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CARDIOVASCUL AR 22 Heart SYSTEM O U T L I N E 22.1 Overview of the Cardiovascular System 657 22.1a Pulmonary and Systemic Circulations 657 22.1b Position of the Heart 658 22.1c Characteristics of the Pericardium 659 22.2 Anatomy of the Heart 660 22.2a Heart Wall Structure 660 22.2b External Heart Anatomy 660 22.2c Internal Heart Anatomy: Chambers and Valves 660 22.3 Coronary Circulation 666 22.4 How the Heart Beats: Electrical Properties of Cardiac Tissue 668 22.4a Characteristics of Cardiac Muscle Tissue 22.4b Contraction of Heart Muscle 669 22.4c The Heart’s Conducting System 670 668 22.5 Innervation of the Heart 672 22.6 Tying It All Together: The Cardiac Cycle 673 22.6a Steps in the Cardiac Cycle 673 22.6b Summary of Blood Flow During the Cardiac Cycle 673 22.7 Aging and the Heart 677 22.8 Development of the Heart 677 MODULE 9: C ARDIOVA SCUL AR SYSTEM mck78097_ch22_656-682.indd 656 2/14/11 4:29 PM Chapter Twenty-Two n chapter 21, we discovered the importance of blood and the myriad of substances it carries. To maintain homeostasis, blood must circulate continuously throughout the body. The continual pumping action of the heart is essential for maintaining blood circulation. If the heart fails to pump adequate volumes of blood, cells are deprived of needed oxygen and nutrients, waste products accumulate, and cell death occurs. In a healthy, 80-kilogram resting adult, the heart beats about 75 times per minute (about 4500 times per hour or 108,000 times per day). The amount of blood pumped from one ventricle per minute (about 5.25 liters [L] at rest) is called the cardiac output. When the body is more active, and the cells need oxygen and nutrients delivered at a faster pace, the heart can increase its output up to five- or six-fold. I 22.1 Overview of the Cardiovascular System 657 which carry blood back to the heart. The differences between these types of vessels are discussed in chapter 23. Most arteries carry blood high in oxygen (except for the pulmonary arteries, as explained later), while most veins carry blood low in oxygen (except for the pulmonary veins). The arteries and veins entering and leaving the heart are called the great vessels because of their relatively large diameter. The heart exhibits several related characteristics and functions: ■ ■ ■ Learning Objectives: 1. Identify and describe the basic features of the cardiovascular system. 2. Describe and trace the general patterns of the pulmonary and systemic circulations. 3. Identify the position and location of the heart. 4. Discuss the structure and function of the pericardium. Heart The heart’s anatomy ensures the unidirectional flow of blood through it. Backflow of blood is prevented by valves within the heart. The heart acts like two side-by-side pumps that work at the same rate and pump the same volume of blood; one directs blood to the lungs for gas exchange, while the other directs blood to body tissues for nutrient and respiratory gas delivery. The heart develops blood pressure through alternate cycles of heart wall contraction and relaxation. Blood pressure is the force of the blood pushing against the inside walls of the vessels. A minimum blood pressure is essential for pushing blood through the blood vessels. 22.1a Pulmonary and Systemic Circulations As the center of the cardiovascular system, the heart connects to blood vessels that transport blood between the heart and all body tissues. The two basic types of blood vessels are arteries (ar t́ er-ē), which carry blood away from the heart, and veins (vān), The cardiovascular system consists of two circulations: the pulmonary circulation and the systemic circulation (figure 22.1). The pulmonary (pŭl m ́ ō-nār-ē; pulmo = lung) circulation consists of the chambers on the right side of the heart (right atrium and right ventricle) as well as the pulmonary arteries and veins. This circulation conveys blood to the lungs via pulmonary arteries to reduce carbon dioxide and replenish oxygen levels in the blood before returning to Lung Capillaries Figure 22.1 Pulmonary veins Pulmonary Pulmonary arteries circulation Right atrium Right ventricle Vessels transporting oxygenated blood Vessels transporting deoxygenated blood Left atrium Left ventricle Aorta to systemic arteries Systemic veins Systemic circulation Cardiovascular System. The cardiovascular system is composed of the pulmonary circulation and the systemic circulation. The pulmonary circulation pumps blood from the right side of the heart through pulmonary vessels, to the lungs, and back to the left side of the heart. The systemic circulation pumps blood from the left side of the heart, through systemic vessels in peripheral tissues, and back to the right side of the heart. Capillaries Vessels involved in gas exchange mck78097_ch22_656-682.indd 657 2/14/11 4:29 PM 658 Chapter Twenty-Two Heart Trachea Sternal angle Left lung Right lung 2nd rib Aortic arch Left phrenic nerve Superior border Superior vena cava Right border Left border Sternum Right phrenic nerve Ascending aorta Pulmonary trunk Anterior interventricular artery Left ventricle Diaphragm Inferior border Right atrium Right coronary artery Diaphragm Right ventricle (a) Borders of the heart (b) Heart and lungs, anterior view Mediastinum Left lung Posterior Ascending aorta Pleura (cut) Pericardium (cut) Apex of heart Diaphragm (cut) Thoracic vertebra Right lung Left lung Aortic arch (cut) Heart (in mediastinum) Sternum Anterior (c) Serous membranes of the heart and lungs (d) Cross-sectional view Figure 22.2 Heart Position Within the Thoracic Cavity. The heart is in the mediastinum of the thoracic cavity. (a) An anterior view shows the position of the heart posterior to the anterior thoracic cage. The borders of the heart are labeled. (b) Cadaver photo of the heart within the mediastinum (anterior view). (c) Serous membranes (pericardium and pleura) surround the heart and lungs, respectively. (d) A cross-sectional view depicts the heart’s relationship to the other organs in the thoracic cavity. the heart in pulmonary veins. Blood returns to the left side of the heart, where it then enters the systemic circulation. The systemic (sis-tem ́ i k) circulation consists of the chambers on the left side of the heart (left atrium and left ventricle), along with all the other named blood vessels. It carries blood to all the peripheral organs and tissues of the body. Blood that is high in oxygen (oxygenated) from the left side of the heart is pumped into the aorta, the largest systemic artery in the body, and then into smaller systemic arteries. Gas is exchanged with tissues from the body’s smallest vessels, called capillaries. Systemic veins then carry blood that is low in oxygen (deoxygenated) and high in carbon dioxide and waste products back to the heart. Most veins merge and drain into the superior and inferior venae cavae (vē ́nē ca ́vē; sing., vena cava), which drain blood into the right atrium. There, the blood enters the pulmonary circulation, and the cycle repeats. mck78097_ch22_656-682.indd 658 W H AT D O Y O U T H I N K ? 1 ● We previously mentioned that arteries tend to carry oxygenated blood, but the pulmonary arteries are the exception. Why are the pulmonary arteries carrying deoxygenated blood? 22.1b Position of the Heart The heart is located left of the body midline posterior to the sternum in the mediastinum (figure 22.2). The heart is slightly rotated such that its right side or right border (primarily formed by the right atrium and ventricle) is located more anteriorly, while its left side or left border (primarily formed by the left atrium and ventricle) is located more posteriorly. The posterosuperior surface of the heart, formed primarily by the left atrium, is called the base. The pulmonary veins that enter the left atrium border this base. The superior border is formed by the great arterial trunks (ascending aorta and pulmonary trunk) and the superior 2/14/11 4:29 PM Chapter Twenty-Two Fibrous pericardium Parietal layer of serous pericardium Pericardial cavity Visceral layer of serous pericardium (epicardium) Fibrous pericardium it beats. The pericardial cavity is a potential space with just a thin lining of serous fluid. However, it may become a real space as described in the Clinical View: “Pericarditis.” W H AT D I D Y O U L E A R N? 1 ● 2 ● 3 ● 4 ● What is the basic distinction between arteries and veins? Contrast the pulmonary and systemic circulations. What is the difference between the base of the heart and its apex? Identify the layers of the pericardium. Why is the pericardial cavity described as a potential space? To demonstrate the almost frictionless movement of the heart within the pericardial sac, try the following demonstration: Pericardial cavity Heart wall Myocardium Endocardium Figure 22.3 Pericardium. The pericardium consists of an outer fibrous pericardium and an inner serous pericardium. The serous pericardium consists of a parietal layer, which adheres to the fibrous pericardium, and a visceral layer, which forms the epicardium of the heart. The space between the parietal and visceral layers of the serous pericardium is called the pericardial cavity. vena cava. The inferior, conical end is called the apex (ā ́peks; tip). It projects slightly anteroinferiorly toward the left side of the body. The inferior border is formed by the right ventricle. 22.1c Characteristics of the Pericardium The heart is contained within the pericardium (per-i-kar ́dē-um ̆ ), a fibrous sac and serous lining (figure 22.3). The pericardium restricts the heart’s movements so that it doesn’t bounce and move about in the thoracic cavity, and prevents the heart from overfilling with blood. The pericardium is composed of two parts. The outer portion is a tough, dense connective tissue layer called the fibrous pericardium. This layer is attached to both the diaphragm and the base of the great vessels. The inner portion is a thin, doublelayered serous membrane called the serous pericardium. The serous pericardium may be subdivided into (1) a parietal layer of serous pericardium that lines the inner surface of the fibrous pericardium, and (2) a visceral layer of serous pericardium (also called the epicardium) that covers the outside of the heart. The parietal and visceral layers reflect (fold back) along the great vessels, where these layers become continuous with one another. The thin space between the parietal and visceral layers of the serous pericardium is the pericardial cavity, into which serous fluid is secreted to lubricate the serous membranes and facilitate the almost frictionless, continuous movement of the heart when mck78097_ch22_656-682.indd 659 659 Study Tip! Parietal layer of serous pericardium Visceral layer of serous pericardium (epicardium) Heart 1. Obtain two glass microscope slides. Place them together and then try to slide them back-and-forth past each other. You will find that they stick together (even if they are very clean) and do not move relative to one another very easily! 2. Now, take two similar glass slides and place a very small drop of water onto the surface of one slide. Place the slides together, as before, and then try to slide them back-and-forth past each other. We predict that you have demonstrated to yourself that glass slides move easily past each other when a water drop is present between them. This ease of movement of two opposing surfaces parallels the sliding movement of the parietal and visceral pericardial surfaces when a thin layer of serous fluid is present between them. CLINICAL VIEW Pericarditis Pericarditis (per ́ i-kar-dı̄ ́t is; peri = around, kardia = heart, ites = inflammation) is an inflammation of the pericardium typically caused by viruses, bacteria, or fungi. Whatever the cause of pericarditis, the pericardium is inflamed. The inflammation causes an increase in capillary permeability. Thus, the capillaries become more “leaky,” resulting in fluid accumulation in the pericardial cavity. At this point, the potential space of the pericardial cavity becomes a real space as it fills with fluid and pus. In severe cases, the excess fluid accumulation limits the heart’s movement and keeps it from filling with an adequate amount of blood. The heart is unable to pump blood, leading to a medical emergency called cardiac tamponade and resulting in heart failure and death. Pericarditis typically occurs between the ages of 20 and 50. Fever and chest pain are frequent symptoms. Pericarditis pain is located over the center or left side of the chest, and may extend to the neck or left shoulder. Patients often describe the pain as piercing or “knifelike,” and say that breathing worsens it. In contrast, pain from a myocardial infarction typically is described as crushing. But although the two conditions are different, the diagnosis of myocardial infarction and pericarditis often may be confused, especially by the patients experiencing the symptoms. A helpful diagnostic finding in pericarditis is friction rub, a crackling or scraping sound heard with a stethoscope that is caused by the movement of the inflamed pericardial layers against each other. The inflammation results in the loss of the lubricating action of the serous membranes. 2/14/11 4:29 PM 660 Chapter Twenty-Two Heart 22.2 Anatomy of the Heart Learning Objectives: 1. Describe the external anatomy of the heart and its major vessels. 2. Observe and identify the internal anatomic characteristics of each heart chamber. 3. Distinguish how valves regulate blood flow through the heart. The heart is a relatively small, conical organ approximately the size of a person’s clenched fist. In the average normal adult, it weighs about 250 to 350 grams, but certain diseases may cause heart size to increase dramatically. 22.2a Heart Wall Structure The heart wall consists of three distinctive layers: an external epicardium, a middle myocardium, and an internal endocardium (figure 22.4). The epicardium (ep-i-kar ́dē-ŭm; epi = upon, kardia = heart) is the outermost heart layer and is also known as the visceral layer of the serous pericardium. The epicardium is composed of a serous membrane and areolar connective tissue. As we age, more fat is deposited in the epicardium, and so this layer becomes thicker and more fatty. The myocardium (mı̄-ō-kar ́dē-ŭm; mys = muscle) is the middle layer of the heart wall and is composed of cardiac muscle tissue. The myocardium is the thickest of the three heart wall layers. It lies deep to the epicardium and superficial to the endocardium. The myocardial layer is where myocardial infarctions (heart attacks) occur. The arrangement of cardiac muscle in the heart wall permits the compression necessary to pump large volumes of blood out of the heart. The internal surface of the heart and the external surfaces of the heart valves are covered by endocardium (en-dō-kar ́dē-um ̆ ; endon = within). The endocardium is composed of a simple squamous epithelium, called an endothelium, and a layer of areolar connective tissue. Figure 22.4 Organization of the Heart Wall. The heart wall is composed of an outer epicardium (visceral layer of the serous pericardium), a middle myocardium (cardiac muscle), and an inner endocardium (composed of areolar connective tissue and an endothelium). 22.2b External Heart Anatomy The heart is composed of four hollow chambers: two smaller atria and two larger ventricles (figure 22.5). The left and right atria (ā ́trē-ă; sing., atrium; entrance hall) are thin-walled chambers located superiorly. The anterior part of each atrium is a wrinkled, flaplike extension, called an auricle (au ́ri-kl; auris = ear) because it resembles an ear. The atria receive blood returning to the heart through both circulations: The right atrium receives blood from the systemic circulation, and the left atrium receives blood from the pulmonary circulation. Blood that enters an atrium is passed to the ventricle on the same side of the heart. The left and right ventricles (ven ́tri-kl; venter = belly) are the inferior chambers. Two large arteries, the pulmonary trunk and the aorta (ā-ōr ́ta )̆ , exit the heart at its superior border. The pulmonary trunk carries blood from the right ventricle into the pulmonary circulation, while the aorta conducts blood from the left ventricle into the systemic circulation. Both ventricles pump the same volume of blood per minute. The atria are separated from the ventricles externally by a relatively deep coronary sulcus (or atrioventricular sulcus) that extends around the circumference of the heart. The anterior interventricular (in-ter-ven-trik ́ū-lar̆ ) sulcus and the posterior interventricular sulcus are located between the left and right ventricles on the anterior and posterior surfaces of the heart, respectively. These sulci extend inferiorly from the coronary sulcus toward the heart apex. All sulci house blood vessels packed in adipose connective tissue. These vessels supply and drain the heart (discussed later in this chapter). 22.2c Internal Heart Anatomy: Chambers and Valves Figure 22.6 depicts the internal anatomy and structural organization of the four heart chambers: the right atrium, right ventricle, left atrium, and left ventricle. Each of these chambers plays a role in the continuous process of blood circulation. Important to their function are valves, epithelium-lined dense connective tissue cusps that permit the passage of blood in one direction and Simple squamous epithelium Areolar connective tissue and fat Epicardium (visceral layer of serous pericardium) Myocardium (cardiac muscle) Areolar connective tissue Endocardium Endothelium mck78097_ch22_656-682.indd 660 2/14/11 4:29 PM Chapter Twenty-Two Heart 661 Aortic arch Superior vena cava Ligamentum arteriosum Ascending aorta Left pulmonary artery Pulmonary trunk Branches of the right pulmonary artery Left pulmonary veins Right pulmonary veins Auricle of left atrium Left coronary artery Auricle of right atrium Circumflex artery Right atrium Great cardiac vein Right coronary artery (in coronary sulcus) Anterior interventricular artery Marginal artery Right ventricle In anterior interventricular sulcus Left ventricle Small cardiac vein Inferior vena cava Apex of heart Descending aorta Aortic arch Branches of the right pulmonary artery Ligamentum arteriosum Ascending aorta Left pulmonary vein Right pulmonary vein Pulmonary trunk Auricle of left atrium Left coronary artery Auricle of right atrium Right atrium Right coronary artery (in coronary sulcus) Anterior interventricular artery Marginal artery In anterior interventricular sulcus Left ventricle Right ventricle Apex of heart (a) Anterior view Figure 22.5 External Anatomy and Features of the Heart. (a) An illustration and a cadaver photo show the heart chambers and associated vessels and the apex of the heart in an anterior view. (continued on next page) mck78097_ch22_656-682.indd 661 2/14/11 4:29 PM 662 Chapter Twenty-Two Heart Aortic arch Descending aorta Superior vena cava Left pulmonary artery Right pulmonary artery Left pulmonary veins Right pulmonary veins Left atrium (forms base of heart) Coronary sinus (in coronary sulcus) Right atrium Inferior vena cava Right coronary artery Left ventricle Posterior interventricular artery Middle cardiac vein Apex of heart In posterior interventricular sulcus Right ventricle Aortic arch Left pulmonary artery Branches of right pulmonary artery Left pulmonary veins (collapsed) Right pulmonary veins (collapsed) Left atrium (forms base of heart) Right atrium Coronary sinus (in coronary sulcus) Inferior vena cava Left ventricle Right coronary artery Posterior interventricular artery Middle cardiac vein Apex of heart In posterior interventricular sulcus Right ventricle (b) Posterior view Figure 22.5 External Anatomy and Features of the Heart. (continued) (b) An illustration and a cadaver photo show the heart chambers and associated vessels and the base of the heart in a posterior view. mck78097_ch22_656-682.indd 662 2/14/11 4:29 PM Chapter Twenty-Two Heart 663 Aortic arch Ligamentum arteriosum Ascending aorta Left pulmonary artery Superior vena cava Pulmonary trunk Right pulmonary artery Left pulmonary veins Right pulmonary veins Left atrium Right auricle Aortic semilunar valve Fossa ovalis Interatrial septum Opening for coronary sinus Left atrioventricular valve Pulmonary semilunar valve Right atrium Opening for inferior vena cava Right atrioventricular valve Trabeculae carneae Interventricular septum Chordae tendineae Left ventricle Papillary muscle Septomarginal trabecula Right ventricle Inferior vena cava Descending aorta Aortic arch Ligamentum arteriosum Ascending aorta Superior vena cava Pulmonary trunk Right auricle Right atrium Fossa ovalis Interatrial septum Pectinate muscle Opening for inferior vena cava Pulmonary semilunar valve Right coronary artery Interventricular septum Right atrioventricular valve Left ventricle Chordae tendineae Trabeculae carneae Papillary muscle Right ventricle Coronal section, anterior view Figure 22.6 Internal Anatomy of the Heart. An illustration and a cadaver photo reveal the internal structure of the heart, including the valves and the musculature of the heart wall. mck78097_ch22_656-682.indd 663 2/14/11 4:29 PM 664 Chapter Twenty-Two Heart Table 22.1 Heart Valves Heart Valve Location Structure Function Right atrioventricular valve Between right atrium and right ventricle Three triangular-shaped cusps of dense connective tissue covered by endothelium; chordae tendineae attached to free edges Prevents backflow of blood into right atrium when ventricles contract Pulmonary semilunar valve Between right ventricle and pulmonary trunk Three semilunar cusps of dense connective tissue covered by endothelium; no chordae tendineae Prevents backflow of blood into right ventricle when ventricles relax Left atrioventricular valve Between left atrium and left ventricle Two triangular-shaped cusps of dense connective tissue covered by endothelium; chordae tendineae attached to free edges Prevents backflow of blood into left atrium when ventricles contract Aortic semilunar valve Between left ventricle and ascending aorta Three semilunar cusps of dense connective tissue covered by endothelium; no chordae tendineae Prevents backflow of blood into left ventricle when ventricles relax prevent its backflow (table 22.1). Valve cusps are the tapering projection of a cardiac valve, also called flaps or leaflets. When the flaps of the valves are forced closed during the cardiac cycle, they produce sounds: “lubb-dupp” heart sounds. The first sound heard with a stethoscope is the result of the AV valves closing; producing a “lubb” sound. The second sound is produced when the semilunar valves close; producing a “dupp” sound. (See Clinical View, “Heart Sounds,” on page 666 for a more detailed description.) Fibrous Skeleton The fibrous skeleton of the heart is located between the atria and the ventricles, and is formed from dense regular connective tissue (figure 22.7). The fibrous skeleton performs the following functions: ■ ■ ■ ■ Separates the atria and ventricles. Anchors heart valves by forming supportive rings at their attachment points. Provides electrical insulation between atria and ventricles. This insulation ensures that muscle impulses are not spread randomly throughout the heart, and thus prevents all of the heart chambers from beating at the same time. Provides a rigid framework for the attachment of cardiac muscle tissue. Right Atrium The right atrium receives venous blood from the systemic circulation and the heart muscle itself. Three major vessels empty into the right atrium: (1) The superior vena cava drains blood from the head, neck, upper limbs, and superior regions of the trunk; (2) the inferior vena cava drains blood from the lower limbs and trunk; and (3) the coronary sinus drains blood from the heart wall. The interatrial (in-ter-ā ́trē-a ̆l) septum forms a thin wall between the right and left atria. The posterior atrial wall is smooth, but the auricle and anterior wall exhibit obvious muscular ridges, called pectinate (pek ́ti-nāt; teeth of a comb) muscles. The structural differences in the anterior and posterior walls occur because the two walls formed from separate structures during embryonic development. Inspection of the interatrial septum reveals an oval depression called the fossa ovalis, also called the oval fossa. It occupies the former location of the fetal foramen ovale, which shunted blood from the right atrium to the left atrium during fetal life, as described later in this chapter. Separating the right atrium from the right ventricle is the right atrioventricular opening. This opening is covered by a right atrioventricular (AV) valve (also called the tricuspid valve, since mck78097_ch22_656-682.indd 664 Posterior Left atrioventricular valve Right atrioventricular valve Aortic semilunar valve Fibrous skeleton Openings to coronary arteries Pulmonary semilunar valve Anterior Figure 22.7 Fibrous Skeleton of the Heart. Four thick regions of strong dense regular connective tissue encircle the four heart valves and the origins of the pulmonary trunk and the aorta. This fibrous skeleton isolates the atria from the ventricles (so muscle impulses won’t randomly pass between them), stabilizes the heart valves, and provides an attachment site for cardiac muscle. This is a superior view of a transverse section. it has three triangular cusps). Deoxygenated venous blood flows from the right atrium, through the right atrioventricular opening when the valve is open, into the right ventricle. The right AV valve is forced closed when the right ventricle begins to contract, preventing blood from flowing back into the right atrium. Right Ventricle The right ventricle receives deoxygenated venous blood from the right atrium. An interventricular septum forms a thick wall between the right and left ventricles. The internal wall surface of each ventricle displays characteristic large, smooth, irregular muscular ridges, called the trabeculae carneae (tra -̆ bek ́ū-lē; trabs = beam; kar ́nē-ē; carne = flesh) (see figure 22.6). The right ventricle typically has three cone-shaped, muscular projections called papillary (pap ́i-lar̆ -ē; papilla = nipple) muscles, which anchor numerous thin strands of collagen fibers called chordae tendineae (kōr ́dē ten ́di-nē-ē). The chordae tendineae 2/14/11 4:29 PM Chapter Twenty-Two Heart 665 CLINICAL VIEW Valve Defects and Their Effects on Circulation Structural damage to the heart valves can impair blood circulation and lead to serious health problems. Damage may result from developmental abnormalities, infection, hypertension, or other cardiovascular problems. Valvular insufficiency, also termed valvular incompetence, occurs when one or more of the cardiac valves leaks (called “regurgitant flow”) because the valve cusps do not close tightly enough. Inflammation or disease may cause the free edges of the valve cusps to become scarred and constricted, allowing blood to regurgitate back through the valve. As the heart works to overcome the effect of the backflow, blood must be forced through the valve openings and this effort may cause heart enlargement. As a result, the heart must work harder to circulate the normal amount of blood. Valvular stenosis (ste-nō ́sis; narrowing) is scarring of the valve cusps so that they become rigid or partially fused and cannot open attach to the lower surface of cusps of the right AV valve and prevent the valve from everting and flipping into the atrium when the right ventricle is contracting. A muscle bundle called the septomarginal trabecula (see figure 22.6), or moderator band, connects the base of the anterior papillary muscle to the interventricular septum. At its superior end, the right ventricle narrows into a smooth-walled, conical region called the conus arteriosus. Beyond the conus arteriosus is the pulmonary semilunar valve, which marks the end of the right ventricle and the entrance into the pulmonary trunk. The pulmonary trunk divides shortly into right and left pulmonary arteries, which carry deoxygenated blood to the lungs. Semilunar valves are located within the walls of both ventricles immediately before the connection of the ventricle to the pulmonary trunk and aorta (see figure 22.6). Each valve is composed of three thin, half-moon-shaped, pocketlike semilunar cusps. As blood is pumped into the arterial trunks, it pushes against the cusps, forcing the valves open. When ventricular contraction ceases, blood is prevented from flowing back into the ventricles from the arterial trunk by first entering the pockets of the semilunar valves between the cusps and the chamber wall. This causes the cusps to “fill and expand” and meet at the artery center, effectively blocking blood backflow. completely. A stenotic valve is narrowed and presents resistance to the flow of blood, so that output from the affected chamber decreases. Often the affected chamber hypertrophies and dilates—both conditions that may have deleterious consequences. Heart function may become so reduced that the rest of the body cannot receive adequate blood flow. A primary cause of valvular stenosis is rheumatic heart disease. Rheumatic (roo-mat ́ ik) heart disease may follow a streptococcal infection of the throat. It results when antibodies produced to kill the bacteria cross-react with the body’s own connective tissue, thereby initiating an autoimmune disease. All parts of the heart are subject to injury, but the endocardium, the valve cusps, and the left AV valve are typically most affected. Significantly scarred and narrow valves must be surgically repaired or replaced. Patients with a history of rheumatic heart disease must take antibiotics before undergoing dental or medical procedures that are likely to introduce bacteria into the bloodstream. when the valve is open, into the left ventricle. The left AV valve is forced closed when the left ventricle begins to contract, preventing blood backflow into the left atrium. Left Ventricle The left ventricular wall is typically three times thicker than the right ventricular wall (figure 22.8). The left ventricle requires thicker walls to generate enough pressure to force the oxygenated blood that has returned to the heart from the lungs into the aorta and then through the entire systemic circulation. (The right ventricle, in Left Atrium Once gas exchange occurs in the lungs, the oxygenated blood travels through the pulmonary veins to the left atrium (see figure 22.6). The smooth posterior wall of the left atrium contains openings for approximately four pulmonary veins. Sometimes two of these vessels fuse prior to reaching the left atrium, thus decreasing the number of openings through the atrial wall. Like the right atrium, the left atrium also has pectinate muscles along its anterior wall as well as an auricle. Separating the left atrium from the left ventricle is the left atrioventricular opening. This opening is covered by the left atrioventricular (AV) valve (also called the bicuspid valve, since it has two triangular cusps). This valve is also sometimes called the mitral (mı̄ ́tra ̆l) valve, because the two triangular cusps resemble a miter (the headpiece worn by a bishop). Oxygenated blood flows from the left atrium, through the left atrioventricular opening mck78097_ch22_656-682.indd 665 Left ventricular wall Right ventricular wall Figure 22.8 Comparison of Right and Left Ventricular Wall Thickness. The wall of the left ventricle is about three times thicker than that of the right ventricle, because the left ventricle must generate a force sufficient to push blood through the systemic circulation to capillary beds. 2/14/11 4:29 PM 666 Chapter Twenty-Two Heart CLINICAL VIEW Heart Sounds By using a stethoscope, a physician can discern four normal heart sounds with each contraction: the two familiar sounds referred to as “lubb-dupp” and two minor sounds. The lubb sound signifies the closing of the AV valves, while the dupp sound signifies the closing of the semilunar valves. These heart sounds provide clinically important information about heart activity and the action of heart valves. The minor sounds are caused by contraction of the atria and flow of blood into the ventricles. usually the result of turbulence of the blood as it passes through the heart, and may be caused by valvular leakage, decreased valve flexibility or a misshapen valve. Sometimes heart murmurs are of little consequence, but all of them need to be evaluated to rule out a more serious heart problem. The place where sounds from each AV valve and each semilunar valve may best be heard does not correspond with the location of the valve (see figure) because some overlap of valve sounds occurs near their anatomic locations. ■ ■ ■ ■ The aortic semilunar valve is best heard in the second intercostal space to the right of the sternum. The pulmonary semilunar valve is best heard in the second intercostal space to the left of the sternum. The right AV valve is best heard by the right side of the inferior end of the sternum. The left AV valve is best heard near the apex of the heart (at the level of the left fifth intercostal space, about 9 centimeters from the midline of the sternum). Often, abnormal heart sounds, generally called a heart murmur, are the first indication of heart problems. These sounds may be heard before, during, or after normal heart sounds. A heart murmur is contrast, merely has to pump blood to the nearby lungs.) The trabeculae carneae in the left ventricle are more prominent than in the right ventricle. Typically, two large papillary muscles project from the ventricle’s inner wall and anchor the chordae tendineae that attach to the cusps of the left AV valve. At the superior end of the ventricular cavity, the aortic semilunar valve marks the end of the left ventricle and the entrance into the aorta (see figure 22.7). W H AT D I D Y O U L E A R N? 5 ● 6 ● 7 ● Where is the coronary sulcus located? What are two similarities and two differences between the right and left ventricles? What is the composition and function of the chordae tendineae? 22.3 Coronary Circulation Learning Objective: 1. Identify and describe the location, origins, and branches of the coronary blood vessels. Left and right coronary arteries travel within the coronary sulcus of the heart to supply the heart wall (figure 22.9a). These arteries are the only branches of the ascending aorta. The openings for these arteries are located in the wall of the ascending aorta immediately superior to the aortic semilunar valve. mck78097_ch22_656-682.indd 666 Pulmonary semilunar valve Aortic semilunar valve Left atrioventricular valve Right atrioventricular valve Actual location of heart valve Area where valve sound is best heard Heart valve locations and auscultation (listening) sites. The right coronary artery typically branches into the right marginal artery, which supplies the right border of the heart, and the posterior interventricular artery, which supplies the posterior surface of both the left and right ventricles. The left coronary artery typically branches into the anterior interventricular artery (also called the left anterior descending artery), which supplies the anterior surface of both ventricles and most of the interventricular septum, and the circumflex (ser ́ k u m ̆ fleks; around) artery, which supplies the left atrium and ventricle. This arterial pattern can vary greatly among individuals. For example, some people may have a posterior interventricular artery that is a branch of the left coronary artery. Knowledge of this variation is essential when treating individuals for coronary artery disease. The coronary arteries are considered functional end arteries (see chapter 23). In other words, while the left and right coronary arteries share some tiny connections, called anastomoses, functionally they act like end arteries, which have no anastomoses and are the “end of the line” when it comes to arterial blood flow. The anastomoses allow the coronary arteries to shunt a tiny amount of blood from one artery to another. However, if one of the arteries becomes blocked, as happens with coronary artery disease, these anastomoses are too tiny to shunt sufficient blood from one artery to the other. For example, if a branch of the left coronary artery becomes blocked, the right coronary artery cannot shunt enough blood to the part of the heart supplied by this branch. As a result, the part of the heart 2/14/11 4:29 PM Chapter Twenty-Two Heart 667 Aortic arch Superior vena cava Pulmonary trunk Left coronary artery Aortic semilunar valve Left atrium Right atrium Circumflex artery Anterior interventricular artery Right coronary artery Branches of right coronary artery Posterior interventricular artery Branches of left coronary artery Right marginal artery Right ventricle Left ventricle (a) Coronary arteries Aortic arch Superior vena cava Pulmonary trunk Left atrium Right atrium Coronary sinus Middle cardiac vein Great cardiac vein Small cardiac vein Right ventricle Left ventricle (b) Coronary veins Figure 22.9 Coronary Circulation. Anterior view of (a) coronary arteries and (b) coronary veins that transport blood to and from the cardiac muscle tissue. wall that was supplied by the branch of the left coronary artery dies due to lack of blood flow to the tissue. Venous return occurs through one of several cardiac veins (figure 22.9b). The great cardiac vein runs alongside the anterior interventricular artery; the middle cardiac vein runs alongside the posterior interventricular artery; and the small cardiac vein travels close to the right marginal artery. These cardiac veins all drain into the coronary sinus, a large vein that lies in the posterior aspect of mck78097_ch22_656-682.indd 667 the coronary sulcus. The coronary sinus drains directly into the right atrium of the heart. Because the ventricular myocardium is compressed during contraction, most coronary flow occurs during ventricular relaxation. Normally, flow is evenly distributed throughout the thickness of the myocardium. Under certain circumstances, however, coronary flow may be reduced, especially to the regions immediately external to the endocardium. Situations related to 2/14/11 4:29 PM 668 Chapter Twenty-Two CLINICAL VIEW: IEW: Heart In Depth Angina Pectoris and Myocardial Infarction The most common cause of death in the United States is coronary atherosclerosis (ath ́er-ō-skler-ō ́sis; athere = gruel, sclerosis = hardness), or coronary heart disease (see Clinical View, “Atherosclerosis,” in chapter 23). This condition is characterized by narrowing of the coronary arteries that reduces blood flow to the myocardium and gives rise to chest pain. Coronary atherosclerosis can lead to either angina pectoris or myocardial infarction. Angina pectoris (an ́ j ı̄-nă, an-ji ́na) is not a disease, it is actually a symptom of coronary artery disease caused by narrowing or blockage of coronary arteries. Generally it results from strenuous activity, when workload demands on the heart exceed the ability of the narrowed coronary vessels to supply blood. The pain from angina is typically referred along the sympathetic pathways (T1–T5 spinal cord segments), so an individual may experience pain in the chest region or down the left arm, where the T1 dermatome is located. The pain diminishes shortly after the person stops inadequate coronary blood flow include tachycardia (tak ́i-kar ́dē-a ̆; tachys = quick), an increased heart rate that shortens diastole, and hypotension (low blood pressure), which reduces the ability of blood to flow through the ventricular myocardium. W H AT D O Y O U T H I N K ? 2 ● Do the coronary arteries fill with blood when the ventricles contract or when they relax? W H AT D I D Y O U L E A R N? 8 ● Why are coronary arteries considered functional end arteries? the exertion, and normal blood flow to the heart is restored. Although many people are successfully treated for years with medications that cause temporary vascular dilation, such as nitroglycerine, the prognosis and longterm therapy for angina depend on the severity of the vascular narrowing. Myocardial infarction (in-fark ́shŭn) (MI), commonly called a heart attack, is a potentially fatal condition resulting from sudden and complete occlusion (blockage) of a coronary artery. A region of the myocardium is deprived of oxygen, and some of this tissue may die (necrose). The symptoms of MI are often different for men and women. Most men experiencing an MI report a sudden, excruciating, and crushing substernal chest pain, and marked sweating, Many women, however, have relatively little chest pain. When they experience pain, they describe it as an “aching,” “tightness,” or “pressure,” rather than pain, and the main locations are in the back and high chest. In addition, women more often experience shortness of breath, but because they do not exhibit the traditional or classic symptoms that some doctors expect, it is likely for a female to be sent home from the ER with an incorrect diagnosis of heartburn or anxiety, rather than MI. cells that usually house one or two central nuclei and numerous mitochondria for ATP supply (figure 22.10). Cardiac muscle cells are arranged in spiral bundles and wrapped around and between the heart chambers. Cardiac muscle tissue exhibits some characteristics that are similar to those of skeletal muscle and others that are different (table 22.2). For example, the cells in both tissue types are striated, with extensive capillary networks that supply needed nutrients and oxygen. However, cardiac and smooth muscle differ in the following ways: ■ ■ The sarcoplasmic reticulum in cardiac muscle is less extensive and not as organized as in skeletal muscle. Cardiac muscle has no terminal cisternae, while skeletal muscle does. 22.4 How the Heart Beats: Electrical Properties of Cardiac Tissue Table 22.2 Comparison of Cardiac and Skeletal Muscle Cardiac Muscle Skeletal Muscle Learning Objectives: Cells are short and branching Cells are long and cylindrical One or two nuclei in the center of the cell Multiple nuclei at the periphery of the cell Cells joined by intercellular junctions in intercalated discs Cells do not have specialized intercellular junctions The efficient pumping of blood through the heart and blood vessels requires precisely coordinated contractions of the heart chambers. These contractions are made possible by the properties of cardiac muscle tissue and by specialized cells in the heart, known collectively as its conducting system. Functional contractile unit is the sarcomere Functional contractile unit is the sarcomere T-tubules overlie Z discs T-tubules overlie A band/I band junctions Composed of thick and thin filaments Composed of thick and thin filaments 22.4a Characteristics of Cardiac Muscle Tissue Contains sarcoplasmic reticulum but less than in skeletal muscle Contains sarcoplasmic reticulum but more than in cardiac muscle The myocardium is composed of cardiac muscle, which was described briefly in chapters 4 (see table 4.14) and 10 (see table 10.6). Recall that cardiac muscle cells are relatively short, branched More mitochondria than in skeletal muscle Fewer mitochondria than in cardiac muscle 1. Distinguish between, and compare, cardiac muscle and skeletal muscle. 2. Trace the conduction of muscle impulses along muscle fibers. 3. Describe autorhymicity and the heart’s conducting system. mck78097_ch22_656-682.indd 668 2/14/11 4:29 PM Chapter Twenty-Two Intercalated disc Openings of transverse tubules Heart 669 Intercalated disc Desmosome Gap junction Cardiac muscle cell Sarcolemma Nucleus Mitochondrion (a) Cross section of cardiac muscle cell Sarcomere Intercalated discs Striations Sarcolemma Transverse tubule Sarcoplasmic reticulum Nucleus Mitochondrion Myofibrils LM 1000x Z disc I band H zone M line Z disc (c) Longitudinal section of cardiac muscle I band A band (b) Cardiac muscle cell, longitudinal view Figure 22.10 Organization and Histology of Cardiac Muscle. Cardiac muscle cells form the myocardium. (a) Individual cells are relatively short, branched, and striated. They are connected to adjacent cells by intercalated discs. (b) Transverse tubules are invaginations of the sarcolemma that surround myofibrils and overlie the Z disc. The sarcoplasmic reticulum is meager in cardiac muscle compared to its abundance in skeletal muscle. (c) Light micrograph of cardiac muscle in longitudinal section. ■ ■ ■ Cardiac muscle lacks the extensive association of smooth endoplasmic reticulum (SER) and transverse tubules (T-tubules) present in skeletal muscle. In cardiac muscle, T-tubules overlie Z discs instead of the junctions of A bands and I bands as seen in skeletal muscle. The T-tubules in cardiac muscle have a less extensive distribution and a reduced association with SER compared mck78097_ch22_656-682.indd 669 to those in skeletal muscle, allowing for both the delayed onset and prolonged contraction of cardiac muscle tissue. 22.4b Contraction of Heart Muscle Cardiac muscle cells contract as a single unit because muscle impulses (changes in voltage potential across the sarcolemma [see chapter 10]) are distributed immediately and simultaneously 2/14/11 4:29 PM 670 Chapter Twenty-Two Heart Superior vena cava Right atrium Left atrium Sinoatrial node (pacemaker) Internodal pathway Internodal pathway Atrioventricular node Atrioventricular bundle (bundle of His) Interventricular septum Right bundle Purkinje fibers Purkinje fibers Left bundles Atrioventricular node Atrioventricular bundle 1 Muscle impulse is generated at the sinoatrial node. It spreads throughout the atria and to the atrioventricular node by the internodal pathway. 2 Atrioventricular node cells delay the muscle impulse as it passes to the atrioventricular bundle. Atrioventricular bundle Interventricular septum Left and right bundle branches 3 The atrioventricular bundle (bundle of His) conducts the muscle impulse into the interventricular septum. Purkinje fibers 4 Within the interventricular septum, the right and left bundles split from the atrioventricular bundle. 5 The muscle impulse is delivered to Purkinje fibers in each ventricle and distributed throughout the ventricular myocardium. Figure 22.11 Conducting System of the Heart. Modified cardiac muscle fibers initiate the heartbeat and then spread and conduct the impulse throughout the heart. throughout all cells of first the atria and then the ventricles. Neighboring cardiac muscle cells in the walls of heart chambers have formed specialized cell–cell contacts called intercalated discs (in-ter ́ k a -̆ lā-ted disk), which electrically and mechanically link the cells together and permit the immediate passage of muscle impulses (figure 22.10). Within the intercalated discs, gap junctions increase the flow of ions between the cells as the muscle impulse moves along the sarcolemma. The gap junctions of intercalated discs provide a low-resistance pathway across the membranes of adjoining cardiac muscle cells, allowing the unrestricted passage of ions required for the synchronous beating of cardiac muscle cells. Numerous desmosomes (see figure 4.1) within the intercalated discs prevent cardiac muscle cells from pulling apart. Therefore, cardiac muscle cells function as a single, coordinated unit; the precisely timed stimulation and response by cardiac muscle cells of both the atria and the ventricles are dependent upon these structural features. mck78097_ch22_656-682.indd 670 22.4c The Heart’s Conducting System The heart exhibits autorhythmicity, meaning that the heart itself (not external nerves) is responsible for initiating the heartbeat. Certain cardiac muscle cells are specialized to initiate and conduct muscle impulses to the contractile muscle cells of the myocardium. Collectively, these specialized cells are called the heart’s conducting system (figure 22.11). The heartbeat is initiated by the specialized cardiac muscle cells of the sinoatrial (SA) node, which are located in the posterior wall of the right atrium, adjacent to the entrance of the superior vena cava. The cells of the SA node act as the pacemaker, the rhythmic center that establishes the pace for cardiac activity. Under the influence of parasympathetic innervation, SA node cells initiate impulses 70 to 80 times per minute. The muscle impulse travels from the SA node to the atrioventricular (AV) node. The AV node is located in the floor of the right atrium between the right AV valve and the opening for the coronary sinus. The AV node normally slows conduction of the impulse as it 2/14/11 4:29 PM Chapter Twenty-Two Heart 671 CLINICAL VIEW The Electrocardiogram Electrical currents within the heart can be detected during a routine physical examination using monitoring electrodes attached to the skin—usually at the wrist, ankles, and six separate locations on the chest. The electrical signals are collected and charted as an electrocardiogram (ē-lek-trō-kar ́ dē-ō-gram; gramma = drawing), also called an ECG or EKG. When readings from the different electrodes are compared, they collectively provide an accurate, comprehensive assessment of the electrical activity of the heart. An ECG provides a composite tracing of all muscle impulses generated by myocardial cells. It records the wave of change in voltage (potential) across the sarcolemma that (1) originates in the SA node, (2) radiates through both of the atria to the AV node, (3) passes through the AV node and the interventricular septum to the heart apex, and (4) stimulates the Purkinje fibers in the ventricular myocardium. The progress of impulse transmission through the various parts of the conducting system is mirrored in an electrocardiogram. A typical ECG tracing for one heart cycle has three principal deflections: a P wave above the baseline, a QRS complex that begins (Q) and ends (S) with small downward deflection from the baseline and has a large deflection (R) above the baseline, and a T wave above the baseline (see figure). These waves are indicators of depolarization and repolarization within specific regions of the heart: 1. The P wave is generated when the impulse originating in the SA node depolarizes the cells of the atria. 2. The QRS complex identifies the beginning of depolarization of the ventricles. Simultaneously, the atria repolarize; however, this repolarization signal is masked by the electrical activity of the ventricles. 3. The T wave is a small, rounded peak that denotes ventricular repolarization. 0.8 second R Millivolts +1 1 P wave 3 T wave 0 Q S 2 QRS complex -1 The events of a single cardiac cycle as recorded on an electrocardiogram. travels from the atria to the ventricles, providing a delay between activation and contraction of the ventricles. Recall that the fibrous skeleton insulates the atria from the ventricles to prevent random muscle impulses from spreading between the atria and the ventricles. There is an opening in the fibrous skeleton that allows the AV node to communicate with the next part of the conduction system, the atrioventricular (AV) bundle. This bundle, also known as the bundle of His, receives the muscle impulse from the AV node and extends into the interventricular septum before dividing into left and right bundles. These bundles conduct the impulse to conduction fibers called Purkinje (pur̆ -kin ́ jē) cells that begin within the apex of the heart and extend through the walls of the ventricles. Purkinje fibers are larger than other cardiac muscle cells. Muscle impulse conduction mck78097_ch22_656-682.indd 671 along the Purkinje fibers is extremely rapid, consistent with the large size of the cells, and the impulse spreads immediately throughout the ventricular myocardium. W H AT D I D Y O U L E A R N? 9 ● 10 ● 11 ● What are three of the ways in which cardiac muscle differs from skeletal muscle? Which component of the conducting system is located in the floor of the right atrium, between the right AV valve and the coronary sinus opening? What is meant by the term autorhythmic when used to describe the heart? 2/14/11 4:29 PM 672 Chapter Twenty-Two Heart CLINICAL VIEW Cardiac Arrhythmia Cardiac arrhythmia (ă-rith ́mē-ă; a = not, rhythmos = rhythm), also called dysrhythmia, is any abnormality in the rate, regularity, or sequence of the cardiac cycle. Several common arrhythmias have been described: ■ ■ ■ Atrial flutter occurs when the atria attempt to beat at a rate of 200 to 400 times per minute, and as a consequence literally bombard the AV node with muscle impulses. Abnormal muscle impulses flow continuously through the atrial conduction system, thus stimulating the atrial musculature and AV node over and over. This condition may persist for years, and frequently degenerates into atrial fibrillation. Atrial fibrillation (fı̄-bri-lā ́shŭn) differs from atrial flutter in that the muscle impulses are significantly more chaotic, leading to an irregular heart rate. The ventricles respond by increasing and decreasing contraction activities, which may lead to serious disturbances in the cardiac rhythm. Premature ventricular contractions (PVCs) often result from stress, stimulants such as caffeine, or sleep deprivation. They occur either singly or in rapid bursts due to abnormal impulses 22.5 Innervation of the Heart Learning Objective: 1. Describe and explain how the sympathetic and parasympathetic divisions of the autonomic nervous system regulate heart rate. The heart is innervated by the autonomic nervous system (figure 22.12). This innervation consists of sympathetic and initiated within the AV node or the ventricular conduction system. All of us experience an occasional PVC, and they are not detrimental unless they occur in great numbers. Most PVCs go unnoticed, although occasionally one is perceived as the heart “skipping a beat” and then “jumping” in the chest. A more serious arrhythmia is ventricular fibrillation, a rapid, repetitious movement of the ventricular muscle that replaces normal contraction. This is a life-threatening condition caused by scattered impulses originating at different times and places throughout the entire myocardium. Because the contractions of a heart in fibrillation are uncoordinated, the heart does not pump blood, and blood circulation stops. This cessation of cardiac activity is called cardiac arrest. Fibrillation almost certainly results in death unless the normal rhythmic contractions of the heart are promptly restored. To restore normal heart contractions, medical personnel apply a strong electrical shock to the skin of the chest using paddle electrodes. The electrical current passes through the chest wall to completely and immediately depolarize the entire myocardium. This procedure is analogous to pushing the reset button on a computer—and as in rebooting the computer, the hope is that when the heart begins to function again, it will work as intended. parasympathetic components, collectively referred to as the coronary plexus. The innervation by autonomic centers in the brainstem doesn’t initiate a heartbeat, but it can increase or decrease the rate of the heartbeat. Sympathetic innervation arises from the T1–T5 segments of the spinal cord. Preganglionic axons enter the sympathetic trunk and ascend into the thoracic and cervical portions, where they synapse on ganglionic neurons. Postganglionic axons project from the superior, middle, and inferior cervical ganglia and the T1–T5 ganglia, Autonomic centers Parasympathetic Vagal nucleus Medulla oblongata Sympathetic Figure 22.12 Autonomic Innervation of the Heart. The amount of blood pumped from the heart and the heart rate are modified by autonomic centers in the brainstem. (Left) Sympathetic stimulation is carried through the cardiac nerves. (Right) Parasympathetic stimulation is carried by vagus nerves. mck78097_ch22_656-682.indd 672 Cervical sympathetic ganglion Vagus nerve (CN X) Spinal cord Sympathetic preganglionic axon Parasympathetic preganglionic axon Sympathetic postganglionic axon Cardiac nerve 2/14/11 4:29 PM Chapter Twenty-Two and travel directly to the heart through cardiac nerves. Sympathetic innervation increases the rate and the force of heart contractions. Parasympathetic innervation comes from the medulla oblongata via the left and right vagus nerves (CN X). As the vagus nerves descend into the thoracic cavity, they give off branches that supply the heart. Parasympathetic innervation decreases the heart rate, but generally tends to have no effect on the force of contractions. Although there is a particularly rich sympathetic and parasympathetic innervation of the SA and AV nodes, the working myocardial cells are also supplied by both types of autonomic axons. Study Tip! Here is one way to help you remember the effects of sympathetic and parasympathetic innervation on the heart: Sympathetic innervation Speeds the heart rate. Parasympathetic innervation does the oPposite (decreases the heart rate). W H AT D I D Y O U L E A R N? 12 ● How does the sympathetic division of the autonomic nervous system affect the heart rate? 22.6 Tying It All Together: The Cardiac Cycle Learning Objectives: 1. Identify and describe the events in the cardiac cycle. 2. Trace the pattern of blood flow through the heart. A cardiac cycle is the time from the start of one heartbeat to the initiation of the next. During a single cardiac cycle, all chambers within the heart experience alternate periods of contraction and relaxation. The contraction of a heart chamber is called systole (sis ́tō-lē). During this period, the contraction of the myocardium forces blood either into another chamber (from atrium to ventricle) or into a blood vessel (from a ventricle into the attached large artery). The relaxation phase of a heart chamber is termed diastole (dı̄-as ́tō-lē; dilation). During this period between contraction phases, the myocardium of each chamber relaxes, and the chamber fills with blood. At the beginning of the cardiac cycle, the left and right atria contract simultaneously. When the atria contract (atrial systole), blood is forced into the ventricles through the open AV valves. During this time, blood is still returning to the atria in the superior vena cava, inferior vena cava, and coronary sinus (right atrium) and pulmonary veins (left atrium). After the atria begin to relax (atrial diastole), left and right ventricular contraction (ventricular systole) occurs (figure 22.13a). Thus, only two of the four chambers (either the atria or the ventricles) contract at the same time. When the ventricles contract, the atrioventricular openings close as blood pushes against the cusps of the AV valves, and their edges meet to form a seal. Papillary muscles and the chordae tendineae prevent these valve cusps from everting. The semilunar valves are forced open, and blood enters the pulmonary trunk and the aorta. When the ventricles are relaxing during the cardiac cycle (ventricular diastole), most of the blood flows passively from the relaxing atria into the ventricles through the open mck78097_ch22_656-682.indd 673 Heart 673 AV valves (figure 22.13b). Therefore, for the last half of the cardiac cycle, all four chambers are in diastole together. W H AT D O Y O U T H I N K ? 3 ● What could happen if the AV valves were to evert into the atria? 22.6a Steps in the Cardiac Cycle The events in a normal cardiac cycle are shown in figure 22.14 and described here: 1. Atrial systole occurs at the beginning of the cardiac cycle. It is a brief contraction of the atrial myocardium initiated by the heart pacemaker. Contraction of the atria finishes filling the ventricles through the open AV valves while the ventricles are in diastole. The semilunar valves remain closed. 2. Early ventricular systole is the beginning of the ventricular contraction. The atria remain in diastole. The AV valves are forced closed (producing the “lubb” sound), and the semilunar valves remain closed. 3. Late ventricular systole occurs later in the ventricular contraction. The atria remain in diastole, and the AV valves remain closed. Pressure on blood in the ventricles forces the semilunar valves to open, and blood is ejected into the arterial trunks. 4. Early ventricular diastole is the start of ventricular relaxation. The atria remain in diastole. The semilunar valves close to prevent blood backflow into the ventricles (producing the “dupp” sound). The AV valves remain closed. 5. Late ventricular diastole is a continuation of ventricular relaxation and an important time for ventricular filling. The atria remain in diastole. The AV valve opens, and passive filling of the ventricle from the atria begins and continues as most of the ventricular filling occurs. The semilunar valves remain closed. 22.6b Summary of Blood Flow During the Cardiac Cycle As the heart chambers cyclically contract and relax, pressure on the blood in the chambers alternately increases and decreases. Table 22.3 illustrates the flow of blood through the four heart chambers during the cardiac cycle. As you learn this sequence, keep the following general principles in mind: ■ ■ ■ Blood flows from veins into the atria under low pressure. Blood only passes from the atria into the ventricles if the AV valves are open. Most of the ventricular filling is passive (about 70%), occurring when both chambers are relaxing (in diastole) and the atrial pressure is greater than the ventricular pressure. Filling of the final 30% of the ventricles occurs when the atria contract (in systole). Ventricular contraction (systole) increases pressure on the blood within the ventricles. When ventricular pressure rises significantly, the AV valves close, and the semilunar valves are forced open due to increased pressure in the ventricles, allowing blood to enter the large arterial trunks. W H AT D I D Y O U L E A R N? 13 ● 14 ● Distinguish between systole and diastole. What events occur in the ventricles during late ventricular diastole? 2/14/11 4:29 PM 674 Chapter Twenty-Two Heart (a) Ventricular Systole (Contraction) Aortic arch Blood flow into ascending aorta Ascending aorta Pulmonary trunk Blood flow into right atrium Blood flow into pulmonary trunk Right atrium Left atrium Ventricular contraction pushes blood against the open AV valves, causing them to close. Contracting papillary muscles and the chordae tendineae prevent valve flaps from everting into atria. Ventricles contract, forcing semilunar valves to open and blood to enter the pulmonary trunk and the ascending aorta. Semilunar valves open Atrioventricular valves closed Right ventricle Cusp of atrioventricular valve Left ventricle Cusp of semilunar valve Blood in ventricle Posterior Left AV valve (closed) Right AV valve (closed) Left ventricle Right ventricle Aortic semilunar valve (open) Pulmonary semilunar valve (open) Anterior Transverse ansverse secti section Figure 22.13 Ventricular Systole and Ventricular Diastole. (a) The semilunar valves open during ventricular systole to allow blood to flow into the large arteries; the AV valves close during ventricular systole to prevent backflow of blood into the atria. (b) During ventricular diastole, the AV valves open to allow blood to enter the ventricles from the atria; the semilunar valves remain closed to prevent backflow of blood into the ventricles from the large arteries. Transverse sections in both (a) and (b) show a superior view. mck78097_ch22_656-682.indd 674 2/14/11 4:29 PM Chapter Twenty-Two Heart 675 (b) Ventricular Diastole (Relaxation) Aortic arch Blood flow into right atrium Blood flow into left ventricle Right atrium Left atrium During ventricular relaxation, some blood in the ascending aorta and pulmonary trunk flows back toward the ventricles, filling the semilunar valve cusps and forcing them to close. Blood flow into right ventricle Ventricles relax and fill with blood both passively and then by atrial contraction as AV valves remain open. Semilunar valves closed Atrioventricular valves open Atrium Right ventricle Cusp of atrioventricular valve Left ventricle Blood Cusps of semilunar valve Chordae tendineae Papillary muscle Posterior Left AV valve (open) Right AV valve (open) Left ventricle Right ventricle Aortic semilunar valve (closed) Pulmonary semilunar valve (closed) Anterior Transverse section mck78097_ch22_656-682.indd 675 2/14/11 4:29 PM 676 Chapter Twenty-Two Heart Atria contract Atria relax Atria relax Semilunar valves open AV valves closed All valves closed AV valves open Ventricles contract Ventricles contract 2 Early ventricular systole Atria relax; ventricles begin to contract; AV valves are forced closed (lubb sound); semilunar valves still closed 3 Late ventricular systole Atria continue to relax; ventricles contract; AV valves remain closed; semilunar valves are forced open Ventricles relax 1 Atrial systole Atria contract; AV valves are open, semilunar valves are closed Phase Atrial systole Structure Atria Late ventricular systole Early ventricular systole Early ventricular diastole Late ventricular diastole Contract Relax Relax Ventricles Relax Contract Relax AV valves Open Semilunar valves Closed Closed Time (seconds) 0.0 0.1 Open Open 0.2 0.3 Closed 0.4 0.5 Atria relax 0.6 0.7 0.8 Atria relax Semilunar valves closed AV valves open All valves closed Ventricles relax 5 Late ventricular diastole Atria and ventricles relax; atria continue passively filling with blood; AV valves open and ventricles begin to passively fill; semilunar valves remain closed Ventricles relax 4 Early ventricular diastole Atria and ventricles relax; AV valves remain closed and semilunar valves close (dupp sound); atria continue passively filling with blood Figure 22.14 Cardiac Cycle. The cardiac cycle consists of all of the events that occur with a single heartbeat: contraction (systole) and relaxation (diastole) of all four heart chambers. mck78097_ch22_656-682.indd 676 2/14/11 4:29 PM Chapter Twenty-Two Table 22.3 Systemic veins Heart Blood Flow Through the Heart Superior and inferior venae cavae Right atrium Right atrioventricular valve Right ventricle Pulmonary semilunar valve Gas and nutrient exchange in peripheral tissues Systemic arteries 677 Pulmonary trunk and arteries Gas exchange in the lungs Aortic semilunar valve Aorta Left ventricle Left atrioventricular valve Left atrium Pulmonary veins Chamber of the Heart Receives Blood From Sends Blood To Valves Through Which Blood Flows Right atrium Superior vena cava, inferior vena cava, coronary sinus Right ventricle Right AV valve Right ventricle Right atrium Pulmonary trunk (blood enters vessels of pulmonary circulation) Pulmonary semilunar valve Left atrium Pulmonary veins Left ventricle Left AV valve Left ventricle Left atrium Ascending aorta (blood enters vessels of systemic circulation) Aortic semilunar valve 22.7 Aging and the Heart 22.8 Development of the Heart Learning Objective: Learning Objective: 1. Explain how heart function changes as we age. A healthy heart is capable of quickly and efficiently altering both the heartbeat rate and the volume of blood pumped during either an increase or decrease in activity. Consuming a diet low in saturated fats, abstaining from smoking, and exercising regularly help maintain a strong, vigorous heart. However, the decreased flexibility and elasticity of connective tissue that occur with aging can cause the heart valves to become slightly inflexible. As a result, a heart murmur may develop, and blood flow through the heart may be altered. Decreased conducting system efficiency reduces the heart’s ability to pump the extra blood needed during stress and exercise. In addition, the muscular wall of the ventricle increases in thickness when high blood pressure causes the ventricles to work harder to pump blood into the arterial trunks. Consequently, the ventricular myocardium undergoes hypertrophy. Hypertrophy of the heart may have different causes (such as hypertension or narrowing of vessels connected to the heart), but cardiac muscle cells always thicken and lose the normal arrangement of formed cell bundles. Thus, although the heart is enlarged, it works less efficiently. W H AT D I D Y O U L E A R N? 15 ● What are some age-related changes that result in altered heart functions? mck78097_ch22_656-682.indd 677 1. Trace and describe the formation of postnatal heart structures from the primitive heart tube. Development of the heart commences in the third week, when the embryo becomes too large to receive its nutrients through diffusion alone. At this time, the embryo needs its own blood supply, heart, and blood vessels for transporting oxygen and nutrients through its growing body. The steps involved in heart development are complex, because the heart must begin working before its development is complete. By day 19 (middle of week 3), two heart tubes (or endocardial tubes) form from mesoderm in the embryo. By day 21, these paired tubes fuse, forming a single primitive heart tube (figure 22.15). This tube develops the following named expansions that ultimately give rise to postnatal heart structures (listed from inferior to superior): sinus venosus, primitive atrium, primitive ventricle, and bulbus cordis (bŭl ́bŭs kōr ́dis). The sinus venosus and primitive atrium form parts of the left and right atria. The primitive ventricle forms most of the left ventricle. The bulbus cordis may be further subdivided into a trabeculated part of the right ventricle, which forms most of the right ventricle; the conus cordis, which forms the outflow tracts for the ventricles; and the truncus arteriosus, which forms the ascending aorta and pulmonary trunk. Table 22.4 lists these primitive heart tube components and the structures they develop into. By day 22, the primitive heart begins to beat and begins its process of bending and folding. The heart folding is complete by 2/14/11 4:29 PM 678 Chapter Twenty-Two Heart Aortic arch 1 Aortic arch 2 Truncus arteriosus Truncus arteriosus Bulbus cordis Bulbus cordis Primitive ventricle Fusing paired heart tubes form a primitive heart tube Primitive atrium Sinus venosus Primitive ventricle Primitive atrium Unfused heart tubes Sinus venosus (a) 21 days: Paired heart tubes fuse. (b) 22 days: Primitive heart tube begins to fold. (c) 28 days: S-shaped heart tube completes folding. Figure 22.15 Development of the Heart. The heart develops from mesoderm. By day 19, paired heart tubes are present in the cardiogenic region of the embryo. (a) These paired tubes fuse by day 21 to form a primitive heart tube. (b) The primitive heart tube bends and folds upon itself, beginning on day 22. (c) By day 28, the heart tube is S-shaped. Table 22.4 Primitive Heart Tube Components and Their Postnatal Structures Superior vena cava Heart Tube Component Postnatal Derivative Sinus venosus Superior vena cava, coronary sinus, smooth posterior wall of right atrium Primitive atrium Anterior muscular portions of left and right atria Primitive ventricle Most of left ventricle Foramen ovale Trabeculated part of right ventricle Most of right ventricle Endocardial cushion Conus cordis Outflow tracts from ventricles to aorta and pulmonary trunk Truncus arteriosus Ascending aorta, pulmonary trunk Bulbus cordis Septum secundum Blood flow Right atrium Septum primum Left atrium Left ventricle Right ventricle Interventricular septum Inferior vena cava the end of the week (figure 22.15b). The bulbus cordis is pulled inferiorly, anteriorly, and to the embryo’s right, while the primitive ventricle moves left to reposition. The primitive atrium and sinus venosus reposition superiorly and posteriorly. Thus, by day 28 the heart tube is S-shaped (figure 22.15c). The next major steps in heart development occur during weeks 5–8, when the single heart tube becomes partitioned into four chambers (two atria and two ventricles), and the main vessels entering and leaving the heart form. The common atrium is subdivided into a left and right atrium by an interatrial septum, which consists of two parts (septum primum and septum secundum) that partially overlap. These two parts connect to tissue masses called endocardial cushions. An opening in the septum secundum (which is covered by the septum primum) is called the foramen ovale (ō-val ́ē) (figure 22.16). Because the embryonic lungs are mck78097_ch22_656-682.indd 678 Early week 7 (43 days) Figure 22.16 Interatrial Septum. The interatrial septum is formed by two overlapping septa (primum and secundum). The foramen ovale is a passageway that detours blood away from the pulmonary circulation into the systemic circulation prior to birth. 2/14/11 4:29 PM Chapter Twenty-Two not functional, much of the blood is shunted away from the lungs and to the rest of the body. Since it is the right side of the heart that sends blood to the lungs, blood is shunted from the right atrium to the left atrium by traveling through the foramen ovale and pushing the septum primum to the left. Blood cannot flow back from the left atrium to the right, because the septum primum’s movement to the right is stopped when it comes against the septum secundum. Thus, the septum primum acts as a unidirectional flutter valve. When the baby is born and the lungs are fully functional, the blood from the left atrium pushes the septum primum and secundum together, creating a closed interatrial septum. The only remnant of the embryonic opening is an oval-shaped depression in the interatrial septum called the fossa (fos ́a ̆; trench) ovalis. Left and right ventricles are partitioned by an interventricular septum that grows superiorly from the floor of the ventricles. The AV valves, papillary muscles, and chordae tendineae all form from portions of the ventricular walls as well. The superior part of the interventricular septum develops from the Heart 679 aorticopulmonary septum, which is a spiral-shaped mass that also subdivides the truncus arteriosus into the pulmonary trunk and the ascending aorta. Many congenital heart malformations result from incomplete or faulty development during these early weeks. For example, in an atrial septal defect the postnatal heart still has an opening between the left and right atria. Thus, blood from the left atrium (the higher-pressure system) is shunted to the right atrium (the lower-pressure system). This can lead to enlargement of the right side of the heart. Ventricular septal defects can occur if the interventricular septum is incompletely formed. A common malformation called tetralogy of Fallot occurs when the aorticopulmonary septum divides the truncus arteriosus unevenly. As a result, the patient has a ventricular septal defect, a very narrow pulmonary trunk (pulmonary stenosis), an aorta that overlaps both the left and right ventricles, and an enlarged right ventricle (right ventricular hypertrophy). A good knowledge of heart development is essential in understanding these congenital heart malformations. Clinical Terms bradycardia (brad-ē-kar ́dē-ă; bradys = slow) Slowing of the heartbeat, usually described as less than 50 beats per minute. cardiomyopathy (kar ́dē-ō-mı̄-op ́ă-thē) Another term for disease of the myocardium; causes vary and include thickening of the ventricular septum (hypertrophy), secondary disease of the myocardium, or sometimes a disease of unknown cause. endocarditis (en ́dō-kar-dı̄ ́tis) Inflammation of the endocardium. Types include bacterial (caused by the direct invasion of bacteria), chorditis (affecting the chordae tendineae), infectious (caused by microorganisms), mycotic (due to infection by fungi), and rheumatic (due to endocardial involvement as part of rheumatic heart disease). ischemia (is-kē ́mē-ă; ischo = to keep back) Inadequate blood flow to a structure caused by obstruction of the blood supply, usually due to arterial narrowing or disruption of blood flow. myocarditis (mı̄ ́ō-kar-dı̄ ́tis) Inflammation of the muscular walls of the heart. This uncommon disorder is caused by viral, bacterial, or parasitic infections, exposure to chemicals, or allergic reactions to certain medications. Chapter Summary 22.1 Overview of the Cardiovascular System 657 ■ Arteries carry blood away from the heart, and veins return blood to the heart. ■ Heart functions include one-directional blood flow through the cardiovascular system, coordinated side-by-side pumps, and generation of pressure to drive blood through blood vessels. 22.1a Pulmonary and Systemic Circulations ■ The pulmonary circulation conveys blood to and from the lungs, and the systemic circulation carries blood to and from all the organs and tissues. 22.1b Position of the Heart 658 ■ The heart is located posterior to the sternum in the mediastinum. ■ Its base is the posterosuperior surface formed primarily by the left atrium; the apex is in the conical, inferior end. 22.1c Characteristics of the Pericardium 22.2 Anatomy of the Heart 660 657 659 ■ The pericardium that encloses the heart has an outer fibrous portion and an inner serous portion. ■ The pericardial cavity is a thin space between the layers of the serous pericardium. Pericardial fluid produced by the serous membranes lubricates the surfaces to reduce friction. ■ The heart is a small, cone-shaped organ. 22.2a Heart Wall Structure ■ 660 The heart wall has an epicardium (visceral pericardium), myocardium (thick layer of cardiac muscle), and endocardium (thin endothelium and areolar connective tissue). 22.2b External Heart Anatomy 660 ■ The heart has four chambers: Two smaller atria receive blood returning to the heart, and two larger ventricles pump blood away from the heart. ■ Around the circumference of the heart, a deep coronary sulcus separates the atria and ventricles. Shallow sulci extend from the coronary sulcus on the anterior and posterior surfaces between the ventricles. Coronary vessels lie within the sulci. (continued on next page) mck78097_ch22_656-682.indd 679 2/14/11 4:29 PM 680 Chapter Twenty-Two Heart Chapter Summary (continued) 22.2 Anatomy of the Heart 660 (continued) 22.3 Coronary Circulation 666 22.4 How the Heart Beats: Electrical Properties of Cardiac Tissue 668 22.2c Internal Heart Anatomy: Chambers and Valves 660 ■ Dense regular connective tissue forms the fibrous skeleton of the heart that separates the atria and the ventricles. This skeleton (1) separates atria and ventricles, (2) anchors heart valves, (3) electrically insulates atria and ventricles, and (4) provides for attachment of cardiac muscle tissue. ■ The right atrium receives blood from the superior vena cava, the inferior vena cava, and the coronary sinus. ■ Atria are separated by an interatrial septum, and ventricles are separated by an interventricular septum. ■ Four pulmonary veins empty into the left atrium. ■ Thick-walled ventricles receive blood from the atria through open AV valves. The free edges of the AV valve cusps are prevented from everting into the atria by the chordae tendineae. ■ Trabeculae carneae are large, irregular muscular ridges on the inside of the ventricle wall. ■ Papillary muscles are cone-shaped projections on the inner surface of the ventricles that anchor the chordae tendineae. ■ Semilunar valves are located within the wall of each ventricle near its connection to a large artery. The right ventricle houses the pulmonary semilunar valve, and the left ventricle houses the aortic semilunar valve. ■ The major coronary artery branches are the anterior interventricular and circumflex arteries from the left coronary artery, and the right marginal and posterior interventricular arteries from the right coronary artery. ■ Venous return is through the cardiac veins into the coronary sinus, which drains into the right atrium of the heart. 22.4a Characteristics of Cardiac Muscle Tissue 668 ■ Cardiac muscle cells are small and branched. They form sheets of cardiac muscle tissue arranged into spiral bundles wrapped around and between the heart chambers. ■ Intercalated discs tightly link the muscle cells together and permit the immediate passage of muscle impulses. 22.4b Contraction of Heart Muscle 669 ■ The heart exhibits autorhythmicity. Its stimulus to contract is initiated by cells of the sinoatrial (SA) node in the roof of the right atrium. ■ Muscle impulses travel to the atrioventricular (AV) node and then through the AV bundle within the interventricular septum to Purkinje fibers in the heart apex. 22.4c The Heart’s Conducting System 670 22.5 Innervation of the Heart 672 ■ Sympathetic and parasympathetic innervation of the heart have opposing influences on heart rate. Autonomic innervation increases or decreases the rate of the heartbeat, but does not initiate it. 22.6 Tying It All Together: The Cardiac Cycle 673 ■ A cardiac cycle is the period of time from the start of one heartbeat to the beginning of the next. ■ All chambers within the heart experience alternate periods of contraction (systole) and relaxation (diastole) during a single cycle. 22.6a Steps in the Cardiac Cycle 673 22.6b Summary of Blood Flow During the Cardiac Cycle 673 ■ Cyclic contraction and relaxation of heart chambers result in the pumping of blood out of the chambers and the subsequent refilling of the chambers with blood for the next cycle. 22.7 Aging and the Heart 677 ■ Decreased flexibility of heart structures as we age reduces the efficiency of cardiac function. Increased blood pressure results in hypertrophy of the myocardium. 22.8 Development of the Heart 677 ■ Development of the heart commences in the third week; two heart tubes form and fuse to become a single primitive heart tube. ■ The primitive heart tube develops expansions that later form postnatal heart structures. mck78097_ch22_656-682.indd 680 2/14/11 4:29 PM Chapter Twenty-Two Heart 681 Challenge Yourself Matching Match each numbered item with the most closely related lettered item. ______ 1. right marginal artery ______ 2. pulmonary veins ______ 3. left AV valve ______ 4. diastole cells ______ 5. SA node ______ 6. venae cavae ______ 7. intercalated disc ______ 8. right AV valve ______ 9. circumflex artery ______ 10. systole a. veins that carry blood to right atrium b. period of relaxation c. contains three cusps; also known as tricuspid valve d. specialized junction between cardiac muscle cells e. veins that carry blood to left atrium f. period of contraction g. branch of right coronary artery h. origin of heartbeat i. branch of left coronary artery j. also known as bicuspid or mitral valve Multiple Choice Select the best answer from the four choices provided. ______ 1. Muscle impulses are spread rapidly between cardiac muscle cells by a. sarcomeres. b. intercalated discs. c. chemical neurotransmitters. d. AV valves. ______ 2. Venous blood from the heart wall enters the right atrium through the a. superior vena cava. b. coronary sinus. c. inferior vena cava. d. pulmonary veins. ______ 3. How is blood prevented from flowing into the right ventricle from the pulmonary trunk? a. closing of the right AV valves b. opening of the pulmonary semilunar valve c. contraction of the right atrium d. closing of the pulmonary semilunar valve mck78097_ch22_656-682.indd 681 ______ 4. Which of the following is the correct circulatory sequence for blood to pass through part of the heart? a. R. atrium → right AV valve → R. ventricle → pulmonary semilunar valve b. R. atrium → left AV valve → R. ventricle → pulmonary semilunar valve c. L. atrium → right AV valve → L. ventricle → aortic semilunar valve d. L. atrium → left AV valve → L. ventricle → pulmonary semilunar valve ______ 5. The pericardial cavity is located between the a. fibrous pericardium and the parietal layer of the serous pericardium. b. parietal and visceral layers of the serous pericardium. c. visceral layer of the serous pericardium and the epicardium. d. myocardium and the visceral layer of the serous pericardium. ______ 6. In the developing heart, the atria form from the primitive atrium and the a. sinus venosus. b. bulbus cordis. c. primitive ventricle. d. conus cordis. ______ 7. The irregular muscular ridges in the ventricular walls are the a. papillary muscles. b. trabeculae carneae. c. chordae tendineae. d. moderator bands. ______ 8. Sympathetic innervation of cardiac muscle originates from a. CN X (vagus nerve). b. L1–L2 segments of the spinal cord. c. the AV node. d. T1–T5 segments of the spinal cord. ______ 9. When the ventricles contract, all of the following occur except a. closing of the AV valves. b. blood ejecting into the pulmonary trunk and aorta. c. closing of the semilunar valves. d. opening of the semilunar valves. ______ 10. The thickest part of the heart wall is the a. pericardium. b. epicardium. c. myocardium. d. endocardium. 2/14/11 4:29 PM 682 Chapter Twenty-Two Heart Content Review 1. What are the differences between the pulmonary and systemic circulations? 2. What chamber walls primarily form the anterior side of the heart? What chamber walls form the posterior side of the heart? 3. Compare the structure, location, and function of the parietal and visceral layers of the serous pericardium. 4. Where is the fibrous skeleton of the heart located? What are its functions? 5. Why are the chordae tendineae required for the proper functioning of the AV valves? 6. Explain why the walls of the atria are thinner than those of the ventricles, and why the walls of the right ventricle are relatively thin when compared to the walls of the left ventricle. 7. Identify and compare the branches of the right and left coronary arteries. In general, what portions of the heart does each branch supply? 8. Compare cardiac and skeletal muscle. In what ways are these muscle types similar? In which ways are they different? 9. Describe the functional differences in the effects of the sympathetic and parasympathetic divisions of the autonomic nervous system on the activity of cardiac muscle. 10. What are the phases of the cardiac cycle? Describe each phase with respect to which heart chambers are in systole or diastole, which valves are open or closed, and blood flow into or out of the chambers. Developing Critical Reasoning 1. It was the end of the semester, and Huang had begun to prepare for his final examinations. Unfortunately, he still had to work his full-time job. In order to find sufficient time to study, he stayed up late and drank large amounts of coffee to stay alert. One evening during a very late study session, Huang felt a pounding in his chest and thought he was having a heart attack. His roommate took Huang to the emergency room. After an examination and interview by the physician, Huang was told that he probably had a cardiac arrhythmia. What was the most probable cause of the arrhythmia? 2. Josephine is a 55-year-old overweight woman who has a poor diet and does not exercise. One day while walking briskly, she experienced pain in her chest and down her left arm. Her doctor told her she was experiencing angina due to heart problems. Josephine asks you to explain what causes angina, and why she was feeling pain in her arm even though the problem was with her heart. What do you tell her? Answers to “What Do You Think?” 1. The pulmonary arteries carry blood low in oxygen to the lungs because the lungs are responsible for replenishing the oxygen supplies in the blood. After deoxygenated blood travels to the lungs, the pulmonary capillaries are involved in gas exchange—that is, carbon dioxide is removed from the blood, and oxygen enters the blood. Then the pulmonary veins carry the newly oxygenated blood back to the heart. 3. If the AV valves were to evert into the atria, some of the blood from the ventricles would be pushed into the atria, resulting in inefficient pumping of blood. When the heart has to work harder to pump the blood out of the heart, clinical problems result (see Clinical View: “Valve Defects and Their Effects on Circulation” for further information). 2. When the ventricles contract, they also constrict the coronary arteries. Thus, the coronary arteries can fill with blood only when the ventricles are relaxed. www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more. mck78097_ch22_656-682.indd 682 2/14/11 4:29 PM