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Biomedicine and Health: The Cardiovascular System

Biomedicine and Health: The Cardiovascular System

Introduction

In modern science the brain is the key to our identity. The head is the seat of thought and the emotions. Through the nerves of the brain and spinal cord we learn about the world. These concepts seem so obvious that it is hard to imagine that the enthronement of the nervous system is not “natural.” Yet this neurological supremacy is relatively recent, dating only to the seventeenth century. Before this time and back to the first human records, the heart and blood vessels were more often regarded as the source of life, thought, and feeling. To this day vestiges of these ancient ideas have a rugged existence in popular culture.

Historical Background and Scientific Foundations

The beating heart of a freshly killed animal was surely known to ancient hunters, and scholars debate whether a Paleolithic cave painting in Pindal, Spain, from 30,000 BC, depicts a heart in a creature that is clearly a mammoth.

The importance of the heart to ancient cultures is revealed in two Egyptian papyri: the Ebers papyrus and the Edwin Smith papyrus. Both date from around 1600 BC, but Egyptologists are confident that the material they contain is very much older. These texts contain fragments of a document “The Beginning of the Physicians Secret: Knowledge of the Heart's Movement and Knowledge of the Heart.” This treatise describes how the body contains a system of vessels springing from the heart. These vessels carry all the body's fluids: blood, urine, tears, milk, sperm, and so on. The heart's action in propelling these fluids to the body parts can be felt in the pulse. In the pulse, the document says, the heart “speaks out of the vessels of every limb.” The heart was considered the seat of thought and feeling. It is not, of course, surprising that the Egyptians envisaged the body as a great network of canals—after all, the land around the Nile was covered with a similar system of irrigation channels.

The ancient Greeks differed over the priority of the heart or the brain. Some distributed their responsibilities. In the later half of the fifth century BC, the philosopher Philolaus of Tarentum (fl. fifth century BC), a follower of Greek philosopher and mathematician Pythagoras (580–500 BC), wrote: “The brain is the seat of mind, the heart of the soul and of sensation.” The works of the philosopher Empedocles (c.490–c.430 BC) were to have considerable importance for later concepts of the role of the heart and vessels. Empedocles considered that the heart distributed inspired air or pneuma to the parts. Pneuma, however, was more than just air, it was soul, the vital spirit and life force pervading the universe (in Christian theology pneumatology is the study of the Holy Spirit).

The works associated with the Greek physician Hippocrates (c.460–375 BC) (but which are clearly from many different authors and sects) generally supported the brain theory. The Hippocratic authors showed little interest in internal anatomy, but in a treatise named On the Heart, the author had obviously dissected animal hearts and knew the Empedoclean tradition. It was his opinion that the heart was composed of two auricles and two ventricles. He theorized that the right ventricle supplies blood to the lungs in exchange for air. The left ventricle contains only air and is the seat of intelligence.

The most influential, and arguably the greatest, ancient writer on natural philosophy was Aristotle (384–322 BC), an indefatigable dissector and theorist of animal form. He regarded the heart as the prime organ of the body: the seat of intelligence, the first part to develop, the last to die. The brain, he believed, served only to cool the heart. Although the four-chambered view held by the Hippocratic writer is similar to our own, nature does not dictate we divide the heart in this way. It is perfectly possible to “see” the heart differently. Indeed Aristotle did so, declaring it had but three chambers.

In the Hellenistic period, Alexandria became the great centre of ancient scholarship. Here human dissection was carried out by the physician Herophilus of Chalcedon (330–260 BC) and anatomist Erasistratus of Ceos (fl. c.250 BC), who first claimed that medicine should be based on knowledge gained by observation of the hidden structure of the human frame. Nothing like this appears in the Hippocratic texts. Herophilus championed the brain as the supreme bodily organ. He distinguished between arteries and veins. Erasistratus made arteries and veins the center of his theorizing. Blood, he said, was carried in the veins. Air is taken in by the lungs and in the heart is changed to vital spirit (a kind of pneuma) and then sent through the arteries to the parts. In the brain, vital spirit is converted into another pneuma, animal spirit, which flows through the nerves.

Erasistratus synthesized many of the traditions already noted, but his importance lies in the fact that his works were built on by Galen of Pergamum (AD 129–c.216), a Greek physician who practiced in Rome. Galen was a superb anatomist, and his writings formed the basis of Western medicine until the seventeenth century. His firsthand knowledge of the body's interior was derived almost exclusively from animals, however; Roman law did not permit human dissection.

Galen believed that the human venous system originated in the liver, and that the arterial system was based in the heart. The connections between these systems were minimal. Galen explained that, after digestion, food was absorbed from the intestines and slowly made its way to the liver, being converted into blood en route. From the liver, blood passed into the veins and oozed outwards through the whole body, traveling upwards from the liver through the right auricle of the heart. A small amount flowed to the right ventricle where two things happened. First, a quantity of blood was expelled and flowed to the lung to provide nourishment. The blood went no further. Second, a tiny amount trickled through pores in the heart's septum to the left ventricle. Here it mixed with air coming from the lung to make animal spirit, which flowed into the arteries to be distributed to the body's organs. It was the source of their vitality. So, apart from an anatomical connection of the veins with the right side of the heart and a small amount of blood crossing the septum, what we would call arterial and venous systems had little to do with each other. No blood ever returned to the heart. Like the Egyptians, Galen seemed to think of the blood itself as food, not a nutrient-carrying agent. Like Nile water, it slowly oozed down irrigation channels disappearing into the soil that was the body'ssubstance.

Galen's books were written in Greek. With the fall of the Roman Empire they all but disappeared in the West. They were translated into Arabic and related languages in the great age of Islam, then translated from Arabic into Latin in the revival of European culture that began in the eleventh century. It could fairly be said that Galen knew more about anatomy and physiology of the heart and blood vessels than anybody in the West until the Renaissance.

Something, however, was new in the Middle Ages that was to have a profound effect on knowledge of the heart and blood vessels: Human dissection began to take place. This, coupled with the growth of naturalistic illustration and the discovery of Galen's original Greek texts, led to the gradual rejection of ancient authority. In sixteenth-century Italian universities there were many anatomists, but the greatest of these, contemporaries agreed, was Andreas Vesalius (1514–1564), a Flemish professor at Padua.

Vesalius's fame lay in three things. First he was a very skillful dissector of the human corpse. Second, his 1543 text On the Fabric of the Human Body was the most beautifully illustrated anatomical work that had ever been seen (or has ever been seen, many would say). Third, and most important, although Vesalius was a follower of Galen, he began to question some of Galen's findings, noting that they could be detected in animals but not humans. Significantly, he doubted the existence of the pores in the heart. Vesalius's work began the gradual rejection of Galenic anatomy from the mid-sixteenth century onwards.

The first serious challenge to Galen's authority and his account of the heart's action came from Realdus Columbus (1516–1559), Vesalius's assistant and successor at Padua. On the basis of dissection and animal experiment, Columbus claimed Galen had mistakenly correlated the heart's contraction (systole) with filling and expansion (diastole) with expelling blood. The reverse, he said, was the case (which we now know to be true). More significant, Columbus maintained that blood from the right ventricle passes through the lungs to the left side. The pulmonary vein was filled with blood, not air. By the end of the sixteenth century many of Galen's observations were being questioned, and anatomical structures that were apparently unknown to him were being described. Among these were the valves of the veins, described by yet another anatomist at Padua, Fabricius ab Aquapendente (1537–1619).

The valves were to appear as important evidence for a radically new idea of blood flow that appeared in a publication by English physician William Harvey (1578–1657). In 1628 Harvey published An Anatomical Dissertation of the Movement of the Heart and Blood in Animals. This book, which claimed that the blood circulated, that is, continually returned to the heart from the periphery, confronts the historian with a deep paradox. Long hailed as a revolutionary text, it is in fact, the work of a very conservative figure.

Harvey was conservative socially and intellectually during a period of profound upheaval in both realms—he lived through the scientific and Puritan revolutions. He was a fellow of the Royal College of Physicians, an institution deeply committed to preserving its elite privileges and defending Galenic medicine. He was also a royalist. More curious, he was an Aristotelian in the days when Aristotle's philosophy was being dethroned. The paradoxes go even deeper; for he studied Aristotle at Padua, where he also learned to revere experimental knowledge—the very thing that was to overthrow ancient authority. Nonetheless, in the same way that Galen must be seen as the anatomist who made Vesalius's work possible, so it was that Aristotle's philosophy inspired Harvey's studies.

Harvey repeatedly cites Aristotle as his master, and in a number of ways he made Harvey's discovery possible. First, Aristotle described the goal of philosophy as the search for final causes (the purpose for which things existed), and Harvey repeatedly said that what drove his enquiries was the search for the final causes of the heart's motion and the movement of the blood. Second, the heart, as already noted, was the supreme organ of the body for Aristotle. Third, Aristotle conceived of circular motion as perfect, and Harvey says the blood moves “in a circle.” Finally Aristotle himself was a tireless dissector of animals and repeatedly urged the observation of nature on his followers.

Aristotelian though he was, however, Harvey brought something modern to his studies: experiments on live animals, while Aristotle dissected the dead. Harvey experimented on hundreds of animals and lots of different species: frogs, snakes, snails, fish, lobsters, and so on. Harvey claims that experiments on cold-blooded animals (whose hearts beat far slower than those of mammals) proved the truth of Realdus Columbus's assertion: Galen had got systole and diastole back to front. Harvey also says that calculation of cardiac output after measuring the amount forced out in each beat in a dying animal shows that an enormous quantity of blood leaves the heart in, say, a minute. Where does it all go? The solution, he suggests, is that blood is ejected from the right ventricle, travels through the lung to the left side of the heart from where it is propelled into the arteries, then returns to the right side of the heart through the veins. He then describes a series of clamping experiments which, he says, prove his view to be correct. Finally, in an arresting picture, he illustrates what he conceives to be the use of the venous valves described by Fabricius. They prevent venous blood, which is flowing centrally, from returning to the periphery.

Harvey's work is an outstanding example of a banal truth in the history of science. Sudden, bold, radical claims about nature, whether of a theoretical sort or an assertion of fact based on experiment, are always greeted with caution if not outright rejection. What is an unproblematic truth for one generation was deeply troublesome to its predecessor (witness Copernicus's solar system, Lavoisier's oxygen, Darwin's evolution by natural selection). It took 30 years and a great deal of intellectual battling for Harvey's claim to be accounted a fact. Not only that, this happened because in those 30 years Galen and Aristotle were being kicked out of medicine in general: the mechanical philosophy, chemistry, and experiment were in. The men who supported Harvey's view had no time for his Aristotelian gibberish as they saw it. Harvey's view of the motion of the blood and his experimental means of proving it were part of their new dynamic account of nature and how she (nature was almost always she) should be unveiled (a common metaphor at the time).

In actuality, even when Harvey's claim was accepted, and had indeed become a totem of the new science, medical practice scarcely changed a jot. Harvey and his publications, however, did inspire a great deal of interesting physiological and chemical study, especially in England, notably on the nature of air and the function of the lungs. Atmospheric pressure was a central debating point in the new science, and the study of air, with the newly invented air pump, was an important focus of experimental investigation. In Oxford (where Harvey retired during the English Civil War [1642–1651]) and London (at the newly established Royal Society) a circle using this device developed around the Robert Boyle (1627–1691), an enthusiast for mechanical and chemical study. Boyle and Robert Hooke (1635–1703), Curator of Experiments at the Royal Society, placed animals in an air pump and observed their behavior under varying air pressures. Hooke and British physician Richard Lower (1631–1691) conducted experiments on dogs, exposing their lungs and artificially ventilating them with bellows. Lower, a passionate supporter of the idea of the circulation, reported that blood changed color from dark blue to bright red as it passed through the lungs; it did not do so in the heart as was previously held to be the case. A younger collaborator of Lower, chemist John Mayow (1640–1679), on the basis of experiments and chemical theory, postulated that there were “elastic” particles in the air that were necessary to support combustion and life and that were taken into the blood during respiration. Curiously, as noted, at the moment in which the nature of the cardiovascular system was being paraded as one of the great discoveries of the new science, its role in understanding the workings of the body and mind was being taken over by the brain and spinal cord. It was not until the early nineteenth century that investigation of the heart was once again to figure as a totem of medical progress. This time, however, the developments were in France and in clinical medicine. Paris, in the years following the French Revolution of 1789, saw profound changes in the practice of medicine.

Until that time medical theory had largely been constructed to serve an elite clientele treated by an equally elite body of medical practitioners. It was individual-oriented and based on general disturbances of the patient's body fluids (humors). In the massive, overcrowded hospitals of Paris, French physicians (and especially surgeons) created a medicine that would serve their armies in the European wars. This medicine was based on the idea that morbid change in the body was local, confined to organs.

In the old medicine, doctors never examined their aristocratic patients. In the new one, the naked bodies of the poor were fair game: touched, looked at, and listened to. It was in this context that French physician René Théophile-Hyacinthe Laennec (1781–1826) introduced the stethoscope in 1819. By this time dissection had produced a very detailed knowledge of the heart's anatomy, and post-mortems had resulted in many descriptions of diseases of the valves, but how the normal or sick heart behaved in life was still in question. Gradually, over the nineteenth century, heart sounds were correlated with the opening and closing of the valves in health and disease. It is hard to imagine today how common valve disease once was. It is a sequel to the once-prevalent rheumatic fever, and many famous physicians of the past made their reputations in the diagnosis of valve disorders.

The major transformations in the understandings of the cardiovascular system in the nineteenth and early twentieth century, however, came mainly from the new discipline of physiology, which was based largely on animal experiments. One of the things that made the discipline so powerful was a new tool: the graphic method, a technology based on a device called the kymograph, first constructed in 1846 by a German physiologist, Karl Ludwig (1816–1895). The kymograph is a rotating drum covered with smoked paper (heavily impregnated with carbon granules). When an animal movement, such as the heart beat, or a physical parameter, for instance the pressure of the blood in an artery, is transmitted by some means to a pointer, this records the function as a permanent tracing on the paper. In the nineteenth century the graphic method usually required experimenting on an animal and connecting an organ directly to the kymograph by rubber tubing or, in the case of a muscle or nerve, by wires, since the signals are electrical. But the principle of the method does not require invasion of the body nor does the recorder have to be smoked paper.

At the end of the nineteenth century researchers began to investigate means to record human functions noninvasively. The first results were the sphygmomanometer for recording blood pressure by applying an inflatable cuff to the arm. Far more complex was the electrocardiograph, introduced into medicine by the Dutch doctor and physiologist Willem Einthoven (1860–1927) in 1903, after the electrical activity of the hearts of experimental animals had already been recorded. The electrocardiograph allowed the human heart's electrical pathways to be mapped harmlessly, although this work was carried out in conjunction with animal experiments.

In the twentieth century the cathode ray tube replaced the kymograph, as transducers made it possible to convert physical signals (blood pressure for example) into electrical ones. Today, data logging has made computer printouts one of the commonest forms of graphic recording. When a patient enters a modern cardiology department and undergoes a battery of tests, or is monitored in an intensive care unit after a heart attack, however technologically sophisticated the apparatus appears, many of the investigations performed and the recordings made are based on the principle behind Ludwig's kymograph.

One of the most valuable means for studying the heart was also devised in the early twentieth century. Cardiac catheterization coupled with graphic recoding is used to produce fundamental physiological information, and in the clinic it has become a cornerstone test of heart function. Like electrocardiography, it was first attempted when clinicians were trying to bring laboratory methods to the bedside. Unlike electrocardiography, it was invasive and dangerous. This procedure was first carried out by German physician Werner Forssmann (1904–1979) in 1929. He performed it on himself, feeding a catheter from his arm into the right atrium of his heart. He walked to the radiology department and had a chest x ray, which showed the catheter in place. He was fired from the hospital but won the 1956 Nobel Prize for physiology or medicine.

Cardiac catheterization, coupled with the injection of dyes—angiocardiography—became central to preoperative assessment for heart surgery, which began in the United States and Great Britain just after World War II (1939–1945). In those days nearly every heart operation was for rheumatic valve disease; today coronary artery bypass surgery for coronary artery disease is one of the commonest reasons.

The development of cardiac surgery was propelled by the invention of the heart-lung machine in the United States in the 1950s. Like neuroscience and neurology, cardiovascular physiology and clinical cardiology are sciences and medical specialties now practiced on an industrial scale, with scientists specializing their research in areas like capillary circulation, the neural control of blood pressure, the splanchnic (visceral) circulation, the cerebral circulation, and so on.

Cardiovascular Components: The Heart

Located in the thoracic cavity, the heart is a four-chambered muscular organ that serves as the primary pump or driving force within the circulatory system. The heart contains a special form of muscle, appropriately named cardiac muscle, that has intrinsic contractility (i.e., is able to beat on its own, without nervous system control).

The heart is divided into chambers: two upper (superior) atrial chambers and two thicker-walled, heavily muscular inferior ventricular chambers. The right and left sides of the heart are divided by a thick septum. The right side of the heart is on the same side of the heart as is the right arm of the patient. The atrial and ventricular chambers on each side of the septum constitute separate collection and pumping systems for the pulmonary (right side) and systemic circulation (left side). The coronary sulcus or grove separates the atria from the ventricles. The left and right side atrial and ventricular chambers each are separated by a series of one way valves that, when properly functioning, allow blood to move in one direction but prohibit it from regurgitating (flowing back through the valve).

Deoxygenated blood—returned to the heart from the systemic circulatory venous system—enters the right atrium of the heart through the superior and inferior vena cava. Auricles lie on each atrium and are most visible when the atria are drained and deflated. The auricles (so named because they resembled ear flaps) allow for greater atrial expansion. Pectinate muscles on the auricles assist with atrial contraction. Small contractions within the right atrium, and pressure differences caused by evacuation of blood in the lower (inferior) right ventricle, cause this deoxygenated blood to move through the tricuspid valve during diastole (the portion of the heart's contractile cycle between contractions, and a period of lower pressure as compared to systole) into the right ventricle. When the heart contracts, a sweeping wave of pressure forces open the pulmonic semilunar valve, which allows blood to rush from the right ventricle into the pulmonary artery, where it travels to the lungs for oxygenation and other gaseous exchanges.

Freshly oxygenated blood returns to the heart from the pulmonary circulation through the pulmonary vein, which then empties into the left atrium. During diastole, the oxygenated blood moves from the left atrium into the left ventricle through the mitral valve. During systolic contraction, the oxygenated blood is pumped under high pressure through the semilunar aortic valve into the aorta and thus enters the systemic circulatory system.

As the volume and pressure rise during the filling of the right and left ventricles, the increased pressure snaps shut the flaps of the atrioventricular valves (tricuspid and mitral valves) anchored by fibrous connection to the left and right ventricles. The pressure in the ventricles seals the valves, and, as the pressure increases during systole, the valves' seals become further compressed. A prolapse in one of the valves (a pushing through of one of the cusps) leads to blood flow back through the valve. The cusps are held against prolapse by the chordae tendineae, thin cords that attach the cusps to papillary muscles.

The heart and great vessels attached to it are encased within a multi-layered pericardium. The outer layer is fibrous and covers a double-membraned inner sac-like structure termed the pericardial cavity, which is filled with pericardial fluid. The pericardial fluid acts to reduce friction between the heart, the pericardial membranes, and the thoracic wall as the heart contracts and expands during the cardiac cycle.

The heart muscle is composed of three distinct layers. The outermost layer, the outer epicardium, is separated from the inner endocardium by the middle pericardium. The outer epicardium is continuous and in some places the same as the visceral pericardium. The epicardium protects the heart and is invested with capillaries, nerves, and lymph vessels. The middle myocardium is a think layer of cardiac muscle. The innermost endocardium contains connective tissue and Purkinje fibers. The endocardium is continuous with the lining of the great vessels attached to the heart, and it lines all valve and cardiac inner surfaces.

Heart muscle does not directly take up oxygen from the blood it pumps. A specialized set of vessels (e.g., the left and right coronary arteries and their branches) supply oxygenated blood to the heart muscle and constitute the coronary circulation. A heart attack occurs whenever blood flow is occluded (blocked).

Various intrinsic, neural, and hormonal factors act to influence the rhythm control and impulse conduction within the heart. Although cardiac muscle can contract on its own, the sino-atrial node on the right atrium acts to send out signals that regulate and coordinate contractions. The sino-atrial node and atrioventricular nodes act as a pacemaker for the heart. Cardiac arrhythmias result from abnormalities in the rhythm or rate of heat contractions (heart beats).

The fossa ovalis is a remnant of the embryonic foramen ovale, which allows blood to flow between the left and right atria in the developing fetus.

Modern Understandings: The Developing Heart and Changes at Birth

The developing fetal heart accounts for a large percentage of the volume of the early thorax. About 20 days after fertilization, the heart develops from the fusion of paired endothelial tubes into a single tube. Heart growth subsequently involves the growth, expansion, and partitioning of this tube into four chambers separated by thickened septa of cardiac muscle and valves. Atrial development is initially more advanced than ventricular development. The left and right atria develop while the primitive ventricle remains a single chamber. As atrial separation nears completion, the left and right ventricles begin to form, then continue until the heart consists of its fully developed four-chambered structure.

Although the majority of the heart develops from mesoderm (splanchnic mesoderm) near the neural plate and sides of the embryonic disk, there are also contributions from neural crest cells that help form the valves.

Three systems initially return venous blood to the primitive heart. Regardless of the source, this venous blood returns to sinus venosus. Vitelline veins return blood from the yolk sac; umbilical veins return oxygenated blood from the placenta. The left umbilical vein enlarges and passes through the embryonic liver before continuing on to become the inferior vena cava, which fuses with a common chambered sinus venosus and the right atrium of the heart. Especially early in development, venous return also comes via the cardinal system. The anterior cardinals drain venous blood from the developing head region. Subcardinal veins return venous blood from the developing renal and urogenital system, while supracardinals drain the developing body wall.

The anterior veins empty into the common cardinals that terminate in the sinus venosus.

Movement of blood through the early embryonic vascular system begins as soon as the primitive heart tubes form and fuse. Contractions of the primitive heart begin early in development, as early as the initial fusion of the endothelial channels that fuse to form the heart.

The heart and the atrial tube that form the aorta develop by the compartmentalization of the primitive cardiac tube. Six separate septae are responsible for the portioning of the heart and the development of the walls of the atria and ventricles. A septum primum divides the primitive atria into left and right chambers. The septum secundum (second septum) grows along the same course of the primary septum to add thickness and strength to the partition. There are two holes in these septae through which blood passes, the foramen secundum and the foramen ovale. Specialized endocardinal tissue develops into the atrioventricular septum that separates the atrium and ventricles. The mitral and tricuspid valves also develop from the atrioventricular septum.

As development proceeds, the interventricular septum becomes large and muscular to separate the ventricles and provide strength to these high-pressure contractile chambers. The interventricular septum also has a membranous portion.

Initially, there is only a common truncus arteriosus as a channel for ventricular output. The truncus eventually separates into the pulmonary trunk and the ascending aorta.

Blood oxygenated in the placenta returns to the heart via the inferior vena cave into the right atrium. A valve-like flap in the wall at the juncture of the inferior vena cava and the right atrium directs the majority of the flow of oxygenated blood through the foramen ovale, then allows blood to flow from the right atrium to the left. Although there is some mixing with blood from the superior vena cava, the directed flow of oxygenated blood across the right atrium caused by the valve of the inferior vena cava means that deoxygenated fetal blood returning via the superior vena cava still ends up moving into the right ventricle.

While in the uterus, the lungs are non-functional. Accordingly, another shunt, the ductus arteriosis (also spelled ductus arteriosus) provides a diversionary channel that allows fetal blood to cross between the pulmonary artery and aorta and thus largely bypass the rudimentary pulmonary system.

Because only a small amount of blood returns from the pulmonary circulation, almost all of the blood in the fetal left atrium comes through the foramen ovale. The relatively oxygen-rich blood then passes through the mitral value into the left ventricle. Contractions of the heart, whether in the single primitive ventricle or from the more developed left ventricle, then pump this oxygenated blood into the fetal systemic arterial system.

In response to inflation of the lungs and pressure changes within the pulmonary system, both the foramen ovale and the ductus arteriosis normally close at birth to establish the normal adult circulatory pattern whereby blood flows into the right atrium, though the tricuspid valve into the right ventricle. The right atrium pumps blood into the pulmonary artery and pulmonary circulation for oxygenation in the lungs. Oxygenated blood returns to the left atrium by pulmonary veins. After collecting in the left atrium, blood flows through the mitral value into the left atrium where it is then pumped into the systemic circulation via the ascending aorta.

Physiological Control of the Cardiovascular System and Cardiac Cycle

In the modern era it is understood that the rhythmic control of the cardiac cycle and its accompanying heartbeat rely on the regulation of impulses generated and conducted within the heart. Regulation of the cardiac cycle is also achieved via the autonomic nervous system. The sympathetic and parasympathetic divisions of the autonomic system regulate heart rhythm by affecting the same intrinsic impulse conducting mechanisms that lie within the heart in opposing ways.

Cardiac muscle is self-contractile because it is capable of generating a spontaneous electrochemical signal as it contracts. This signal induces surrounding cardiac muscle tissue to contract, and a wave-like contraction of the heart can result from the initial contraction of a few localized cardiac cells.

The cardiac cycle describes the normal rhythmic series of cardiac muscular contractions. The cardiac cycle can be subdivided into the systolic and diastolic phases. Systole occurs when the ventricles of the heart contract, and diastole occurs between ventricular contractions when the right and left ventricles relax and fill. The sino-atrial node (S-A node) and atrioventricular node (AV node) of the heart act as pacemakers of the cardiac cycle.

The contractile systolic phase begins with a localized contraction of specialized cardiac muscle fibers within the sino-atrial node. The S-A node is composed of nodal tissue that contains a mixture of muscle and neural cell properties. The contraction of these fibers generates an electrical signal that then propagates throughout the surrounding cardiac muscle tissue. In a contractile wave originating at the S-A node, the right atrium muscle contracts (forcing blood into the right ventricle) and then the left atrium contracts (forcing blood into the left ventricle).

Intrinsic regulation is achieved by delaying the contractile signal at the atrioventricular node. This delay also allows the complete contraction of the atria so that the ventricles receive the minimum amount of blood to make their own contractions efficient. A specialized type of neuro-muscular cells, named Purkinje cells, form a system of fibers that covers the heart and conveys the contractile signal from S-A node (which is also a part of the Purkinje system or subendocardial plexus). Because the Purkinje fibers are slower in passing electrical signals (action potentials) than are neural fibers, the delay allows the atria to finish their contractions prior to ventricular contractions. The signal delay by the AV node lasts about a tenth (0.1) of a second.

The contractile signal then continues to spread across the ventricles via the Purkinje system. The signal travels away from the AV node via the bundle of His before it divides into left and right bundle branches that travel down their respective ventricles.

Extrinsic control of the heart rate and rhythm is achieved via autonomic nervous system (ANS) impulses (regulated by the medulla oblongata) and specific hormones that alter the contractile and or conductive properties of heart muscle. ANS sympathetic stimulation via the cervical sympathetic chain ganglia acts to increase heart rate and increase the force of atrial and ventricular contractions. In contrast, parasympathetic stimulation via the vagal nerve slows the heart rate and decreases the vigor of atrial and ventricular contractions. Sympathetic stimulation also increases the conduction velocity of cardiac muscle fibers. Parasympathetic stimulation decreases conduction velocity.

The regulation in impulse conduction results from the fact that parasympathetic fibers utilize acetylcholine, a neurotransmitter hormone that alters the transmission of an action potential by altering membrane permeability to specific ions, e.g., potassium ions (K +). In contrast, sympathetic postganglionic neurons secrete the neurotransmitter norepinephrine, which alters membrane permeability to sodium (Na +) and calcium ions (Ca2+).

The ion permeability changes result in parasympathetic induced hypopolarization and sympathetic induced hyperpolarization.

Additional hormonal control is achieved principally by the adrenal glands (specifically the adrenal medulla), which release both epinephrine and norepinephrine into the blood when stimulated by the sympathetic nervous system. As part of the fight or flight reflex, these hormones increase heart rate and the volume of blood ejected into the cardiovascular system during the cardiac cycle.

The electrical events associated with the cardiac cycle are measured with an electrocardiogram (EKG). Disruptions in the impulse conduction system of the heart result in arrhythmias.

Cardiovascular Components: The Pulmonary System

The pulmonary circulatory system delivers deoxygenated blood from the right ventricle of the heart to the lungs and returns oxygenated blood from the lungs to the left atrium of the heart. At its most minute level, the alveolar capillary bed, the pulmonary circulatory system is the principle point of gas exchange between blood and air that moves in and out of the lungs during respiration.

The pulmonary vascular circuit begins with pulmonary arteries that branch from the pulmonary trunk, leaving the right ventricle of the heart. Venous blood collected from the systemic and coronary circulation collects in the left atrium, and, during the diastolic portion of the cardiac cycle, flows into the right ventricle. As the heart contracts during systole, the semilunar valves that comprise the pulmonary valve separating the pulmonary trunk from the right ventricle open to allow blood to be rapidly pumped into the pulmonary system. As the pressure drops, the pulmonary valve closes to prevent backflow into the heart.

At about the level of the fifth or sixth thoracic vertebrae, the main pulmonary arterial trunk divides (bifurcates) into the left and right pulmonary arteries that travel to the corresponding lung. Upon entering the lungs, the pulmonary arteries rapidly divide to form a complex branch of pulmonary arterioles and ultimately a fine capillary bed that surrounds and supplies the alveoli.

Within the alveoli, gas exchange takes place. The principle exchanges involved allow the uptake of oxygen by hemoglobin-carrying red blood cells and the discharge of carbon dioxide—a metabolic waste product—into the respiratory air. During respiration, there is a subsequent gaseous exchange between the air within the respiratory system and air in the environment, which allows a continual supply of oxygen to cells, while at the same time venting toxic carbon dioxide.

Lung capillaries ultimately fuse into venules and pulmonary veins. Pulmonary veins located in the portioned lobes of the lung usually unite to form a single efferent outgoing (efferent) vein from each lobe of the lung. Eventually, the pulmonary veins from the middle and upper lobes of the right lung (along with three lobes) fuse (anastomose) to create a pair of right pulmonary veins—an inferior pulmonary vein and a superior pulmonary vein. Matched by paired superior and inferior left pulmonary veins, the four pulmonary veins travel separately to individually enter the left atrium of the heart.

Cardiovascular Components: Arteries

Arteries are blood vessels that transport oxygenated blood from the heart to other organs and systems throughout the body.

A typical artery contains an elastic arterial wall that can be divided into three principal layers, although the absolute and relative thickness of each layer varies with the type or diameter of artery. The outer layer is termed the tunica adventia, the middle layer is termed the tunica media, and an inner layer is the tunica intima. These layers surround a lumen, or opening, that varies in size with the particular artery, through which blood passes.

Arteries of varying size comprise a greater arterial blood system that includes, in descending diameter, the aorta, major arteries, smaller arteries, arterioles, metaarterioles, and capillaries. It is only at the level of the capillary that branches of arteries become thin enough to permit gas and nutrient exchange. As the arterial system progresses toward the smaller diameter capillaries, there is a general and corresponding increase in the number of branches and total area of lumen available for blood flow. As a result, the rate of flow slows as blood approaches the capillary beds. This slowing is an important feature that enables efficient exchange of gases—especially oxygen.

In larger arteries, the outer, middle, and inner endothelial and muscle layers are supported by elastic fibers and serve to channel the high pressure and high rate of blood flow. A difference in the orientation of cells within the layers (e.g., the outer endothelial cells are oriented longitudinally, while the middle layer smooth muscle cells run in a circumference around the lumen) also contributes both strength and elasticity to arterial structure.

The aorta and major arteries are highly elastic and contain walls with high amounts of elastin. During heart systole (contraction of the heart ventricles), the arterial walls expand to accommodate the increased blood flow. Correspondingly, the vessels contract during diastole, and this contraction also serves to drive blood through the arterial system.

In the systemic arterial network that supplies oxygenated blood to the body, aortas are regions of the large-lumened singular artery arising from the left ventricle of the heart. Starting with the ascending aorta that arises from the left ventricle, the aortas form the main trunk of the systemic arterial system. Before the ascending aorta curves into the aortic arch, right and left coronary arteries branch off to supply the heart with oxygenated blood. Before the aortic arch turns to continue downward (inferiorly) as the descending aorta, it gives rise to a number of important arteries. Branching either directly off of—or from a trunk communicating with the aortic arch—is a brachiocephalic trunk that branches into the right subclavian and right common carotid artery that supply oxygenated blood to the right sight of the head and neck, as well as portions of the right arm.

The aortic arch also gives rise to the left common carotid artery that, along with the right common carotid artery, branches into the external and internal carotid arteries to supply oxygenated blood to the head, neck, brain.

The left subclavian artery branches from the aortic arch and—with the right subclavian arising from the brachiocephalic trunk—supplies blood to neck, chest (thoracic wall), central nervous system, and arms via axillary, brachial, and vertebral arteries.

In the chest (thoracic region), the continuation of the aortic arch—the descending aorta—is specifically referred to as the thoracic aorta. The thoracic aorta is the trunk of arterial blood supply to the thoracic region. Parietal branches of arteries derived from the thoracic aorta supply blood to the walls of thoracic organs and cavities. Visceral arterial branches supply blood to interior thoracic organs.

As the thoracic aorta passes through an opening in the diaphragm (aortic hiatus) to become the abdominal aorta, parietal and visceral branches supply oxygenated blood to abdominal organs and structures. The abdominal aorta ultimately branches into left and right common iliac arteries that then branch into internal and external iliac arteries, supplying oxygenated blood to the organs and tissues of the lower abdomen, pelvis, and legs.

In the pulmonary arterial system, the pulmonary trunk arises from the right ventricle of the heart to divide into left and right pulmonary arteries that supply deoxygenated (unaerated) blood to the lungs (by a number of pulmonary branch arteries) to different regions of lung tissue.

Cardiovascular Components: Veins and the Venous System

Veins are vessels designed to collect and return blood, including deoxygenated hemoglobin, from tissues to the heart. Veins and the venous vascular system can be divided into three separate systems, depending on anatomical relationships and function. Initially, veins can be divided into systemic and pulmonary systems. The veins that drain the heart, comprising the coronary venous system, may be described as an independent venous system, or be considered a subset of the systemic vascular system. The systemic veins transport venous blood—deoxygenated when compared with arterial blood—from the body to the heart. The pulmonary veins return freshly oxygenated blood from the lungs to the heart so that it may be pumped into the systemic arterial system.

Veins can also be described by their anatomical position. Deep veins run in organs or connective tissues that support organs, muscle, or bone. Superficial veins are those that drain the outer skin and fascia.

In contrast to arteries, veins often run a more convoluted course, with frequent branching and fusions with other veins (anastomoses) that make the tracing of the venous system less straightforward than mapping the arterial system. In addition, there are reservoirs or pools (sinus) that collect venous return from multiple sources. Many veins contain valves that assure a unidirectional (one way) flow of venous blood toward the heart.

The systemic venous system can be roughly divided into groups depending on the region they drain, and the vessel through which they return blood to the heart.

The first systemic venous group consists of veins that drain the head, neck, thorax, and upper limbs. These veins ultimately return blood to the heart through the superior vena cava.

Veins that drain the abdomen, pelvis, and lower limbs return blood through the inferior vena cava. Both the superior and the inferior vena cava return deoxygenated blood to the right atrium of the heart. The coronary sinus collects blood from a number of cardiac veins before returning blood to the right atrium near the point where the inferior vena cava enters the right atrium.

The pulmonary veins return blood oxygenated in the lungs to the left atrium. There are four major pulmonary veins, each lung being drained by a pair of pulmonary veins. Akin to the drainage of a land basin from streams into a larger river system, smaller venules arise from the lung alveolar capillary bed, then the venules fuse to form single veins that separately drain isolated lobes of the lung. The veins from the upper and middle lobes of the three-lobed right lung fuse to create a pair of veins, a superior and inferior pulmonary vein, that separately transport blood to the left atrium.

At a microscopic or histological level, veins have thinner walls than do arteries. They are more elastic and capable of a wider range of lower pressure volume transformations. The elasticity is a result of the fact that veins have less subendothelial connective tissue in their vascular walls. In addition, the tunica media and tunica adventitia are often indistinguishable layers, or are poorly developed when compared with arterial linings.

Venules drain capillaries and capillary beds. The venules ultimately fuse (coalesce) into veins that, as they increase in size, also increase in organization and differentiation of their vascular walls. In general, the larger the vein, the more likely it is to be invested or surrounded with smooth muscle tissue. Valves are generally absent from the largest veins and the pulmonary veins.

Veins also serve in fluid uptake and can receive lymph fluid from lymphatic vessels. The major lymphatic duct, for example, drains into the fused vein formed from the fusion of the subclavian and left internal jugular vein.

Blood Pressure The pressure exerted by the blood inside the arteries is termed blood pressure. Several factors are accountable for its levels: the heart rate, volume and viscosity of blood pumped per beat, force of the heartbeat, elasticity and resistance of vessel walls, and the resistance of the capillary bed (i.e., the network of capillary vessels that permeates tissues). Capillaries are minute blood vessels connecting the arterioles to either veins or lymphatic vessels. Other factors with a role in blood pressure levels are the balance between potassium and sodium levels and the action of pressure-controlling hormones.

As the heart contracts and relaxes in a pulsatile rhythm, systole, i.e., the contraction of the left ventricle that ejects blood into the aorta, distending its walls, exerts a pressure level of 120 mm Hg, whereas diastole (i.e., momentary heart relaxation) has a level pressure of 80 mm Hg, due to the elastic recoil of arterial walls. As the blood flows along the systemic vessels, the pressure gradually falls to almost 0 mm Hg when the blood reaches the end of the cava vein, as is emptied into the right coronary atrium. In the capillary bed, the average pressure is about 17 mm Hg, varying from about 35 mm Hg in the arteriolar ends to 10 mm Hg near the venous ends.

Blood pressure measurements are usually done in millimeters of mercury (mm Hg) with a mercury manometer used for this purpose. The mercury manometer measures the force of blood against any unit area of the vessel wall. However, it is only useful for measuring stable pressures. When it is necessary to monitor unstable blood pressure, oscillating rapidly, electronic pressure transducers are utilized. These convert pressure into electrical signals, which are recorded at high speed.

Modern Cultural Connections

By the mid-twentieth century, surgeons began successfully transplanting hearts and other human organs in order to save the lives of patients whose organs were failing from disease. These procedures were at first sensational, sparking debate among the medical community and the general public. In order for a transplantation to take place, a donor was required, and this raised both new challenges to immunologists to thwart rejected tissue as well as ethical questions regarding what defined death.

Within fifty years, kidney, liver, and heart transplants moved from experimentation to mainstream medical treatment for patients with failing organs and few other options to regain health.

Although the transplant barrier was actually broken in 1954, with the first successful human kidney transplant, heart transplantation captured world-wide attention in 1967, when South African surgeon Christiaan Barnard (1922–2001) performed the world's first human transplant. Barnard and his team of physicians and nurses at Groote Schuur Hospital in Cape Town, South Africa, removed the heart of a 55-year-old man and replaced it with the healthy heart of a 25-year-old woman who had died earlier of injuries sustained in an automobile accident. The patient survived 18 days after the transplant, dying from pneumonia as a result of an immune system suppressed to prevent rejection of the donor heart.

IN CONTEXT: ARTIFICIAL HEART ADVANCES

In 1966, Michael DeBakey (1908–) implanted a partial artificial heart. In 1969, Denton Cooley (1920–) and his surgical team at the Texas Heart Institute implanted a pneumatically driven Dacron-lined plastic heart, designed by Argentine-born Domingo Liotta (1924–), as a temporary measure to keep a patient alive until a heart transplant could be performed.

From 1982 to 1985, American cardiothoracic surgeon William DeVries (1943–) carried out a series of implants of a device called the Jarvik-7 artificial heart (named after one of its designers, Dr. Robert Jarvik [1946–]). The first heart was implanted in Barney Clark, who lived for 112 days before dying of complications caused by the device. Mechanical and other problems ultimately stopped the use of the Jarvik-7. In 2000, the modified version of the Jarvik heart (the Jarvik 2000) was implanted. This was the first completely artificial heart to be installed.

In July of 2001, surgeons at Jewish Hospital in Louisville, Kentucky, implanted an artificial heart (the AbioCor® heart) in Robert Tools, a 59-year-old man whose own heart was damaged and failing. The AbioCor® heart was the first self-contained artificial heart to be implanted in a human. It was also the smallest artificial heart yet devised, being about the size of a softball. Tools suffered a serious stroke and died in November 2001. Nonetheless, his progress after the installation of the artificial heart was encouraging. In 2006 the AbioCor® heart was approved by the U.S. Food and Drug Administration (FDA) for a limited market. As of early 2008, the AbioCor® II was in development. It is planned to be a smaller device, allowing a wider range of patients, with a longer duration than the first AbioCor® heart.

In 2004, the CardioWest® Temporary Total Artificial Heart (TAH) was developed by researchers from the University of Arizona. The TAH was the first implantable artificial heart to be approved by the FDA. This device marked the first time a mechanical device became available on a broad basis to replace temporarily a weakened, dying heart. The device replaces the lower half of the heart for desperately sick patients as they wait in hospitals for heart transplants.

Barnard performed his second heart transplant in 1968. The patient achieved notoriety as a symbol of hope for victims of heart disease and spurred the transplantation process, surviving 563 days after the operation.

In 1968, American surgeons started heart transplant operations in the United States. Surgeons continued to develop and refine surgical techniques for the burgeoning field of heart transplantation. It was not until the early 1980s, however, with the advent of cyclosporin and other next-generation of anti-rejection drugs, that the heart transplant procedure became widely accepted.

By the turn of the century, heart transplantation had evolved from an experimental operation to an established treatment for advanced heart disease, with over 2,000 performed yearly in the United States.

As organ transplant procedures increased and became standard treatment for otherwise fatal illnesses, both the medical community and the public at large considered ethical issues related to by organ donation. The National Transplantation Act, passed by Congress in 1984, mandated a centralized system for sharing available organs, along with a scientific register to collect and report transplant data. The Act also made illegal the sale or purchase of organs.

A national system was established to match donors and recipients, and is managed by the United Network for Organ Sharing (UNOS). UNOS members work with all transplant centers in the United States to ensure that the limited supply of organs is distributed fairly to patients in need regardless of age, sex, race, lifestyle, or financial or social status. Through the UNOS Organ Center, organ donors are matched to waiting recipients every day of the year, around the clock. Organ sharing is based upon scientific criteria including the recipient's acuity (urgency state) of the disease process, compatibility

of body size and blood chemistries, as well as length of time on the waiting list. At the close of the twentieth century, new laws designed to remove any geographical bias in organ allocation were under consideration. The Scientific Registry maintained by UNOS contains data on every solid organ transplant since 1987. The registry is one of the most comprehensive data analysis systems targeting a single therapy in the world. Patient confidentiality is maintained with a number system, and scientists are able to quickly exchange information vital to the progress of transplantation.

With increases in the number of transplants performed, UNOS and public health organizations have attempted to raise public awareness of the importance of organ donation. States included organ donor status on citizens' drivers licenses, and a universal donor card was widely publicized. By the 1990s, most states had passed legislation requiring medical personnel to approach all potential donor patients, or, if the patient is unable, their families for a donation decision. The criteria for brain death was clarified in 1981 by a presidential commission on medical ethics to allay public concern regarding time of death and organ recovery. All major religions in the United States voiced opinions encouraging personal choice and organ donation. In spite of these efforts, the demand for donated organs far outnumbered the supply. By the turn of the century, over 67,000 Americans were on the UNOS national patient waiting list. Transplantation procedures were quickly growing. In addition to heart transplants, organs and tissues were needed for additional types of transplants added to the medical arsenal against disease: lung, pancreas, bone marrow, small intestine, cornea—all were considered an acceptable part of medical treatment.

In efforts to potentially ease the shortage, some scientists have experimented with xenotransplantation, or transplanting an organ of another species into a human. A celebrated xenotransplantation case was that of “Baby Faye,” into whom a baboon heart was transplanted in 1984 at the Loma Linda Medical Center in California. Baby Faye's baboon heart functioned for 20 days. Xenotransplantation remains experimental, as are artificial mechanical organs—scientists continue to study both of these measures as potential “bridges” to serve a critically ill patient until a donor organ can be located.

CPR Revives Health Consciousness

Modern cardiopulmonary resuscitation (CPR) was created in 1960 when the three techniques, mouth-to-mouth resuscitation, the head tilt, and chest compressions, were united. The American Heart Association formally endorsed CPR in 1963, and founded a committee dedicated to its study. By 1966, the National Academy of Sciences reported recommendations to standardize the performance and training of CPR, allowing CPR to become the standard first-line treatment for hospitalized victims of cardiac arrest. The same year, deaths from cardiac arrest in hospitals began to decline.

Heart disease was the leading cause of death in America by 1960, due in part to Americans' changing lifestyles. Nutritional habits changed as processed, convenience, and fast foods contributed a higher fat and salt content in the diet. Simultaneously, increased urbanization led to a more sedentary lifestyle. As a result, more Americans suffered myocardial infarctions (heart attacks), and most of these occurred outside the hospital. Coupled with trauma suffered from automobile accidents on increasingly crowded roadways, the need for CPR outside the hospital was quickly realized.

In 1966, a national standard for training ambulance personnel was created, which included CPR, and led to the rise of emergency medical technicians (EMTs). EMTs could provide CPR at the scene and en route to the hospital. American physician and engineer Eugene Nagel devised a method to deliver advanced emergency care in the field while allowing the physician to monitor it from the hospital in 1968. Nagel developed the first portable telemetry machine (used to determine the heart rhythm and relay it to a physician at the hospital) and trained rescue firemen in Miami in its use. In 1969, Nagel's “paramedics” performed CPR on a 60-year-old man, then used the portable defibrillator to shock the fibrillating man's heart back to a normal rhythm. Today, the paramedic system of providing advanced emergency medical care is in use throughout the developed world.

Seattle physician Leonard Cobb, after analyzing Seattle emergency response data, found the sooner CPR was started, the more favorable were the chances for survival. Cobb embarked upon an ambitious program to train 100,000 Seattle area citizens in a modified version of CPR, reasoning that bystanders can become the first responders to a medical emergency. Although the medical community was skeptical at first, by 1973 the American Heart Association defined the standards for basic CPR, and by 1974, the American Red Cross conducted popular CPR classes for the public throughout the Unites States. Community based CPR continues today as one of America's most far-reaching public health initiatives of the twentieth century.

IN CONTEXT: DENTON A. COOLEY (1920–), SURGEON AND TECHNOLOGY PIONEER

Denton A. Cooley (1920–) is a pioneer in the evolution of modern cardiovascular (heart-circulatory system) surgery. Cooley was the first surgeon to implant an entire artificial heart in a human awaiting heart transplantation. He also performed what many argue was the first “successful” human heart transplant in the United States. Cooley is founder of the world-renowned Texas Heart Institute, where more open-heart surgeries and heart diagnostic procedures have been performed than in any other facility in the world. Cooley's high-profile surgeries sparked both praise and ethical debate in the medical community, and broadened the boundaries of surgical practice.

Cooley earned his M.D. degree from Johns Hopkins University in 1944. During his residency at Hopkins, Cooley assisted Alfred Blalock (1899–1964) and Helen B. Taussig (1898–1986) during what came to be known as the “blue-baby” operation. This involved an anastomosis, or surgical joining, of the left subclavian artery (which supplies blood to the neck and arms) and the left pulmonary artery to cure Tetralogy of Fallot. In this congenital (inborn) defect, there are four different malformations of the heart, and blood flow to the lung is thus impeded. The resulting lack of oxygenation of the blood gives a bluish tinge to the skin of affected babies. Fifteen-month-old Eileen Saxon was the first to undergo the blue-baby operation.

The operation, a milestone surgical procedure, was also made possible by essential technical contribution by African American laboratory researcher Vivien Thomas (1910–1985), who had been thwarted in his pursuit of a medical degree by poverty and the discriminatory polices of the time, but actually pioneered and taught surgical techniques to white surgeons.

Blalock's influence and pioneering work on the frontier of open-heart surgery inspired Cooley to specialize in the field. Cooley earned a reputation of simplifying complex surgical techniques while exercising speed and dexterity in the operating room.

In the early 1950s, Cooley began a collaboration with another cardiovascular surgeon, Michael E. DeBakey (1908–), which sparked major innovations in heart surgery throughout the 1950s. Together the two developed and perfected a heart-lung bypass machine that allowed immobilization of the heart during surgical repair. They also collaborated on surgical techniques to remove aneurysms (weakened areas of the arterial wall) from the aorta and to repair damaged heart valves.

In 1962, Cooley founded the Texas Heart Institute at St. Luke's Episcopal Hospital in Houston. Here Cooley and his colleagues encouraged and refined the development of artificial heart valves, dramatically reducing the mortality rate for valve transplant patients. Cooley's team was the first to successfully remove a pulmonary embolism (blood clot in the lungs), and the team continued to pioneer delicate procedures necessary to correct congenital heart defects in infants and children.

Cooley performed the first successful human heart transplant in the United States on May 3, 1968. His patient, a 47-year-old man, received the heart of a 15-year-old girl who had committed suicide. The donor's brain function had ceased, but her heart was still beating. Although the recipient lived for 204 days, and Cooley received praise from the medical community, an ethical debate ensued as Americans wrestled with the issue of determining the moment death occurs. In 1969, Cooley implanted the first artificial heart in a human. The mechanical heart served as a temporary bridge to a human donor heart, which became available approximately three days later. Although the patient died the day after the human heart was transplanted, this pioneering effort sparked intensive research world-wide in artificial heart support. Procedures and laws governing organ donation, organ harvesting, and allocation of organs for transplant continue as a subject of debate and revision today.

During the 1970s, Cooley turned his attention to the study of coronary artery disease, by then the leading cause of death in the United States. Cooley pioneered many techniques for the coronary bypass operation. Cooley and his associates performed approximately 100,000 open heart operations, more than any other group in the world.

In 1998, President William Clinton presented Cooley with the national Medal of Technology, the nation's highest honor for technical innovation.

Empowered with the new knowledge of CPR, Americans in the mid 1970s began a period of increased health consciousness. Aerobics, exercising in a manner to build cardio-vascular endurance and fitness, was embraced by many Americans who included aerobic exercise into their regular routine for the first time. Running, and particularly jogging, became popular aerobic pastimes. Health and exercise clubs increased in number, and athletic wear became a fashion trend. Americans became familiar with their individual “cholesterol counts” (fatty acids in the blood that are a risk factor for heart disease) and sought ways to improve them through a plethora of self-help publications on the market. The natural foods trend of the 1960s re-emerged, with an emphasis on heart-healthy foods. These trends continue to evolve today. The rate of American deaths from heart disease reached a plateau, then experienced a decline during the 1980s and onward. Further innovations in life-sustaining treatment of heart disease as well as lifestyle improvements are credited with the reduction.

The advent of CPR gave rise to ethical questions, both in the hospital and the community. Prior to 1950, cardiac arrest almost always resulted in death. With CPR and advanced life support, medical personnel have greater options and responsibility to deter or determine when death occurs. The precise incidence of CPR effectiveness is not known, and at times those who are saved by CPR require intensive long-term life support. For these reasons, many with advanced disease or age choose not to undergo CPR and advanced life support. Legislation was passed in most states urging medical personnel to honor a patient's “living will” expressing end-of-life choices. Legislation was also refined to protect and encourage bystanders to render first aid, including CPR if needed, to victims in emergency situations. Most hospitals have ethics committees to define standards of life-sustaining and end-of-life care, and assist medical personnel and their patients dealing with this issue.

See Also Biomedicine and Health: Dissection and Vivisection; Biomedicine and Health: Human Gross Anatomy; Biomedicine and Health: The Brain and Nervous System.

bibliography

Books

Fishman, Alfred P., and Dickinson W. Richards, eds. Circulation of the Blood: Men and Ideas. New York: Oxford University Press, 1964.

French, R.K. The History of the Heart: Thoracic Physiology from Ancient to Modern Times. Aberdeen: Equipress, 1979.

French, R.K. William Harvey's Natural Philosophy. Cambridge, New York: Cambridge University Press, 1994.

Snellen, H.A. History of Cardiology. Rotterdam: Donker Academic Publications, 1984.

Christopher Lawrence

Brenda Wilmoth Lerner

K. Lee Lerner

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