Tortora Heart Anatomy PDF

2568T_c20_717-759.qxd 12/18/07 4:22 PM Page 718 Team B 209:JWQY057:Ch20: 718 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART ANATOMY OF THE HEART 6 cm (2.5 in.) thick, with an average mass of 250 g (8 oz) in...

2568T_c20_717-759.qxd 12/18/07 4:22 PM Page 718 Team B 209:JWQY057:Ch20: 718 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART ANATOMY OF THE HEART 6 cm (2.5 in.) thick, with an average mass of 250 g (8 oz) in adult females and 300 g (10 oz) in adult males. The heart rests  OBJECTIVES on the diaphragm, near the midline of the thoracic cavity. It lies Describe the location of the heart. in the mediastinum (mē-dē-a-STĪ-num), an anatomical region Describe the structure of the pericardium and the heart that extends from the sternum to the vertebral column, the first wall. rib to the diaphragm, and between the lungs (Figure 20.1a). Discuss the external and internal anatomy of the cham- About two-thirds of the mass of the heart lies to the left of the bers of the heart. body’s midline (Figure 20.1b). You can visualize the heart as a cone lying on its side. The pointed apex is formed by the tip of Location of the Heart the left ventricle (a lower chamber of the heart) and rests on the For all its might, the heart is relatively small, roughly the same diaphragm. It is directed anteriorly, inferiorly, and to the left. size (but not the same shape) as your closed fist. It is about The base of the heart is its posterior surface. It is formed by the 12 cm (5 in.) long, 9 cm (3.5 in.) wide at its broadest point, and atria (upper chambers) of the heart, mostly the left atrium. Figure 20.1 Position of the heart and associated structures in the mediastinum (dashed outline). (See Tortora, A Photographic Atlas of the Human Body, Second Edition, Figures 6.5 and 6.6.) The heart is located in the mediastinum, with two-thirds of its mass to the left of the midline. ANTERIOR Sternum Transverse plane Heart Muscle Left lung Right lung Esophagus Aorta View Sixth thoracic vertebra POSTERIOR (a) Inferior view of transverse section of thoracic cavity showing the heart in the mediastinum Superior vena cava Arch of aorta SUPERIOR BORDER Pulmonary trunk RIGHT BORDER Left lung Right lung Pleura (cut to LEFT BORDER reveal lung inside) APEX OF HEART Diaphragm INFERIOR SURFACE (b) Anterior view of the heart in the thoracic cavity ? What is the mediastinum? 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 719 Team B 209:JWQY057:Ch20: ANATOMY OF THE HEART 719 In addition to the apex and base, the heart has several distinct Pericardium surfaces and borders (margins). The anterior surface is deep to the sternum and ribs. The inferior surface is the part of the The membrane that surrounds and protects the heart is the peri- heart between the apex and right border and rests mostly on the cardium ( peri-  around). It confines the heart to its position diaphragm (Figure 20.1b). The right border faces the right lung in the mediastinum, while allowing sufficient freedom of move- and extends from the inferior surface to the base. The left bor- ment for vigorous and rapid contraction. The pericardium con- der, also called the pulmonary border, faces the left lung and ex- sists of two main parts: (1) the fibrous pericardium and (2) the tends from the base to the apex. serous pericardium (Figure 20.2a). The superficial fibrous peri- cardium is composed of tough, inelastic, dense irregular con- nective tissue. It resembles a bag that rests on and attaches to the CLINICAL CONNECTION Cardiopulmonary diaphragm; its open end is fused to the connective tissues of the Resuscitation blood vessels entering and leaving the heart. The fibrous peri- Because the heart lies between two rigid structures—the vertebral cardium prevents overstretching of the heart, provides protec- column and the sternum (Figure 20.1a)—external pressure on the chest tion, and anchors the heart in the mediastinum. (compression) can be used to force blood out of the heart and into the The deeper serous pericardium is a thinner, more delicate circulation. In cases in which the heart suddenly stops beating, cardiopul- membrane that forms a double layer around the heart (Figure monary resuscitation (CPR)—properly applied cardiac compressions, 20.2a). The outer parietal layer of the serous pericardium is performed with artificial ventilation of the lungs via mouth-to-mouth fused to the fibrous pericardium. The inner visceral layer of the respiration—saves lives. CPR keeps oxygenated blood circulating until serous pericardium, also called the epicardium (epi-  on top the heart can be restarted. of), is one of the layers of the heart wall and adheres tightly to In a 2007 Japanese study, researchers found that chest compres- the surface of the heart. Between the parietal and visceral layers sions alone are equally as effective as, if not better than, traditional CPR of the serous pericardium is a thin film of lubricating serous with lung ventilation. This is good news because it is easier for an emer- fluid. This slippery secretion of the pericardial cells, known as gency dispatcher to give instructions limited to chest compressions to pericardial fluid, reduces friction between the layers of the frightened, nonmedical bystanders. As public fear of contracting conta- serous pericardium as the heart moves. The space that contains gious diseases such as hepatitis, HIV, and tuberculosis continues to the few milliliters of pericardial fluid is called the pericardial rise, bystanders are much more likely to perform chest compressions cavity. alone than treatment involving mouth-to-mouth rescue breathing. Figure 20.2 Pericardium and heart wall. The pericardium is a triple-layered sac that surrounds and protects the heart. PERICARDIUM Heart wall ENDOCARDIUM FIBROUS PERICARDIUM Pericardium PARIETAL LAYER OF SEROUS PERICARDIUM Epicardium Coronary blood vessels Myocardium Trabeculae carneae Endocardium Pericardial cavity MYOCARDIUM (CARDIAC MUSCLE) VISCERAL LAYER OF SEROUS PERICARDIUM (EPICARDIUM) (a) Portion of pericardium and right ventricular heart wall showing the divisions of the pericardium and layers of the heart wall F I G U R E 20.2 CO N T I N U E S 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 720 Team B 209:JWQY057:Ch20: 720 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART F I G U R E 20.2 CO N T I N U E D Aorta Parietal layer of serous Superior Heart pericardium vena cava Pulmonary trunk Superficial muscle bundles Pericardial in atria cavity Pericardial Deep muscle Superficial Serous pericardium cavity Visceral layer bundle in muscle of serous ventricle bundles in pericardium ventricles (b) Simplified relationship of the serous pericardium to the heart (c) Cardiac muscle bundles of the myocardium ? Which layer is both a part of the pericardium and a part of the heart wall? muscle. The cardiac muscle fibers swirl diagonally around the CLINICAL CONNECTION Pericarditis heart in bundles (Figure 20.2c). The innermost endocardium Inflammation of the pericardium is called pericarditis (per-i-kar-DĪ-tis). (endo-  within) is a thin layer of endothelium overlying a thin The most common type, acute pericarditis, begins suddenly and has no layer of connective tissue. It provides a smooth lining for the known cause in most cases but is sometimes linked to a viral infection. chambers of the heart and covers the valves of the heart. The en- As a result of irritation to the pericardium, there is chest pain that may docardium is continuous with the endothelial lining of the large extend to the left shoulder and down the left arm (often mistaken for a blood vessels attached to the heart, and it minimizes surface fric- heart attack) and pericardial friction rub (a scratchy or creaking sound tion as blood passes through the heart and blood vessels. heard through a stethoscope as the visceral layer of the serous peri- cardium rubs against the parietal layer of the serous pericardium). Acute pericarditis usually lasts for about one week and is treated with CLINICAL CONNECTION Myocarditis and drugs that reduce inflammation and pain, such as ibuprofen or aspirin. Endocarditis Chronic pericarditis begins gradually and is long-lasting. In one Myocarditis (mı̄-ō-kar-DĪ-tis) is an inflammation of the myocardium that form of this condition, there is a buildup of pericardial fluid. If a great usually occurs as a complication of a viral infection, rheumatic fever, or deal of fluid accumulates, this is a life-threatening condition because exposure to radiation or certain chemicals or medications. Myocarditis the fluid compresses the heart, a condition called cardiac tamponade often has no symptoms. However, if they do occur, they may include (tam-pon-ĀD). As a result of the compression, ventricular filling is de- fever, fatigue, vague chest pain, irregular or rapid heartbeat, joint pain, creased, cardiac output is reduced, venous return to the heart is dimin- and breathlessness. Myocarditis is usually mild and recovery occurs ished, blood pressure falls, and breathing is difficult. Most causes of within two weeks. Severe cases can lead to cardiac failure and death. chronic pericarditis involving cardiac tamponade are unknown, but it is Treatment consists of avoiding vigorous exercise, a low-salt diet, elec- sometimes caused by conditions such as cancer and tuberculosis. trocardiographic monitoring, and treatment of the cardiac failure. Treatment consists of draining the excess fluid through a needle passed Endocarditis (en-dō-kar-DĪ-tis) refers to an inflammation of the endo- into the pericardial cavity. cardium and typically involves the heart valves. Most cases are caused by bacteria (bacterial endocarditis). Signs and symptoms of endocardi- Layers of the Heart Wall tis include fever, heart murmur, irregular or rapid heartbeat, fatigue, loss of appetite, night sweats, and chills. Treatment is with intravenous The wall of the heart consists of three layers (Figure 20.2a): the antibiotics. epicardium (external layer), the myocardium (middle layer), and the endocardium (inner layer). As noted earlier, the outermost epicardium, the thin, transparent outer layer of the heart wall, Chambers of the Heart is also called the visceral layer of the serous pericardium. It is The heart has four chambers. The two superior receiving cham- composed of mesothelium and delicate connective tissue that bers are the atria ( entry halls or chambers), and the two infe- imparts a smooth, slippery texture to the outermost surface of rior pumping chambers are the ventricles ( little bellies). On the heart. The middle myocardium (myo-  muscle), which is the anterior surface of each atrium is a wrinkled pouchlike struc- cardiac muscle tissue, makes up about 95% of the heart and is ture called an auricle (OR-i-kul; auri-  ear), so named because responsible for its pumping action. Although it is striated like of its resemblance to a dog’s ear (Figure 20.3). Each auricle skeletal muscle, cardiac muscle is involuntary like smooth slightly increases the capacity of an atrium so that it can hold a 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 721 Team B 209:JWQY057:Ch20: ANATOMY OF THE HEART 721 Figure 20.3 Structure of the heart: surface features. Throughout this book, blood vessels that carry oxygenated blood (which looks bright red) are colored red, whereas those that carry deoxygenated blood (which looks dark red) are colored blue. Sulci are grooves that contain blood vessels and fat and mark the external boundaries between the various chambers. Brachiocephalic trunk Left common carotid artery Left subclavian artery Superior vena cava Arch of aorta Ligamentum arteriosum Ascending aorta Left pulmonary artery Pulmonary trunk Right pulmonary artery Left pulmonary veins Fibrous pericardium (cut) Right pulmonary veins LEFT AURICLE OF LEFT ATRIUM Branch of left coronary artery RIGHT AURICLE OF RIGHT ATRIUM LEFT VENTRICLE Right coronary artery RIGHT ATRIUM ANTERIOR INTERVENTRICULAR SULCUS CORONARY SULCUS RIGHT VENTRICLE Inferior vena cava Descending aorta (a) Anterior external view showing surface features Left subclavian artery Brachiocephalic trunk Left common carotid artery Arch of aorta Superior vena cava Ligamentum arteriosum Left pulmonary artery Ascending aorta Left pulmonary veins Right pulmonary veins Pulmonary trunk RIGHT AURICLE OF LEFT AURICLE OF LEFT ATRIUM RIGHT ATRIUM RIGHT ATRIUM CORONARY SULCUS LEFT VENTRICLE RIGHT VENTRICLE ANTERIOR INTERVENTRICULAR SULCUS (b) Anterior external view F I G U R E 20.3 CO N T I N U E S 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 722 Team B 209:JWQY057:Ch20: 722 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART F I G U R E 20.3 CO N T I N U E D Left common carotid artery Left subclavian artery Brachiocephalic trunk Arch of aorta Superior vena cava Descending aorta Ascending aorta Left pulmonary artery Right pulmonary artery AURICLE OF LEFT ATRIUM Left pulmonary veins Right pulmonary veins RIGHT ATRIUM LEFT ATRIUM Coronary sinus Right coronary artery (in the coronary sulcus) Inferior vena cava LEFT VENTRICLE Middle cardiac vein RIGHT VENTRICLE POSTERIOR INTERVENTRICULAR SULCUS (c) Posterior external view showing surface features ? The coronary sulcus forms an external boundary between which chambers of the heart? greater volume of blood. Also on the surface of the heart are a the interatrial septum (inter-  between; septum  a dividing series of grooves, called sulci (SUL-sē), that contain coronary wall or partition). A prominent feature of this septum is an oval blood vessels and a variable amount of fat. Each sulcus (SUL- depression called the fossa ovalis, the remnant of the foramen kus) marks the external boundary between two chambers of the ovale, an opening in the interatrial septum of the fetal heart that heart. The deep coronary sulcus (coron-  resembling a crown) normally closes soon after birth (see Figure 21.30 on page 822). encircles most of the heart and marks the external boundary be- Blood passes from the right atrium into the right ventricle tween the superior atria and inferior ventricles. The anterior through a valve that is called the tricuspid valve (trı̄-KUS-pid; interventricular sulcus is a shallow groove on the anterior sur- tri-  three; cuspid  point) because it consists of three leaflets face of the heart that marks the external boundary between the or cusps (Figure 20.4a). It is also called the right atrioventricu- right and left ventricles. This sulcus continues around to the pos- lar valve. The valves of the heart are composed of dense con- terior surface of the heart as the posterior interventricular nective tissue covered by endocardium. sulcus, which marks the external boundary between the ventri- cles on the posterior aspect of the heart (Figure 20.3c). Right Ventricle The right ventricle is about 4–5 mm (0.16–0.2 in.) in average Right Atrium thickness and forms most of the anterior surface of the heart. The right atrium forms the right border of the heart and re- The inside of the right ventricle contains a series of ridges ceives blood from three veins: the superior vena cava, inferior formed by raised bundles of cardiac muscle fibers called tra- vena cava, and coronary sinus (Figure 20.4a) (Veins always re- beculae carneae (tra-BEK-ū-lē KAR-nē-ē; trabeculae  little turn blood to the heart). The right atrium is about 2–3 mm beams; carneae  fleshy; see Figure 20.2a). Some of the trabec- (0.08–0.12 in.) in average thickness. The anterior and posterior ulae carneae convey part of the conduction system of the heart, walls of the right atrium are very different. The posterior wall is which you will learn about later in this chapter (see page 732). smooth; the anterior wall is rough due to the presence of muscu- The cusps of the tricuspid valve are connected to tendonlike lar ridges called pectinate muscles (PEK-tin-āt; pectin  cords, the chordae tendineae (KOR-dē ten-DIN-ē-ē; chord-  comb), which also extend into the auricle (Figure 20.4b). cord; tend-  tendon), which in turn are connected to cone- Between the right atrium and left atrium is a thin partition called shaped trabeculae carneae called papillary muscles (papill-  2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 723 Team B 209:JWQY057:Ch20: Figure 20.4 Structure of the heart: internal anatomy. Blood flows into the right atrium through the superior vena cava, inferior vena cava, and coronary sinus and into the left atrium through four pulmonary veins. Left common carotid artery Left subclavian artery Brachiocephalic trunk Frontal plane Arch of aorta Ligamentum arteriosum Ascending aorta Superior vena cava Left pulmonary artery Right pulmonary artery Pulmonary trunk PULMONARY VALVE Left pulmonary veins LEFT ATRIUM Right pulmonary veins AORTIC VALVE BICUSPID (MITRAL) VALVE Opening of superior vena cava CHORDAE TENDINEAE Fossa ovalis LEFT VENTRICLE RIGHT ATRIUM INTERVENTRICULAR SEPTUM Opening of coronary sinus PAPILLARY MUSCLE Opening of inferior vena cava TRABECULAE CARNEAE TRICUSPID VALVE RIGHT VENTRICLE Inferior vena cava Descending aorta (a) Anterior view of frontal section showing internal anatomy Brachiocephalic trunk Left subclavian artery Left common carotid artery Superior vena cava Arch of aorta Ligamentum anteriosum Right pulmonary vein Ascending aorta Pulmonary trunk RIGHT AURICLE (cut open) Left pulmonary vein Pectinate muscles LEFT AURICLE RIGHT ATRIUM Cusp of tricuspid valve LEFT VENTRICLE Chordae tendineae INTERVENTRICULAR Papillary muscle SEPTUM RIGHT VENTRICLE TRABECULAE CARNEAE (b) Anterior view of partially sectioned heart F I G U R E 20.4 CO N T I N U E S 723 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 724 Team B 209:JWQY057:Ch20: 724 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART F I G U R E 20.4 CO N T I N U E D ANTERIOR Transverse plane Right ventricle Left ventricle View Interventricular septum POSTERIOR (c) Inferior view of transverse section showing differences in thickness of ventricular walls ? How does thickness of the myocardium relate to the workload of a cardiac chamber? nipple). Internally, the right ventricle is separated from the left in the aorta flows into the coronary arteries, which branch from ventricle by a partition called the interventricular septum. the ascending aorta and carry blood to the heart wall. The re- Blood passes from the right ventricle through the pulmonary mainder of the blood passes into the arch of the aorta and de- valve (pulmonary semilunar valve) into a large artery called the scending aorta (thoracic aorta and abdominal aorta). Branches pulmonary trunk, which divides into right and left pulmonary of the arch of the aorta and descending aorta carry blood arteries. Arteries always take blood away from the heart. throughout the body. During fetal life, a temporary blood vessel, called the Left Atrium ductus arteriosus, shunts blood from the pulmonary trunk into The left atrium is about the same thickness as the right atrium the aorta. Hence, only a small amount of blood enters the and forms most of the base of the heart (see Figure 20.1b). It re- nonfunctioning fetal lungs (see Figure 21.30 on page 822). ceives blood from the lungs through four pulmonary veins. Like The ductus arteriosus normally closes shortly after birth, the right atrium, the inside of the left atrium has a smooth poste- leaving a remnant known as the ligamentum arteriosum, rior wall. Because pectinate muscles are confined to the auricle which connects the arch of the aorta and pulmonary trunk of the left atrium, the anterior wall of the left atrium also is (Figure 20.4a). smooth. Blood passes from the left atrium into the left ventricle through the bicuspid (mitral) valve (bi-  two), which, as its Myocardial Thickness name implies, has two cusps. The term mitral refers to the re- and Function semblance of the bicuspid valve to a bishop’s miter (hat), which The thickness of the myocardium of the four chambers varies is two-sided. It is also called the left atrioventricular valve. according to each chamber’s function. The thin-walled atria de- Left Ventricle liver blood under less pressure into the adjacent ventricles. The left ventricle is the thickest chamber of the heart, averaging Because the ventricles pump blood under higher pressure over 10–15 mm (0.4–0.6 in.) and forms the apex of the heart (see greater distances, their walls are thicker (Figure 20.4a). Figure 20.1b). Like the right ventricle, the left ventricle contains Although the right and left ventricles act as two separate pumps trabeculae carneae and has chordae tendinae that anchor the that simultaneously eject equal volumes of blood, the right side cusps of the bicuspid valve to papillary muscles. Blood passes has a much smaller workload. It pumps blood a short distance to from the left ventricle through the aortic valve (aortic semilunar the lungs at lower pressure, and the resistance to blood flow is valve) into the ascending aorta (aorte  to suspend, because the small. The left ventricle pumps blood great distances to all other aorta once was believed to lift up the heart). Some of the blood parts of the body at higher pressure, and the resistance to blood 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 725 Team B 209:JWQY057:Ch20: HEART VALVES AND CIRCULATION OF BLOOD 725 flow is larger. Therefore, the left ventricle works much harder HEART VALVES AND CIRCULATION than the right ventricle to maintain the same rate of blood flow. The anatomy of the two ventricles confirms this functional OF BLOOD difference—the muscular wall of the left ventricle is consider-  OBJECTIVES ably thicker than the wall of the right ventricle (Figure 20.4c). Describe the structure and function of the valves of the Note also that the perimeter of the lumen (space) of the left ven- heart. tricle is roughly circular in contrast to that of the right ventricle, Outline the flow of blood through the chambers of which is somewhat crescent shaped. the heart and through the systemic and pulmonary circulations. Fibrous Skeleton of the Heart Discuss the coronary circulation. In addition to cardiac muscle tissue, the heart wall also contains As each chamber of the heart contracts, it pushes a volume of dense connective tissue that forms the fibrous skeleton of the blood into a ventricle or out of the heart into an artery. Valves heart (Figure 20.5). Essentially, the fibrous skeleton consists of open and close in response to pressure changes as the heart con- four dense connective tissue rings that surround the valves of the tracts and relaxes. Each of the four valves helps ensure the one- heart, fuse with one another, and merge with the interventricular way flow of blood by opening to let blood through and then septum. In addition to forming a structural foundation for the closing to prevent its backflow. heart valves, the fibrous skeleton prevents overstretching of the valves as blood passes through them. It also serves as a point of Operation of the Atrioventricular Valves insertion for bundles of cardiac muscle fibers and acts as an elec- trical insulator between the atria and ventricles. Because they are located between an atrium and a ventricle, the tricuspid and bicuspid valves are termed atrioventricular (AV)  CHECKPOINT valves. When an AV valve is open, the rounded ends of the cusps 1. Define each of the following external features of the project into the ventricle. When the ventricles are relaxed, the heart: auricle, coronary sulcus, anterior interventricular sulcus, and posterior interventricular sulcus. papillary muscles are relaxed, the chordae tendineae are slack, 2. Describe the structure of the pericardium and the layers and blood moves from a higher pressure in the atria to a lower of the wall of the heart. pressure in the ventricles through open AV valves (Figure 3. What are the characteristic internal features of each 20.6a, d). When the ventricles contract, the pressure of the blood chamber of the heart? drives the cusps upward until their edges meet and close the 4. Which blood vessels deliver blood to the right and left opening (Figure 20.6b, e). At the same time, the papillary mus- atria? cles contract, which pulls on and tightens the chordae tendineae. 5. What is the relationship between wall thickness and This prevents the valve cusps from everting (opening into the function among the various chambers of the heart? atria) in response to the high ventricular pressure. If the AV 6. What type of tissue composes the fibrous skeleton of the valves or chordae tendineae are damaged, blood may regurgitate heart? What functions does this tissue perform? (flow back) into the atria when the ventricles contract. Figure 20.5 Fibrous skeleton of the heart. Elements of the fibrous skeleton are shown in capital letters. Fibrous rings support the four valves of the heart and are fused to one another. View Pulmonary valve PULMONARY FIBROUS RING Left coronary artery CONUS TENDON Aortic valve AORTIC FIBROUS RING Transverse plane LEFT FIBROUS TRIGONE Right coronary artery RIGHT FIBROUS TRIGONE Bicuspid valve Tricuspid valve LEFT ATRIOVENTRICULAR RIGHT ATRIOVENTRICULAR FIBROUS RING FIBROUS RING Superior view (the atria have been removed) ? In what two ways does the fibrous skeleton contribute to the functioning of heart valves? 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 726 Team B 209:JWQY057:Ch20: 726 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART Figure 20.6 Responses of the valves to the pumping of the heart. Heart valves prevent the backflow of blood. BICUSPID VALVE CUSPS Open Closed CHORDAE TENDINEAE Slack Taut PAPILLARY MUSCLES Relaxed Contracted (a) Bicuspid valve open (b) Bicuspid valve closed Cusp of tricuspid valve Chordae tendineae Papillary muscle (c) Tricuspid valve open ANTERIOR ANTERIOR Pulmonary Pulmonary valve (closed) Aortic valve valve (open) (closed) Left coronary artery Right coronary Aortic valve artery (open) Bicuspid valve Bicuspid (open) valve (closed) Tricuspid Tricuspid valve valve (open) (closed) POSTERIOR POSTERIOR (d) Superior view with atria removed: pulmonary and aortic (e) Superior view with atria removed: pulmonary and aortic valves closed, bicuspid and tricuspid valves open. valves open, bicuspid and tricuspid valves closed. 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 727 Team B 209:JWQY057:Ch20: HEART VALVES AND CIRCULATION OF BLOOD 727 Operation of the Semilunar Valves CLINICAL CONNECTION Heart Valve Disorders The aortic and pulmonary valves are known as the semilunar When heart valves operate normally, they open fully and close com- (SL) valves (semi-  half; lunar  moon-shaped) because pletely at the proper times. A narrowing of a heart valve opening that they are made up of three crescent moon-shaped cusps (Fig- restricts blood flow is known as stenosis (ste-NŌ-sis  a narrowing); ure 20.6d). Each cusp attaches to the arterial wall by its convex failure of a valve to close completely is termed insufficiency or incom- outer margin. The SL valves allow ejection of blood from the petence. In mitral stenosis, scar formation or a congenital defect heart into arteries but prevent backflow of blood into the ventri- causes narrowing of the mitral valve. One cause of mitral insufficiency, cles. The free borders of the cusps project into the lumen of the in which there is backflow of blood from the left ventricle into the left artery. When the ventricles contract, pressure builds up within atrium, is mitral valve prolapse (MVP). In MVP one or both cusps of the the chambers. The semilunar valves open when pressure in the mitral valve protrude into the left atrium during ventricular contraction. ventricles exceeds the pressure in the arteries, permitting ejec- Mitral valve prolapse is one of the most common valvular disorders, tion of blood from the ventricles into the pulmonary trunk and affecting as much as 30% of the population. It is more prevalent in aorta (Figure 20.6e). As the ventricles relax, blood starts to flow women than in men, and does not always pose a serious threat. In aor- back toward the heart. This backflowing blood fills the valve tic stenosis the aortic valve is narrowed, and in aortic insufficiency cusps, which causes the semilunar valves to close tightly (Figure there is backflow of blood from the aorta into the left ventricle. 20.6d). Certain infectious diseases can damage or destroy the heart Surprisingly perhaps, there are no valves guarding the junc- valves. One example is rheumatic fever, an acute systemic inflamma- tions between the venae cavae and the right atrium or the pul- tory disease that usually occurs after a streptococcal infection of the monary veins and the left atrium. As the atria contract, a small throat. The bacteria trigger an immune response in which antibodies amount of blood does flow backward from the atria into these produced to destroy the bacteria instead attack and inflame the connec- tive tissues in joints, heart valves, and other organs. Even though vessels. However, backflow is minimized by a different mecha- rheumatic fever may weaken the entire heart wall, most often it dam- nism; as the atrial muscle contracts, it compresses and nearly ages the mitral and aortic valves. collapses the venous entry points. ANTERIOR Pulmonary trunk Ascending aorta PULMONARY VALVE Right coronary artery Pectinate muscle of left atrium Pectinate muscle of right atrium Left coronary AORTIC artery VALVE TRICUSPID BICUSPID VALVE (MITRAL) VALVE Coronary sinus POSTERIOR (f) Superior view of atrioventricular and semilunar valves Semilunar cusp of aortic valve ? How do papillary muscles prevent atrioventricular valve cusps from everting (g) Superior view of aortic valve (swinging upward) into the atria? 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 728 Team B 209:JWQY057:Ch20: 728 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART Figure 20.7 Systemic and pulmonary circulations. Systemic and Pulmonary Circulations The left side of the heart pumps In postnatal (after birth) circulation, the heart pumps blood into oxygenated blood into the systemic two closed circuits with each beat—systemic circulation and circulation to all tissues of the body except the air pulmonary circulation ( pulmon-  lung). The two circuits are sacs (alveoli) of the lungs. The right side of the heart arranged in series: The output of one becomes the input of the pumps deoxygenated blood into the pulmonary other, as would happen if you attached two garden hoses (see circulation to the air sacs (alveoli) of the lungs. Figure 21.17 on page 000). The left side of the heart is the pump for systemic circulation; it receives bright red, oxygen-rich blood from the lungs. The left ventricle ejects blood into the aorta (Figure 20.7). From the aorta, the blood divides into sepa- rate streams, entering progressively smaller systemic arteries that carry it to all organs throughout the body—except for the air sacs (alveoli) of the lungs, which are supplied by pulmonary 4. In pulmonary capillaries, blood circulation. In systemic tissues, arteries give rise to smaller- loses CO2 and gains O2 diameter arterioles, which finally lead into extensive beds of systemic capillaries. Exchange of nutrients and gases occurs 3. 5. Pulmonary trunk and Pulmonary veins across the thin capillary walls. Blood unloads O2 (oxygen) and pulmonary arteries (oxygenated blood) picks up CO2 (carbon dioxide). In most cases, blood flows through only one capillary and then enters a systemic venule. Pulmonary valve Venules carry deoxygenated (oxygen-poor) blood away from tis- sues and merge to form larger systemic veins. Ultimately the 2. 6. blood flows back to the right atrium. Right ventricle Left atrium The right side of the heart is the pump for pulmonary circula- tion; it receives all the dark red, deoxygenated blood returning Tricuspid valve Bicuspid valve from systemic circulation. Blood ejected from the right ventricle flows into the pulmonary trunk, which branches into pulmonary 1. 7. arteries that carry blood to the right and left lungs. In pulmonary Right atrium (deoxygenated blood) Left ventricle capillaries, blood unloads CO2 , which is exhaled, and picks up inhaled O2. The freshly oxygenated blood then flows into pul- monary veins and returns to the left atrium. Aortic valve 10. Superior Inferior 8. Aorta and Coronary vena vena systemic Coronary Circulation cava cava sinus arteries Nutrients are not able to diffuse quickly enough from blood in the chambers of the heart to supply all the layers of cells that make up the heart wall. For this reason, the myocardium has its own network of blood vessels, the coronary or cardiac circula- tion (coron-  crown). The coronary arteries branch from the ascending aorta and encircle the heart like a crown encircles the head (Figure 20.8a). While the heart is contracting, little blood 9. In systemic capillaries, blood flows in the coronary arteries because they are squeezed shut. loses O2 and gains CO2 When the heart relaxes, however, the high pressure of blood in the aorta propels blood through the coronary arteries, into capil- Diagram of blood flow laries, and then into coronary veins (Figure 20.8b). ? Which numbers constitute the pulmonary circulation? Coronary Arteries Which constitute the systemic circulation? Two coronary arteries, the right and left coronary arteries, branch from the ascending aorta and supply oxygenated blood to lies in the coronary sulcus and distributes oxygenated blood to the myocardium (Figure 20.8a). The left coronary artery the walls of the left ventricle and left atrium. passes inferior to the left auricle and divides into the anterior in- The right coronary artery supplies small branches (atrial terventricular and circumflex branches. The anterior interven- branches) to the right atrium. It continues inferior to the right tricular branch or left anterior descending (LAD) artery is in auricle and ultimately divides into the posterior interventricular the anterior interventricular sulcus and supplies oxygenated and marginal branches. The posterior interventricular branch blood to the walls of both ventricles. The circumflex branch follows the posterior interventricular sulcus and supplies the 2568T_c20_717-759.qxd 12/18/07 3:31 PM Page 729 Team B 209:JWQY057:Ch20: Figure 20.8 The coronary circulation. The views of the heart from the anterior aspect in (a) and (b) are drawn as if the heart were transparent to reveal blood vessels on the posterior aspect. (See Tortora, A Photographic Atlas of the Human Body, Second Edition, Figures 6.8 and 6.9.) The right and left coronary arteries deliver blood to the heart; the coronary veins drain blood from the heart into the coronary sinus. Arch of aorta Superior Ascending vena cava aorta Pulmonary Pulmonary LEFT trunk trunk CORONARY Left auricle Left auricle RIGHT CORONARY CIRCUMFLEX BRANCH Right atrium ANTERIOR Right INTER- CORONARY atrium VENTRICULAR SMALL SINUS BRANCH CARDIAC GREAT POSTERIOR ANTERIOR CARDIAC MARGINAL INTER- CARDIAC BRANCH VENTRICULAR MIDDLE BRANCH CARDIAC Right Left Left Right ventricle ventricle ventricle ventricle Inferior vena cava (a) Anterior view of coronary arteries (b) Anterior view of coronary veins SUPERIOR Arch of aorta Ascending aorta Left pulmonary artery Pulmonary trunk Left auricle Right auricle RIGHT CORONARY GREAT CARDIAC VEIN ARTERY LEFT CORONARY ARTERY CIRCUMFLEX BRANCH ANTERIOR CARDIAC LEFT MARGINAL VEIN BRANCH Right ventricle Left ventricle MARGINAL BRANCH ANTERIOR INTERVENTRICULAR TRIBUTARY TO GREAT BRANCH CARDIAC VEIN INFERIOR (c) Anterior view ? Which coronary blood vessel delivers oxygenated blood to the walls of the left atrium and left ventricle? 729 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 730 Team B 209:JWQY057:Ch20: 730 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART walls of the two ventricles with oxygenated blood. The mar- CLINICAL CONNECTION Myocardial Ischemia ginal branch in the coronary sulcus transports oxygenated and Infarction blood to the myocardium of the right ventricle. Most parts of the body receive blood from branches of more Partial obstruction of blood flow in the coronary arteries may cause myo- than one artery, and where two or more arteries supply the same cardial ischemia (is-KĒ-mē-a; ische-  to obstruct; -emia  in the blood), a region, they usually connect. These connections, called anasto- condition of reduced blood flow to the myocardium. Usually, ischemia moses (a-nas-tō-MŌ-sēs), provide alternate routes called col- causes hypoxia (reduced oxygen supply), which may weaken cells lateral circuits, for blood to reach a particular organ or tissue. without killing them. Angina pectoris (an-JĪ-na, or AN-ji-na, PEK-to-ris), The myocardium contains many anastomoses that connect which literally means “strangled chest,” is a severe pain that usually branches of a given coronary artery or extend between branches accompanies myocardial ischemia. Typically, sufferers describe it as a of different coronary arteries. They provide detours for arterial tightness or squeezing sensation, as though the chest were in a vise. blood if a main route becomes obstructed. Thus, heart muscle The pain associated with angina pectoris is often referred to the neck, may receive sufficient oxygen even if one of its coronary arteries chin, or down the left arm to the elbow. Silent myocardial ischemia, is partially blocked. ischemic episodes without pain, is particularly dangerous because the person has no forewarning of an impending heart attack. Coronary Veins A complete obstruction to blood flow in a coronary artery may result in a myocardial infarction (in-FARK-shun), or MI, commonly called After blood passes through the arteries of the coronary circula- a heart attack. Infarction means the death of an area of tissue because tion, it flows into capillaries, where it delivers oxygen and nutri- of interrupted blood supply. Because the heart tissue distal to the ents to the heart muscle and collects carbon dioxide and waste, obstruction dies and is replaced by noncontractile scar tissue, the heart and then moves into coronary veins. Most of the deoxygenated muscle loses some of its strength. Depending on the size and location blood from the myocardium drains into a large vascular sinus in of the infarcted (dead) area, an infarction may disrupt the conduction the coronary sulcus on the posterior surface of the heart, called system of the heart and cause sudden death by triggering ventricular the coronary sinus (Figure 20.8b). (A vascular sinus is a thin- fibrillation. Treatment for a myocardial infarction may involve injection walled vein that has no smooth muscle to alter its diameter.) The of a thrombolytic (clot-dissolving) agent such as streptokinase or t-PA, deoxygenated blood in the coronary sinus empties into the right plus heparin (an anticoagulant), or performing coronary angioplasty or atrium. The principal tributaries carrying blood into the coronary coronary artery bypass grafting. Fortunately, heart muscle can remain sinus are the following: alive in a resting person if it receives as little as 10–15% of its normal blood supply. Great cardiac vein in the anterior interventricular sulcus, which drains the areas of the heart supplied by the left coro- nary artery (left and right ventricles and left atrium) Middle cardiac vein in the posterior interventricular sulcus,  CHECKPOINT which drains the areas supplied by the posterior interven- 7. What causes the heart valves to open and to close? tricular branch of the right coronary artery (left and right What supporting structures ensure that the valves ventricles) operate properly? Small cardiac vein in the coronary sulcus, which drains the 8. In correct sequence, which heart chambers, heart valves, right atrium and right ventricle and blood vessels would a drop of blood encounter as it Anterior cardiac veins, which drain the right ventricle and flows from the right atrium to the aorta? open directly into the right atrium 9. Which arteries deliver oxygenated blood to the myocardium of the left and right ventricles? When blockage of a coronary artery deprives the heart muscle of oxygen, reperfusion, the reestablishment of blood flow, may damage the tissue further. This surprising effect is due to the for- mation of oxygen free radicals from the reintroduced oxygen. As CARDIAC MUSCLE TISSUE you learned in Chapter 2, free radicals are electrically charged molecules that have an unpaired electron (see Figure 2.3b on AND THE CARDIAC CONDUCTION page 000). These unstable, highly reactive molecules cause chain SYSTEM reactions that lead to cellular damage and death. To counter the  OBJECTIVES effects of oxygen free radicals, body cells produce enzymes Describe the structural and functional characteristics of that convert free radicals to less reactive substances. Two such cardiac muscle tissue and the conduction system of the enzymes are superoxide dismutase and catalase. In addition, heart. nutrients such as vitamin E, vitamin C, beta-carotene, zinc, and Explain how an action potential occurs in cardiac contrac- selenium serve as antioxidants, which remove oxygen free tile fibers. radicals from circulation. Drugs that lessen reperfusion damage Describe the electrical events of a normal electrocardio- after a heart attack or stroke are currently under development. gram (ECG). 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 731 Team B 209:JWQY057:Ch20: CARDIAC MUSCLE TISSUE AND THE CARDIAC CONDUCTION SYSTEM 731 Histology of Cardiac Muscle Tissue 20.9). They also exhibit branching, which gives individual car- Compared with skeletal muscle fibers, cardiac muscle fibers are diac muscle fibers a “stair-step” appearance (see Table 4.5B on shorter in length and less circular in transverse section (Figure page 138). A typical cardiac muscle fiber is 50–100 µm long and Figure 20.9 Histology of cardiac muscle tissue. (See Table 4.5B on page 138 for a light micrograph of cardiac muscle.) Cardiac muscle fibers connect to neighboring fibers by intercalated discs, which contain desmosomes and gap junctions. Intercalated discs Opening of Desmosomes transverse tubule Gap junctions Mitochondrion Cardiac muscle fiber Nucleus Sarcolemma (a) Cardiac muscle fibers Sarcolemma Transverse Mitochondrion Sarcoplasmic tubule reticulum Nucleus Thin filament Thick filament Z disc M line Z disc H zone I band A band I band Sarcomere (b) Arrangement of components in a cardiac muscle fiber ? What are the functions of intercalated discs in cardiac muscle fibers? 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 732 Team B 209:JWQY057:Ch20: 732 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART has a diameter of about 14 m. Usually one centrally located mic fibers repeatedly generate action potentials that trigger heart nucleus is present, although an occasional cell may have two contractions. They continue to stimulate a heart to beat even after nuclei. The ends of cardiac muscle fibers connect to neighboring it is removed from the body—for example, to be transplanted fibers by irregular transverse thickenings of the sarcolemma into another person—and all of its nerves have been cut. (Note: called intercalated discs (in-TER-kā-lāt-ed; intercalat-  to Surgeons do not attempt to reattach heart nerves during heart insert between). The discs contain desmosomes, which hold the transplant operations. For this reason, it has been said that heart fibers together, and gap junctions, which allow muscle action surgeons are better “plumbers” than they are “electricians.”) potentials to conduct from one muscle fiber to its neighbors. Gap During embryonic development, only about 1% of the cardiac junctions allow the entire myocardium of the atria or the ventri- muscle fibers become autorhythmic fibers; these relatively rare cles to contract as a single, coordinated unit. fibers have two important functions. Mitochondria are larger and more numerous in cardiac mus- 1. They act as a pacemaker, setting the rhythm of electrical cle fibers than in skeletal muscle fibers. In a cardiac muscle excitation that causes contraction of the heart. fiber, they take up 25% of the cytosolic space; in a skeletal mus- cle fiber only 2% of the cytosolic space is occupied by mito- 2. They form the conduction system, a network of specialized chondria. Cardiac muscle fibers have the same arrangement of cardiac muscle fibers that provide a path for each cycle of cardiac actin and myosin, and the same bands, zones, and Z discs, as excitation to progress through the heart. The conduction system skeletal muscle fibers. The transverse tubules of cardiac muscle ensures that cardiac chambers become stimulated to contract in a are wider but less abundant than those of skeletal muscle; the coordinated manner, which makes the heart an effective pump. one transverse tubule per sarcomere is located at the Z disc. The As you will see later in the chapter, problems with autorhythmic sarcoplasmic reticulum of cardiac muscle fibers is somewhat fibers can result in arrhythmias (abnormal rhythms) in which the smaller than the SR of skeletal muscle fibers. As a result, cardiac heart beats irregularly, too fast, or too slow. muscle has a smaller intracellular reserve of Ca2. Cardiac action potentials propagate through the conduction system in the following sequence (Figure 20.10a): CLINICAL CONNECTION Regeneration of 1 Cardiac excitation normally begins in the sinoatrial (SA) Heart Cells node, located in the right atrial wall just inferior and lateral to the opening of the superior vena cava. SA node cells do As noted earlier in the chapter, the heart of a heart attack survivor often not have a stable resting potential. Rather, they repeatedly has regions of infarcted (dead) cardiac muscle tissue that typically are depolarize to threshold spontaneously. The spontaneous de- replaced with noncontractile fibrous scar tissue over time. Our inability polarization is a pacemaker potential. When the pacemaker to repair damage from a heart attack has been attributed to a lack of potential reaches threshold, it triggers an action potential stem cells in cardiac muscle and to the absence of mitosis in mature (Figure 20.10b). Each action potential from the SA node cardiac muscle fibers. A recent study of heart transplant recipients by propagates throughout both atria via gap junctions in the in- American and Italian scientists, however, provides evidence for signifi- tercalated discs of atrial muscle fibers. Following the action cant replacement of heart cells. The researchers studied men who had potential, the atria contract. received a heart from a female, and then looked for the presence of a Y chromosome in heart cells. (All female cells except gametes have two 2 By conducting along atrial muscle fibers, the action poten- X chromosomes and lack the Y chromosome.) Several years after the tial reaches the atrioventricular (AV) node, located in the transplant surgery, between 7% and 16% of the heart cells in the trans- interatrial septum, just anterior to the opening of the coro- planted tissue, including cardiac muscle fibers and endothelial cells in nary sinus (Figure 20.10a). coronary arterioles and capillaries, had been replaced by the recipient’s 3 From the AV node, the action potential enters the atrioven- own cells, as evidenced by the presence of a Y chromosome. The study tricular (AV) bundle (also known as the bundle of His). also revealed cells with some of the characteristics of stem cells in both This bundle is the only site where action potentials can con- transplanted hearts and control hearts. Evidently, stem cells can mi- duct from the atria to the ventricles. (Elsewhere, the fibrous grate from the blood into the heart and differentiate into functional skeleton of the heart electrically insulates the atria from the muscle and endothelial cells. The hope is that researchers can learn ventricles.) how to “turn on” such regeneration of heart cells to treat people with heart failure or cardiomyopathy (diseased heart). 4 After propagating along the AV bundle, the action potential enters both the right and left bundle branches. The bundle branches extend through the interventricular septum toward the apex of the heart. Autorhythmic Fibers: The Conduction System 5 Finally, the large-diameter Purkinje fibers rapidly conduct An inherent and rhythmical electrical activity is the reason for the action potential beginning at the apex of the heart up- the heart’s lifelong beat. The source of this electrical activity is a ward to the remainder of the ventricular myocardium. Then network of specialized cardiac muscle fibers called autorhythmic the ventricles contract, pushing the blood upward toward the fibers (auto-  self) because they are self-excitable. Autorhyth- semilunar valves. 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 733 Team B 209:JWQY057:Ch20: CARDIAC MUSCLE TISSUE AND THE CARDIAC CONDUCTION SYSTEM 733 Figure 20.10 The conduction system of the heart. Autorhythmic fibers in the SA node, located in the right atrial wall (a), act as the heart’s pacemaker, initiating cardiac action potentials (b) that cause contraction of the heart’s chambers. The conduction system ensures that the chambers of the heart contract in a coordinated manner. Frontal plane Left atrium Right atrium 1 SINOATRIAL (SA) NODE 2 ATRIOVENTRICULAR (AV) NODE 3 ATRIOVENTRICULAR (AV) BUNDLE (BUNDLE OF HIS) Left ventricle 4 RIGHT AND LEFT BUNDLE BRANCHES Right ventricle 5 PURKINJE FIBERS (a) Anterior view of frontal section + 10 mV Action potential Membrane potential Threshold – 60 mV Pacemaker potential 0 0.8 1.6 2.4 Time (sec) (b) Pacemaker potentials and action potentials in autorhythmic fibers of SA node ? Which component of the conduction system provides the only electrical connection between the atria and the ventricles? On their own, autorhythmic fibers in the SA node would initi- the heart. Nerve impulses from the autonomic nervous system ate an action potential about every 0.6 second, or 100 times per (ANS) and blood-borne hormones (such as epinephrine) modify minute. This rate is faster than that of any other autorhythmic the timing and strength of each heartbeat, but they do not estab- fibers. Because action potentials from the SA node spread lish the fundamental rhythm. In a person at rest, for example, through the conduction system and stimulate other areas before acetylcholine released by the parasympathetic division of the the other areas are able to generate an action potential at their ANS slows SA node pacing to about every 0.8 second or 75 ac- own, slower rate, the SA node acts as the natural pacemaker of tion potentials per minute (Figure 20.10b). 2568T_c20_717-759.qxd 12/14/07 6:53 PM Page 734 Team B 209:JWQY057:Ch20: 734 CHAPTER 20 THE CARDIOVASCULAR SYSTEM: THE HEART by an action potential from neighboring fibers, its voltage- CLINICAL CONNECTION Artificial Pacemakers gated fast Na channels open. These sodium ion channels If the SA node becomes damaged or diseased, the slower AV node can are referred to as “fast” because they open very rapidly in pick up the pacemaking task. Its rate of spontaneous depolarization is response to a threshold-level depolarization. Opening of 40 to 60 times per minute. If the activity of both nodes is suppressed, these channels allows Na inflow because the cytosol of the heartbeat may still be maintained by autorhythmic fibers in the contractile fibers is electrically more negative than intersti- ventricles—the AV bundle, a bundle branch, or Purkinje fibers. tial fluid and Na concentration is higher in interstitial fluid. However, the pacing rate is so slow (20–35 beats per minute) that Inflow of Na down the electrochemical gradient produces blood flow to the brain is inadequate. When this condition occurs, nor- a rapid depolarization. Within a few milliseconds, the fast mal heart rhythm can be restored and maintained by surgically implant- Na channels automatically inactivate and Na inflow ing an artificial pacemaker, a device that sends out small electrical cur- decreases. rents to stimulate the heart to contract. A pacemaker consists of a battery and impulse generator and is usually implanted beneath the 2 Plateau. The next phase of an action potential in a contrac- skin just inferior to the clavicle. The pacemaker is connected to one or tile fiber is the plateau, a period of maintained depolariza- two flexible leads (wires) that are threaded through the superior vena tion. It is due in part to opening of voltage-gated slow Ca2 cava and then passed into the various chambers of the heart. Many of channels in the sarcolemma. When these channels open, the newer pacemakers, referred to as activity-adjusted pacemakers, calcium ions move from the interstitial fluid (which has a automatically speed up the heartbeat during exercise. higher Ca2 concentration) into the cytosol. This inflow of Ca2 causes even more Ca2 to pour out of the sarcoplasmic reticulum into the cytosol through additional Ca2 channels Action Potential and Contraction in the sarcoplasmic reticulum membrane. The increased of