphatic vessels collect intercellular fluid (tissue fluid), kill foreign organisms, and return it to the circulatory system. The lymphatic system also prevents tissue fluid from accumulating in the tissue spaces. Lymph capillaries pick up the intercellular fluid, now called lymph, and carry it into larger and larger lymph vessels. Inside the lymph vessels, lymph passes through lymph nodes, where lymphocytes attack viruses and bacteria. The lymphatic system transports lymph to the large brachiocephalic veins below the collarbone where it is re-enters the circulatory system. Lymph moves through the lymphatic system by the squeezing action of nearby muscles, for there is no pump in this system. Lymph vessels are equipped with one-way valves that prevent backflow. The spleen, an organ of the lymphatic system, removes old blood cells, bacteria, and foreign particles from the blood.

Resources

BOOKS

Aaronson, P. I. The Cardiovascular System at a Glance. Malden, MA: Blackwell Publishing, 2004.

Mohrman, David E. Cardiovascular Physiology. New York: McGraw-Hill Medical, 2006.

Sutton, Amy L. Blood And Circulatory Disorders Sourcebook. Detroit, MI: Omnigraphics, 2005.

Bernice Essenfeld

Circulatory system

Living things require a circulatory system to deliver food, oxygen , and other needed substances to all cells, and to take away waste products. Materials are transferred between individual cells and their internal environment through the cell membrane by diffusion , osmosis , and active transport. During diffusion and osmosis, molecules move from a higher concentration to a lower concentration. During active transport, carrier molecules push or pull substances across the cell membrane, using adenosine triphosphate (ATP) for energy . Unicellular organisms depend on passive and active transport to exchange materials with their watery environment. More complex multicellular forms of life rely on transport systems that move material-containing liquids throughout the body in specialized tubes. In vascular plants, tubes transport food and water . Some invertebrates rely on a closed system of tubes, while others have an open system. Humans and other higher vertebrates have a closed system of circulation.

Circulation in vascular plants

Water and dissolved minerals enter a plant's roots from the soil by means of diffusion and osmosis. These substances then travel upward in the plant in xylem vessels. The transpiration theory ascribes this ascending flow to a pull from above, caused by transpiration, the evaporation of water from leaves. The long water column stays intact due to the strong cohesion between water molecules. Carbohydrates, produced in leaves by photosynthesis , travel downward in plants in specialized tissue , phloem. This involves active transport of sugars into phloem cells and water pressure to force substances from cell to cell.

Circulation in invertebrates

Animal circulation depends on the contraction of a pump—usually a heart that pumps blood in one direction through vessels along a circulatory path. In a closed path, the network of vessels is continuous. Alternately, an open path has vessels that empty into open spaces in the body. The closed system in the earthworm uses five pairs of muscular hearts (the aortic arches), to pump blood. Located near the anterior or head end of the animal, the aortic arches contract and force blood into the ventral blood vessel that runs from head to tail. Blood then returns back to the hearts in the dorsal blood vessel. Small ring vessels in each segment connect dorsal and ventral blood vessels. As blood circulates throughout the body, it delivers nutrients and oxygen to cells and picks up carbon dioxide and other wastes.

Most arthropods and some advanced molluscs such as squid and octopuses have an open circulatory system. In the grasshopper, a large blood vessel runs along the top of the body, and enlarges at the posterior or tail end to form a tubelike heart. Openings in the heart (ostia) have valves that permit only the entry of blood into the heart. The heart contracts, forcing blood forward in the blood vessel and out into the head region. Outside the heart, the blood goes into spaces that surround the insect's internal organs. The blood delivers food and other materials to cells and picks up wastes. Animals with open circulatory systems depend on the respiratory system to transport oxygen and carbon dioxide. The blood moves slowly from the head to the tail end of the animal. At the posterior, the blood re-enters the heart through the openings. Contraction of muscles helps speed up the blood flow.

Human circulatory system

The human circulatory system is termed the cardiovascular system, from the Greek word kardia, 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 work done by the cardiovascular system is astounding. Each year, the heart pumps more than 1,848 gal (7,000 l) of blood through a closed system of about 62,100 mi (100,000 km) of blood vessels. This is more than twice the distance around the equator of the earth . As blood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands , and delivers these to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these to the lungs and kidneys, where they are excreted. Substances pass out of blood vessels to the cells through the interstitial or tissue fluid which surrounds cells.

The human heart

The adult heart is a hollow cone-shaped muscular organ located in the center of the chest cavity. The lower tip of the heart tilts toward the left. The heart is about the size of a clenched fist and weighs approximately 10.5 oz (300 g). Remarkably, the heart beats more than 100,000 times a day and close to 2.5 billion times in the average lifetime. A triple-layered sac, the pericardium, surrounds, protects, and anchors the heart. A liquid pericardial fluid located in the space between two of the layers, reduces friction when the heart moves.

The heart is divided into four chambers. A partition or septum divides it into a left and right side. Each side is further divided into an upper and lower chamber. The upper chambers, atria (singular atrium), are thin-walled. They receive blood entering the heart, and pump it to the ventricles, the lower heart chambers. The walls of the ventricles are thicker and contain more cardiac muscle than the walls of the atria, enabling the ventricles to pump blood out to the lungs and the rest of the body. The left and right sides of the heart function as two separate pumps. The right atrium receives oxygen-poor blood from the body from a major vein, the vena cava, and delivers it to the right ventricle. The right ventricle, in turn, pumps the blood to the lungs via the pulmonary artery. The left atrium receives the oxygen-rich blood from the lungs from the pulmonary veins , and delivers it to the left ventricle. The left ventricle then pumps it into the aorta, a major artery that leads to all parts of the body. The wall of the left ventricle is thicker than the wall of the right ventricle, making it a more powerful pump able to push blood through its longer trip around the body.

One-way valves in the heart keep blood flowing in the right direction and prevent backflow. The valves open and close in response to pressure changes in the heart. Atrioventricular (AV) valves are located between the atria and ventricles. Semilunar (SL) valves lie between the ventricles and the major arteries into which they pump blood. The "lub-dup" sounds that the physician hears through the stethoscope occur when the heart valves close. The AV valves produce the "lub" sound upon closing, while the SL valves cause the "dup" sound. People with a heart murmur have a defective heart valve that allows the backflow of blood.

The rate and rhythm of the heartbeat are carefully regulated. We know that the heart continues to beat even when disconnected from the nervous system . This is evident during heart transplants when donor hearts keep beating outside the body. The explanation lies in a small mass of contractile cells, the sino-atrial (SA) node or pacemaker , located in the wall of the right atrium. The SA node sends out electrical impulses that set up a wave of contraction that spreads across the atria. The wave reaches the atrio-ventricular (AV) node, another small mass of contractile cells. The AV node is located in the septum between the left and right ventricle. The AV node, in turn, transmits impulses to all parts of the ventricles. The bundle of His, specialized fibers, conducts the impulses from the AV node to the ventricles. The impulses stimulate the ventricles to contract. An electrocardiogram, ECG or EKG, is a record of the electric impulses from the pacemaker that direct each heartbeat. The SA node and conduction system provide the primary heart controls. In patients with disorganized electrical activity in the heart, surgeons implant an artificial pacemaker that serves to regulate the heart rhythm. In addition to self-regulation by the heart, the autonomic nervous system and hormones also affect its rate.

The heart cycle refers to the events associated with a single heartbeat. The cycle involves systole, the contraction phase, and diastole, the relaxation phase. In the heart, the two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The heart cycle consists of a systole and diastole of both the atria and ventricles. At the end of a heartbeat all four chambers rest. The rate of heartbeat averages about 75 beats per minute, and each cardiac cycle takes about 0.8 seconds.

Heart disease is the number one cause of death among people living in the industrial world. In coronary heartdisease (CHD), a clot or stoppage occurs in a blood vessel of the heart. Deprived of oxygen, the surrounding tissue becomes damaged. Education about prevention of CHD helps to reduce its occurrence. We have learned to prevent heart attacks by eating less fat , preventing obesity , exercising regularly, and by not smoking. Medications, medical devices and techniques also help patients with heartdisease. One of these, the heart-lung machine , is used during open-heart and bypass surgery . This device pumps the patient's blood out of the body, and returns it after having added oxygen and removed carbon dioxide. For patients with CHD, physicians sometimes use coronary artery bypass grafting (CABG). This is a surgical technique in which a blood vessel from another part of the body is grafted into the heart. The relocated vessel provides a new route for blood to travel as it bypasses the clogged coronary artery. In addition, cardiologists can also help CHD with angioplasty. Here, the surgeon inflates a balloon inside the aorta. This opens the vessel and improves the blood flow. For diagnosing heartdisease, the echocardiogram is used in conjunction with the ECG. This device uses high frequency sound waves to take pictures of the heart.

Blood vessels

The blood vessels of the body make up a closed system of tubes that carry blood from the heart to tissues all over the body and then back to the heart. Arteries carry blood away from the heart, while veins carry blood toward the heart. Capillaries connect small arteries (arterioles) and small veins (venules). Large arteries leave the heart and branch into smaller ones that reach out to various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, the arterioles branch into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which are small veins. The venules merge to form larger and larger veins that eventually return blood to the heart. The two main circulation routes in the body are the pulmonary circulation, to and from the lungs, and the systemic circulation, to and from all parts of the body. Subdivisions of the systemic system include the coronary circulation, for the heart, the cerebral circulation, for the brain , and the renal circulation, for the kidneys. In addition, the hepatic portal circulation passes blood directly from the digestive tract to the liver.

The walls of arteries, veins, and capillaries differ in structure. In all three, the vessel wall surrounds a hollow center through which the blood flows. The walls of both arteries and veins are composed of three coats. The inner coat is lined with a simple squamous endothelium, a single flat layer of cells. The thick middle coat is composed of smooth muscle that can change the size of the vessel when it contracts or relaxes, and of stretchable fibers that provide elasticity . The outer coat is composed of elastic fibers and collagen . The difference between veins and arteries lies in the thickness of the wall of the vessel. The inner and middle coats of veins are very thin compared to arteries. The thick walls of arteries make them elastic and capable of contracting. The repeated expansion and recoil of arteries when the heart beats creates the pulse. We can feel the pulse in arteries near the body surface, such as the radial artery in the wrist. The walls of veins are more flexible than artery walls and they change shape when muscles press against them. Blood returning to the heart in veins is under low pressure often flowing against gravity. One-way valves in the walks of veins keep blood flowing in one direction. Skeletal muscles also help blood return to the heart by squeezing the veins as they contract. Varicose veins develop when veins lose their elasticity and become stretched. Faulty valves allow blood to sink back thereby pushing the vein wall outward. The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allow the exchange of materials between cells and the blood. Their extensive branching provides a sufficient surface area to pick up and deliver substances to all cells in the body.

Blood pressure is the pressure of blood against the wall of a blood vessel. Blood pressure originates when the ventricles contract during the heartbeat. In a healthy young adult male, blood pressure in the aorta during systole is about 120 mm Hg, and approximately 80 mm Hg during diastole. The sphygmomanometer is an instrument that measures blood pressure. A combination of nervous carbon and hormones help regulate blood pressure around a normal range in the body. In addition, there are local controls that direct blood to tissues according to their need. For example, during exercise , reduced oxygen and increased carbon dioxide stimulate blood flow to the muscles.

Two disorders that involve blood vessels are hypertension and atherosclerosis. Hypertension, or high blood pressure, is the most common circulatory disease. For about 90% of hypertension sufferers, the blood pressure stays high without any known physical cause. Limiting salt and alcohol intake, stopping smoking, losing weight, increasing exercise, and managing stress help reduce blood pressure. Medications also help control hypertension. In atherosclerosis, the walls of arteries thicken and lose their elasticity. Fatty material such as cholesterol accumulates on the artery wall forming plaque that obstructs blood flow. The plaque can form a clot that breaks off, travels in the blood, and can block a smaller vessel. For example, a stroke occurs when a clot obstructs an artery or capillary in the brain. Treatment for atherosclerosis includes medication, surgery, a low-fat, high-fiber diet, and exercise. The type of cholesterol carried in the blood indicates the risk of atherosclerosis. Low density lipoproteins (LDLs) deposit cholesterol on arteries, while high density lipoproteins (HDLs) remove it.

Blood

Blood is liquid connective tissue . It transports oxygen from the lungs and delivers it to cells. It picks up carbon dioxide from the cells and brings it to the lungs. It carries nutrients from the digestive system and hormones from the endocrine glands to the cells. It takes heat and waste products away from cells. The blood helps regulate the body's base-acid balance (pH ), temperature , and water content. It protects the body by clotting and by fighting disease through the immune system .

When we study the structure of blood, we find that it is heavier and stickier than water, has a temperature in the body of about 100.4°F (38°C), and a pH of about 7.4. Blood makes up approximately 8% of the total body weight. A male of average weight has about 1.5 gal (5-6 l) of blood in his body, while a female has about 1.2 gal (4-5 l). Blood is composed of a liquid portion (the plasma ), and blood cells.

Plasma is composed of about 91.5% water which acts as a solvent, heat conductor, and suspending medium for the blood cells. The rest of the plasma includes plasma proteins produced by the liver, such as albumins, that help maintain water balance, globulins, that help fight disease, and fibrinogen, that aids in blood clotting. The plasma carries nutrients, hormones, enzymes, cellular waste products, some oxygen, and carbon dioxide. Inorganic salts, also carried in the plasma, help maintain osmotic pressure. Plasma leaks out of the capillaries to form the interstitial fluid (tissue fluid) that surrounds the body cells and keeps them moist, and supplied with nutrients.

The cells in the blood are erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). More than 99% of all the blood cells are erythrocytes, or red blood cells. Red blood cells look like flexible biconcave discs about 8 nm in diameter that are capable of squeezing through narrow capillaries. Erythrocytes lack a nucleus and therefore are unable to reproduce. Antigens, specialized proteins on the surface of erythrocytes, determine the ABO and Rh blood types. Erythrocytes contain hemoglobin, a red pigment that carries oxygen, and each red cell has about 280 million hemoglobin molecules. An iron ion in hemoglobin combines reversibly with one oxygen molecule , enabling it to pick up, carry and drop off oxygen. Erythrocytes are formed in red bone marrow, and live about 120 days. When they are worn out, the liver and spleen destroy them and recycle their breakdown products. Anemia is a blood disorder characterized by too few red blood cells.

Leukocytes are white blood cells. They are larger than red blood cells, contain a nucleus, and do not have hemoglobin. Leukocytes fight disease organisms by destroying them or by producing antibodies. Lymphocytes are a type of leukocyte that bring about immune reactions involving antibodies. Monocytes are large leukocytes that ingest bacteria and get rid of dead matter . Most leukocytes are able to squeeze through the capillary walls and migrate to an infected part of the body. Formed in the white/yellow bone marrow, a leukocyte's life ranges from hours to years depending on how it functions during an infection . In leukemia , a malignancy of bone marrow tissue, abnormal leukocytes are produced in an uncontrolled manner. They crowd out the bone marrow cells, interrupt normal blood cell production, and cause internal bleeding. Treatment for acute leukemia includes blood transfusions, anticancer drugs, and, in some cases, radiation .

Thrombocytes or platelets bring about clotting of the blood. Clotting stops the bleeding when the circulatory system is damaged. When tissues are injured, platelets disintegrate and release the substance thromboplastin. Working with calcium ions and two plasma proteins, fibrinogen and prothrombin, thromboplastin converts prothrombin to thrombin. Thrombin then changes soluble fibrinogen into insoluble fibrin. Finally, fibrin forms a clot. Hemophilia , a hereditary blood disease in which the patient lacks a clotting factor, occurs mainly in males. Hemophiliacs hemorrhage continuously after injury. They can be treated by transfusion of either fresh plasma or a concentrate of the deficient clotting factor.

The lymphatic system and the circulatory system

The lymphatic system is an open transport system that works in conjunction with the circulatory system. Lymphatic vessels collect intercellular fluid (tissue fluid), kill foreign organisms, and return it to the circulatory system. The lymphatic system also prevents tissue fluid from accumulating in the tissue spaces. Lymph capillaries pick up the intercellular fluid, now called lymph, and carry it into larger and larger lymph vessels. Inside the lymph vessels, lymph passes through lymph nodes, where lymphocytes attack viruses and bacteria. The lymphatic system transports lymph to the large brachiocephalic veins below the collarbone where it is re-enters the circulatory system. Lymph moves through the lymphatic system by the squeezing action of nearby muscles, for there is no pump in this system. Lymph vessels are equipped with one-way valves that prevent backflow. The spleen, an organ of the lymphatic system, removes old blood cells, bacteria, and foreign particles from the blood.

Resources

books

Berne, R.M., and M.N. Levy. Cardiovascular Physiology. St. Louis: C.V. Mosby, 1992.

Guyton & Hall. Textbook of Medical Physiology. 10th ed. New York: W. B. Saunders Company, 2000.

Kapit, Wynn, and Lawrence M. Elson. The Anatomy Coloring Book. New York: Harper & Row, 1995.

periodicals

Acierno, L.J., and T. Worrell. "Profiles in Cardiology: James Bryan Herrick." Clinical Cardiology no. 23 (2000): 230-232.

Fackelmann, K. A. "Immune Cell Triggers Attack on Plaque." Science News (October 22, 1994).

Vogel, Steven. "Nature's Pumps." American Scientist (September/October 1994).

other

Two Hearts That Beat as One. Films for Humanities and Science, 1995. Videocassette.

Bernice Essenfeld

KEY TERMS

Artery

—Vessel that transports blood away from the heart.

Atherosclerosis

—Abnormal narrowing of the arteries of the body that generally originates from the buildup of fatty plaque on the artery wall.

Atrium (plural: atria)

—One of two upper chambers of the heart. They are receiving units that hold blood to be pumped into the lower chambers, the ventricles.

Capillary

—The smallest blood vessel; it connects artery to vein.

Phloem

—Plant tissue that transports food.

Pulmonary route

—Circuit that transports blood to and from the lungs.

Systemic route

—Circuit that transports blood to and from all parts of the body except the lungs.

Vein

—Vessel that transports blood to the heart.

Ventricles

—The two lower chambers of the heart; also the main pumping chambers.

Xylem

—Plant tissue that transports water and minerals upward from the roots.

Circulatory System

The circulatory system is a network that carries blood throughout an animal's body. Described as an internal transport system or a distribution system, the circulatory system maintains a constant flow of blood throughout the body, carrying nutrients and oxygen to the body's tissues and taking away its waste products. It also helps regulate the body's temperature; carries substances, like antibodies and white cells that protect the body from disease; and transports chemicals, such as hormones, that help the body regulate its activities.

All animal cells live in a liquid environment, taking in oxygen and nutrients and creating waste products. All, therefore, require a way to gain access to what they need to grow, to perform their specialized task, to reproduce, and to dispose of their waste products. For any animal larger than a single-celled organism—which meets its needs by passing substances through holes in its membrane (called diffusion)—a complex circulatory system is required in order to make sure that every single cell gets what it needs.

OPEN CIRCULATORY SYSTEM

There are two main types of circulatory systems—an open system and a closed system. Most invertebrates (animals without a backbone) have an open circulatory system. It consists of a fairly simple network of tubes and hollow spaces. In this system, a heart (or a series of hearts) pumps blood out of the vessels (a duct for circulating blood) and into the sinuses or open spaces of an animal's body. Mollusks, like clams, and arthropods, such as crayfish and grasshoppers, have an open circulatory system. In such a system blood flows slowly and under low pressure into the open spaces in these organisms' body cavities. This blood also bathes their cells, allowing them to obtain food and oxygen while eliminating waste. Open circulatory systems pump a bloodlike fluid called "hemolymph" that closely resembles seawater. However, one of the disadvantages of this type of system is that it cannot respond quickly to change, nor can it supply large amounts of oxygen.

CLOSED CIRCULATORY SYSTEM

All vertebrates (animals with a backbone) and some invertebrates, like earthworms, have a closed circulatory system. This means that the blood never leaves their vessels. For vertebrates, blood is pumped by the heart throughout the body via a network of closed vessels that become finer and finer as they get farther away from the heart. Closed systems are more efficient than open ones, since it is easier for them to respond to sudden changes and to alter the distribution of blood.

Although there are invertebrates with closed systems, it is by far the predominant characteristic of vertebrates—whether fish, amphibian, reptile, bird, or mammal. The basic components of the vertebrate circulatory system are the heart, arteries, capillaries, veins, and the blood itself. In the human body, the heart is a hollow, muscular pump that forces blood to move throughout the body. The human heart consists of two pumps that lie side by side. The left and stronger pump receives oxygen-fresh blood from the lungs and pumps it under great pressure to the cells throughout the body. The weaker right side receives "used" blood from the cells and sends it to the lungs to have carbon dioxide removed and to be freshened with oxygen. The heart muscle beats at its own automatic rhythm and pumps in a certain correct sequence. Arteries are blood vessels that carry blood away from the heart. They are specially designed, with thick, strong walls, since the blood they transport is under high pressure. Puncturing an artery can cause blood to spurt and travel through the air. Arteries also serve to smooth out the flow of blood by absorbing much of the rhythmic shock of the pumping heart. Arteries get smaller farther from the heart and are called arterioles.

As oxygen-rich blood continues the one-way trip from the heart and to the cells, the capillaries receive blood from the arterioles and pass it directly to the surrounding tissues and cells. Capillaries are finer than human hair (they are one cell thick) and have very thin walls where the critical exchange of nutrients, oxygen, and waste takes place. Capillaries merge to form venules or tiny veins, which in turn, merge to create large veins that will carry the blood back to the heart. Large veins are thinner than arteries (since the blood is now under much lower pressure) and are the primary blood vessels for the one-way, return trip of blood to the heart. Large veins also have one-way valves that prevent deoxygenated blood (depleted of oxygen and full of carbon dioxide) from flowing backwards. These veins empty into the parts of the heart called the vena cava, which in turn, empty directly into the heart's right atrium and back on to the lungs to start the cycle all over.

The route that blood travels in humans (as well as in birds and other mammals) is called "double circulation," which is in contrast to animals whose blood circulates in a single loop from the heart, around the body, and back again. In a double circulation system blood flows alternately through the lungs and throughout the body in a figure-eight pattern. After completing each loop, blood returns to a different side of the heart. In the human body, an entire double-loop cycle takes less than one minute. While the actual path that blood follows will vary among different groups of vertebrates, all are based on a similar system whether single or double loop. For example, fish have single loop circulation and a four-chambered heart, while amphibians and most reptiles have a three-chambered heart.

WILLIAM HARVEY DISCOVERS BLOOD CIRCULATION

The English physician William Harvey (1578–1657) discovered blood circulation in 1628. He demonstrated, contrary to widely-held beliefs that

WILLIAM HARVEY

English physician and biologist William Harvey (1578–1657) was the first to describe the circulation of the blood through the heart and the blood vessels. As a great experimenter, Harvey was able to prove that blood does not ebb and flow like waves in the body as people believed, but instead travels in a closed, or one-way circle. Many consider Harvey to be the founder of modern physiology (which studies how the different processes in living things work).

William Harvey was born in Folkestone, England, the son of well-to-do parents. After earning his degree from Cambridge in 1597, he did what anyone who wanted the best medical education available did: he went to Padua, Italy. At that time, Padua was able to boast that it had many great scholars, including the young astronomer, Galileo Galilei (1564–1642), on its faculty. Harvey stayed in Padua until he obtained his degree in 1602, and then he moved back to England. Given his father's connections, he soon had important patients, and later became physician to the king. However, Harvey was more interested in experimentation than in his day-to-day medical practice, as evidenced by his claim to have dissected eighty different species of animals by 1616. Harvey always dissected with a purpose, and most often, he focused on learning more about the heart and its vessels. Because of his extensive hands-on experience, Harvey already knew that heart muscle was basically a pump that acted by contracting, which pushed the blood out. He also learned that the valves that separated the heart's two upper and two lower chambers were one-way valves, and that veins (a branching system of vessels through which blood returns to the heart) even had one-way valves. All of this was leading Harvey far away from what most medical men of his time believed.

Traditional medicine was based on the writings of Galen (a.d. 129–c.199) who lived some 1,400 years before Harvey and whose human physiology was based on his dissections of animals. Galen taught that the liver was the important organ for the blood and that the blood that formed there ebbed and flowed throughout the body like the back-and-forth of the oceans' tides. Before Harvey decided to refute Galen, who he knew to be wrong, he performed even more experiments on animal and human hearts. He monitored the beating heart and made rough calculations of the volume of blood leaving the heart per beat and then compared that to total blood volume. It seemed impossible to him that blood could be broken down and reformed, as Galen said, fast enough to account for the amount of blood in the human body. The only explanation was that it must be the same blood moving in circles throughout the body. Finally, in 1628, Harvey published his slim, seventy-two-page book in Holland, in which he argued that blood was in motion all the time and that it circulated. The translated title of his landmark work is On the Motions of the Heart and Blood.

Since Harvey's book was a direct assault on Galen and all those who supported his ancient ideas, it is not surprising that Harvey found himself being ridiculed and condemned. His medical practice suffered for a time, but Harvey refused to be drawn into a defensive debate and instead, let the facts speak for themselves. Throughout this, he retained the king's favor. By his old age, the theory of the circulation of the blood was accepted and he had become a highly respected and even revered man of science. Interestingly, the one part of Harvey's theory that he could not prove was the crucial statement that blood moved from the arteries (a branching system of blood vessels that carry blood away from the heart) to the veins. He was unable to point out any visible connections between the two. However, he knew from his dissections that both vessels divided down to smaller and finer vessels, again and again, and so he simply assumed that they were too small to see with the naked eye. He was, of course, correct and was proven so about twenty-five years later when his countryman, Marcello Malpighi (1628–1694), used his new microscope and discovered extra-fine blood vessels called capillaries that connected the smallest arteries to the smallest veins.

blood flowed from the heart in a continuous, one-way cycle. The current understanding of the circulatory system was born of Harvey's work. It is now known that the circulatory system performs many vital functions in respiration (delivering oxygen to the cells and removing carbon dioxide); in nutrition (carrying nutrients to the cells and liver); and in waste removal (transporting poisons like salts and ammonia to the liver for disposal). The circulatory system also helps the body fight disease by acting as a means of transport for the lymphatic system, which is part of the body's immune system. This system filters harmful substances out of the bloodstream and carries white blood cells that destroy harmful bacteria, viruses, and other invaders.

[See alsoBlood; Heart; Lymphatic System; Respiratory System ]

Circulatory system

The human circulatory system is responsible for delivering food, oxygen, and other needed substances to all cells in all parts of the body while taking away waste products. The circulatory system is also known as the cardiovascular system, from the Greek word kardia, 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. As blood circulates around the body, it picks up oxygen from the lungs, nutrients from the small intestine, and hormones from the endocrine glands, and delivers these to the cells. Blood then picks up carbon dioxide and cellular wastes from cells and delivers these to the lungs and kidneys, where they are excreted.

The human heart

The adult heart is a hollow cone-shaped muscular organ located in the center of the chest cavity. The lower tip of the heart tilts toward the left. The heart is about the size of a clenched fist and weighs approximately 10.5 ounces (300 grams). A heart beats more than 100,000 times a day and close to 2.5 billion times in an average lifetime. The pericardium—a triple-layered sac—surrounds, protects, and anchors the heart. Pericardial fluid located in the space between two of the layers reduces friction when the heart moves.

The heart is divided into four chambers. A 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 receive blood entering the heart and pump it to the ventricles, the lower heart chambers. The walls of the ventricles are thicker and contain more cardiac muscle than the walls of the atria. This enables the ventricles to pump blood out to the lungs and the rest of the body.

The left and right sides of the heart function as two separate pumps. The right atrium receives blood carrying carbon dioxide from the body through a major vein, the vena cava, and delivers it to the right ventricle. The right ventricle, in turn, pumps the blood to the lungs via the pulmonary artery. The left atrium receives the oxygen-rich blood from the lungs from the pulmonary veins, and delivers it to the left ventricle. The left ventricle then pumps it into the aorta, the major artery that leads to all parts of the body. The wall of the left ventricle is thicker than the wall of the right ventricle, making it a more powerful pump, able to push blood through its longer trip around the body.

One-way valves in the heart keep blood flowing in the right direction and prevent backflow. The valves open and close in response to pressure changes in the heart. Atrioventricular valves are located between the atria and ventricles. Semilunar valves lie between the ventricles and the major arteries into which they pump blood. People with a heart murmur have a defective heart valve that allows the backflow of blood.

Words to Know

Artery: Vessel that transports blood away from the heart.

Atherosclerosis: Condition in which fatty material such as cholesterol accumulates on artery walls forming plaque that obstructs blood flow.

Atrium: Receiving chamber of the heart.

Capillary: Vessel that connects artery to vein.

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

Hormones: Chemical messengers that regulate body functions.

Hypertension: High blood pressure.

Sphygmomanometer: Instrument that measures blood pressure in millimeters of mercury.

Systole: Rhythmic contraction of the heat.

Vein: Vessel that transports blood to the heart.

Ventricle: Pumping chamber of the heart.

The heart cycle refers to the events that occur during a single heartbeat. The cycle involves systole (the contraction phase) and diastole (the relaxation phase). In the heart, the two atria contract while the two ventricles relax. Then, the two ventricles contract while the two atria relax. The heart cycle consists of a systole and diastole of both the atria and ventricles. At the end of a heartbeat all four chambers rest. The average heart beats about 75 times per minute, and each heart cycle takes about 0.8 seconds.

Blood vessels

The blood vessels of the body (arteries, capillaries, and veins) make up a closed system of tubes that carry blood from the heart to tissues all over the body and then back to the heart. Arteries carry blood away from the heart, while veins carry blood toward the heart. Large arteries leave

the heart and branch into smaller ones that reach out to various parts of the body. These divide still further into smaller vessels called arterioles that penetrate the body tissues. Within the tissues, the arterioles branch into a network of microscopic capillaries. Substances move in and out of the capillary walls as the blood exchanges materials with the cells. Before leaving the tissues, capillaries unite into venules, which are small veins. The venules merge to form larger and larger veins that eventually return blood to the heart.

The walls of arteries, veins, and capillaries differ in structure. In all three, the vessel wall surrounds a hollow center through which the blood flows. The walls of both arteries and veins are composed of three coats, but they differ in thickness. The inner and middle coats of arteries are thicker than those of veins. This makes arteries more elastic and capable of expanding when blood surges through them from the beating heart. The walls of veins are more flexible than artery walls. This allows skeletal muscles to contract against them, squeezing the blood along as it returns to the heart. One-way valves in the walls of veins keep blood flowing in one direction. The walls of capillaries are only one cell thick. Of all the blood vessels, only capillaries have walls thin enough to allow the exchange of materials between cells and the blood.

Blood pressure is the pressure of blood against the wall of an artery. Blood pressure originates when the ventricles contract during the heartbeat. It is strongest in the aorta and decreases as blood moves through progressively smaller arteries. A sphygmomanometer (pronounced sfigmoe-ma-NOM-i-ter) is an instrument that measures blood pressure in millimeters (mm) of mercury. Average young adults have a normal blood pressure reading of about 120 mm for systolic pressure and 80 mm for diastolic pressure. Blood pressure normally increases with age.

Blood

Blood is liquid connective tissue. It transports oxygen from the lungs and delivers it to cells. It picks up carbon dioxide from the cells and brings it to the lungs. It carries nutrients from the digestive system and hormones from the endocrine glands to the cells. It takes heat and waste products away from cells. It protects the body by clotting and by fighting disease through the immune system.

Blood is heavier and stickier than water, and has a temperature in the body of about 100.4°F (38°C). Blood makes up approximately 8 percent of an individual's total body weight. A male of average weight has about 1.5 gallons (5.5 liters) of blood in his body, while a female has about 1.2 gallons (4.5 liters).

Blood is composed of plasma (liquid portion) and blood cells. Plasma, which is about 91.5 percent water, carries blood cells and helps conduct heat. The three types of cells in blood are red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). More than 99 percent of all the blood cells are red blood cells. They contain hemoglobin, a red pigment that carries oxygen, and each red cell has about 280 million hemoglobin molecules. White blood cells fight disease organisms by destroying them or by producing antibodies. Platelets bring about clotting of the blood.

Circulatory diseases. Two disorders that involve blood vessels are hypertension and atherosclerosis. Hypertension, or high blood pressure, is the most common circulatory disease. In about 90 percent of hypertension sufferers, blood pressure stays high without any known physical cause. Limiting salt and alcohol intake, stopping smoking, losing weight, increasing exercise, and managing stress all help reduce blood pressure. Medications also help control hypertension.

In atherosclerosis, fatty material such as cholesterol accumulates on the artery wall forming plaque that obstructs blood flow. The plaque can form a clot that breaks off, travels in the blood, and can block a smaller vessel. A stroke may occur when a clot obstructs an artery or capillary in the brain. Treatment for atherosclerosis includes medication, surgery, a high-fiber diet low in fat, and exercise.

[See also Blood; Heart; Lymphatic system ]

Circulatory System

Every living organism on Earth, from amoebas to redwoods to whales, has a circulatory system—a means of gathering and transporting nutrients and collecting and removing waste products.

Plants have an elegant system of strawlike tubes called phloem and xylem, which stretch from the roots to the topmost leaves. Stomata, tiny evaporative holes in the leaves, create suction that steadily draws water up the xylem from the roots, allowing plants hundreds of feet tall to circulate nutrients without a pump.

All cells of the simplest animals, such as single-celled amoebas and multicellular flatworms, are close to the surface. In these cells, nutrients wash through the cell fluid, and wastes pass out through a porous outer membrane between the cell and its environment. The cells of larger animals are buried many layers deep, so these animals require a system that connects each cell to the outer world. This system, which consists of the fluid that carries nutrients through vessels that reach every part of the body and the mechanism that powers the flow of nutrients, is called the circulatory system.

The simplest form of circulatory system is an open circulatory system. In an open circulatory system, blood flows through a network of open tubes and hollow spaces, and the movement of the animal itself keeps the blood flowing. In more complex systems, blood is pumped through the body by contractions of the blood vessels. Invertebrates, such as insects and other arthropods , have a central blood vessel that runs down the length of the back. A series of bulbous pumping centers slowly squeeze the blood through a maze of hollow spaces around the body past all the organs.

Vertebrates, including amphibians, reptiles, birds, and mammals, have increasingly complex, closed circulatory systems. Closed circulatory systems consist of an intricate network of vessels filled with blood that delivers nutrients, regulates internal temperature, and takes away waste products. The system is powered by the heart, a muscular pump that never stops working, which continually circulates the blood through the body.

In all vertebrates, the heart is made of involuntary muscle tissue, but the structure is very different in each group. Fish have a two-chambered, single-pump heart. Amphibians have a three-chambered heart that also acts as a single pump. Birds and mammals have a more sophisticated four-chambered, double-pump heart design. One chamber sends blood to the lungs to be purified and reoxygenated, the other sends the enriched blood out into the body. Interestingly, the human embryo goes through every stage of circulatory development, from a passive single-celled heart to a two-, three-, and four-chambered heart.

In adult humans, the circulatory system consists of blood, the heart, and a network of vessels through which the blood travels. Blood is plasma (a watery liquid) that contains billions of molecules of sugars, proteins, hormones , antibodies , and gases. The heart is a strong, muscular, double pump that pushes the blood continuously and automatically around the body through roughly 100,000 kilometers (62,000 miles) of arteries, veins, and capillaries. It takes the blood about one minute to complete a circuit around the body, and this happens about 1,000 times a day.

The human circulatory system has two loops. The shorter pulmonary circulation goes from the lower-right chamber of the heart (the ventricle) through the pulmonary artery to the lungs and back to the upper-left chamber (the atrium) through the pulmonary vein. From there, the newly oxygenated blood descends into the left ventricle through a one-way valve and is pumped into the longer systemic circulation through the main artery of the body, the aorta. The spent blood travels back to the right atrium in two main veins. The superior vena cava drains the upper body, the head, neck, and arms. The inferior vena cava handles the lower body. From the right atrium, the blood flows through the relaxed one-way valve into the right ventricle. Then another pulse of the powerful heart muscle closes the valve and spurts the blood into the pulmonary artery, beginning the cycle again. The sound known as the heart "beat" is the sound of the valves between the atria and ventricles and between the ventricles and arteries as they snap shut to keep the blood from flowing backward.

A healthy, relaxed, adult heart beats about seventy times a minute, pumping blood under high pressure into the thick-walled arteries. The elastic walls of the arteries stretch open to allow the blood to flow in, then squeeze back together to force it along. Arteries branch into narrower and more muscular arterioles. Arterioles branch into finer and finer capillaries, thin-walled, hairlike vessels that interact with surrounding body cells to exchange nutrients and wastes. Capillaries then enlarge into venules, which merge into veins, and carry the spent blood back to the heart. After the blood has traveled through the capillary network the pressure is greatly reduced, and the veins can afford to be much thinner than arteries with weaker muscle fiber. Small, one-way valves inside the veins keep the blood moving against gravity toward the heart.

As well as delivering the supplies that keep cells functioning, the bloodstream regulates body temperature by dissipating heat that builds up in the organs. The contraction or dilation of surface capillaries allows more or less heat to escape the system, depending on whether the body is too hot or too cold. The bloodstream also contains disease-and infection-fighting antibodies.

The lymphatic system is a one-way independent drainage network of fine capillaries primarily involved in fighting disease and infection. White blood cells used in neutralizing bacteria collect in lymph glands. Then normal muscle movement and one-way valves keep the lymph flowing toward the chest, where it drains into two large veins and reenters the blood stream.

see also Blood.

Nancy Weaver

Bibliography

Ballard, Carol. Heart and Circulatory System. Austin, TX: Steck Vaughn, 1997.

Silverstein, Alvin, Virginia, and Robert Alvin. The Circulatory System. New York: Henry Holt & Co., 1994.

Unschuld, Paul U., trans. Nan Ching (Chinese Medicine). Berkeley: University of California Press, 1985.