CARDIOVASCULAR SYSTEM

OUTLINE

22.1 Overview of the Cardiovascular System 657 22.1a Pulmonary and Systemic Circulations 657 22.1b Position of the Heart 658 22 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 Heart 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 668 22.4b Contraction of Heart Muscle 669 22.4c The Heart’s Conducting System 670 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: CARDIOVASCULAR SYSTEM

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n chapter 21, we discovered the importance of blood and the which carry blood back to the heart. The differences between I myriad of substances it carries. To maintain homeostasis, blood these types of vessels are discussed in chapter 23. Most arteries must circulate continuously throughout the body. The continual carry blood high in oxygen (except for the pulmonary arteries, pumping action of the heart is essential for maintaining blood as explained later), while most veins carry blood low in oxygen circulation. If the heart fails to pump adequate volumes of blood, (except for the pulmonary veins). The arteries and veins entering cells are deprived of needed oxygen and nutrients, waste products and leaving the heart are called the great vessels because of their accumulate, and cell death occurs. relatively large diameter. The heart exhibits several related charac- In a healthy, 80-kilogram resting adult, the heart beats about 75 teristics and functions: times per minute (about 4500 times per hour or 108,000 times per day). ■ The heart’s anatomy ensures the unidirectional flow of The amount of blood pumped from one ventricle per minute (about blood through it. Backflow of blood is prevented by valves 5.25 liters [L] at rest) is called the cardiac output. When the body is within the heart. more active, and the cells need oxygen and nutrients delivered at a ■ The heart acts like two side-by-side pumps that work at the faster pace, the heart can increase its output up to five- or six-fold. same rate and pump the same volume of blood; one directs blood to the lungs for gas exchange, while the other directs 22.1 Overview of the Cardiovascular blood to body tissues for nutrient and respiratory gas delivery. ■ The heart develops blood pressure through alternate cycles System of heart wall contraction and relaxation. Blood pressure Learning Objectives: is the force of the blood pushing against the inside walls of the vessels. A minimum blood pressure is essential for 1. Identify and describe the basic features of the pushing blood through the blood vessels. cardiovascular system. 2. Describe and trace the general patterns of the pulmonary 22.1a Pulmonary and Systemic Circulations and systemic circulations. The cardiovascular system consists of two circulations: the pulmo- 3. Identify the position and location of the heart. nary circulation and the systemic circulation (figure 22.1). The 4. Discuss the structure and function of the pericardium. pulmonary (pŭ l ḿō -nā r-ē; pulmo = lung) circulation consists of the As the center of the cardiovascular system, the heart con- chambers on the right side of the heart (right atrium and right ven- nects to blood vessels that transport blood between the heart and tricle) as well as the pulmonary arteries and veins. This circulation all body tissues. The two basic types of blood vessels are arteries conveys blood to the lungs via pulmonary arteries to reduce carbon (ar ter-́ ē ), which carry blood away from the heart, and veins (vā n), dioxide and replenish oxygen levels in the blood before returning to

Lung

Capillaries

Figure 22.1 Cardiovascular System. The cardiovascular system is composed Pulmonary veins Left atrium of the pulmonary circulation and the Pulmonary Pulmonary arteries Left ventricle systemic circulation. The pulmonary Right atrium circulation pumps blood from the right circulation Aorta to Right ventricle systemic Systemic side of the heart through pulmonary arteries circulation vessels, to the lungs, and back to the Systemic left side of the heart. The systemic veins circulation pumps blood from the left side of the heart, through systemic vessels in peripheral tissues, and back Vessels transporting to the right side of the heart. oxygenated blood Capillaries Vessels transporting deoxygenated blood Vessels involved in gas exchange

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Trachea Sternal angle Right lung Left lung 2nd rib Aortic arch

Superior border Left phrenic Superior nerve vena cava Right border Right phrenic Pulmonary Left nerve Sternum trunk border Ascending aorta Diaphragm Anterior interventricular Inferior Right atrium artery border Left ventricle 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)

Thoracic Pericardium vertebra Right lung Left lung (cut) Aortic arch (cut) Apex of heart Heart (in mediastinum)

Diaphragm Sternum (cut) 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 WHAT DO YOU THINK? heart, where it then enters the systemic circulation. ●1 We previously mentioned that arteries tend to carry oxygenated ́ The systemic (sis-tem ik) circulation consists of the cham- blood, but the pulmonary arteries are the exception. Why are the bers on the left side of the heart (left atrium and left ventricle), pulmonary arteries carrying deoxygenated blood? along with all the other named blood vessels. It carries blood to all the peripheral organs and tissues of the body. Blood that 22.1b Position of the Heart is high in oxygen (oxygenated) from the left side of the heart is The heart is located left of the body midline posterior to the pumped into the aorta, the largest systemic artery in the body, sternum in the mediastinum (figure 22.2). The heart is slightly and then into smaller systemic arteries. Gas is exchanged with rotated such that its right side or right border (primarily formed tissues from the body’s smallest vessels, called capillaries. by the right atrium and ventricle) is located more anteriorly, while Systemic veins then carry blood that is low in oxygen (deoxygen- its left side or left border (primarily formed by the left atrium ated) and high in carbon dioxide and waste products back to the and ventricle) is located more posteriorly. The posterosuperior heart. Most veins merge and drain into the superior and inferior surface of the heart, formed primarily by the left atrium, is called venae cavae (vē ńē ca ́vē ; sing., vena cava), which drain blood the base. The pulmonary veins that enter the left atrium border into the right atrium. There, the blood enters the pulmonary cir- this base. The superior border is formed by the great arterial culation, and the cycle repeats. trunks (ascending aorta and pulmonary trunk) and the superior

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it beats. The pericardial cavity is a potential space with just a thin lining of serous fluid. However, it may become a real space Fibrous pericardium as described in the Clinical View: “Pericarditis.”

WHATW DID YOU LEARN? Parietal layer of serous pericardium ●1 What is the basic distinction between arteries and veins? Pericardial cavity ●2 Contrast the pulmonary and systemic circulations. Visceral layer of serous pericardium ●3 What is the difference between the base of the heart and its apex? (epicardium) ●4 Identify the layers of the pericardium. Why is the pericardial cavity described as a potential space?

Fibrous pericardium Parietal layer of Study Tip! serous pericardium To demonstrate the almost frictionless movement of the heart Pericardial cavity within the pericardial sac, try the following demonstration: Visceral layer of 1. Obtain two glass microscope slides. Place them together and serous pericardium (epicardium) then try to slide them back-and-forth past each other. You will Heart wall find that they stick together (even if they are very clean) and Myocardium do not move relative to one another very easily! Endocardium 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. 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, CLINICAL VIEW and a visceral layer, which forms the epicardium of the heart. The space between the parietal and visceral layers of the serous pericardium Pericarditis is called the pericardial cavity. Pericarditis (per ́i-kar-dı¯ ́tis; peri = around, kardia = heart, ites = inflammation) is an inflammation of the pericardium typically vena cava. The inferior, conical end is called the apex (ā peks;́ tip). caused by viruses, bacteria, or fungi. Whatever the cause of peri- It projects slightly anteroinferiorly toward the left side of the body. carditis, the pericardium is inflamed. The inflammation causes an The inferior border is formed by the right ventricle. increase in capillary permeability. Thus, the capillaries become 22.1c Characteristics of the Pericardium more “leaky,” resulting in fluid accumulation in the pericardial cavity. At this point, the potential space of the pericardial cav- The heart is contained within the pericardium (per-i-kar ́dē -ŭm), ity becomes a real space as it fills with fluid and pus. In severe a fibrous sac and serous lining (figure 22.3). The pericardium cases, the excess fluid accumulation limits the heart’s movement restricts the heart’s movements so that it doesn’t bounce and move and keeps it from filling with an adequate amount of blood. The about in the thoracic cavity, and prevents the heart from overfill- heart is unable to pump blood, leading to a medical emergency ing with blood. called cardiac tamponade and resulting in heart failure and death. The pericardium is composed of two parts. The outer por- tion is a tough, dense connective tissue layer called the fibrous Pericarditis typically occurs between the ages of 20 and 50. Fever pericardium. This layer is attached to both the diaphragm and and chest pain are frequent symptoms. Pericarditis pain is located the base of the great vessels. The inner portion is a thin, double- over the center or left side of the chest, and may extend to the layered serous membrane called the serous pericardium. The neck or left shoulder. Patients often describe the pain as piercing serous pericardium may be subdivided into (1) a parietal layer or “knifelike,” and say that breathing worsens it. In contrast, pain of serous pericardium that lines the inner surface of the fibrous from a myocardial infarction typically is described as crushing. But pericardium, and (2) a visceral layer of serous pericardium (also although the two conditions are different, the diagnosis of myocardial called the epicardium) that covers the outside of the heart. The infarction and pericarditis often may be confused, especially by the parietal and visceral layers reflect (fold back) along the great patients experiencing the symptoms. A helpful diagnostic finding vessels, where these layers become continuous with one another. in pericarditis is friction rub, a crackling or scraping sound heard The thin space between the parietal and visceral layers of the with a stethoscope that is caused by the movement of the inflamed serous pericardium is the pericardial cavity, into which serous pericardial layers against each other. The inflammation results in the fluid is secreted to lubricate the serous membranes and facilitate loss of the lubricating action of the serous membranes. the almost frictionless, continuous movement of the heart when

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22.2b External Heart Anatomy 22.2 Anatomy of the Heart The heart is composed of four hollow chambers: two smaller atria Learning Objectives: and two larger ventricles (figure 22.5). The left and right atria (ā tŕē -ă ; sing., atrium; entrance hall) are thin-walled chambers 1. Describe the external anatomy of the heart and its major located superiorly. The anterior part of each atrium is a wrinkled, vessels. flaplike extension, called an auricle (au ri-kl;́ auris = ear) because 2. Observe and identify the internal anatomic characteristics it resembles an ear. The atria receive blood returning to the heart of each heart chamber. through both circulations: The right atrium receives blood from 3. Distinguish how valves regulate blood flow through the heart. the systemic circulation, and the left atrium receives blood from The heart is a relatively small, conical organ approximately the pulmonary circulation. Blood that enters an atrium is passed the size of a person’s clenched fist. In the average normal adult, to the ventricle on the same side of the heart. The left and right it weighs about 250 to 350 grams, but certain diseases may cause ventricles (ven tri-kl;́ venter = belly) are the inferior chambers. Two heart size to increase dramatically. large arteries, the pulmonary trunk and the aorta (ā- ō r t́ă), exit the heart at its superior border. The pulmonary trunk carries blood 22.2a Heart Wall Structure from the right ventricle into the pulmonary circulation, while the The heart wall consists of three distinctive layers: an external aorta conducts blood from the left ventricle into the systemic circu- epicardium, a middle myocardium, and an internal endocardium lation. Both ventricles pump the same volume of blood per minute. (figure 22.4). The atria are separated from the ventricles externally by a The epicardium (ep-i-kar d́ē -ŭm; epi = upon, kardia = heart) is relatively deep coronary sulcus (or atrioventricular sulcus) that the outermost heart layer and is also known as the visceral layer of the extends around the circumference of the heart. The anterior inter- serous pericardium. The epicardium is composed of a serous mem- ventricular (in-ter-ven-trik ́ū-lă r) sulcus and the posterior inter- brane and areolar connective tissue. As we age, more fat is deposited ventricular sulcus are located between the left and right ventricles in the epicardium, and so this layer becomes thicker and more fatty. on the anterior and posterior surfaces of the heart, respectively. The myocardium (mı̄ -ō -kar d́ē -ŭm; mys = muscle) is the middle These sulci extend inferiorly from the coronary sulcus toward the layer of the heart wall and is composed of cardiac muscle tissue. The heart apex. All sulci house blood vessels packed in adipose con- myocardium is the thickest of the three heart wall layers. It lies deep nective tissue. These vessels supply and drain the heart (discussed to the epicardium and superficial to the endocardium. The myocar- later in this chapter). dial layer is where myocardial infarctions (heart attacks) occur. The arrangement of cardiac muscle in the heart wall permits the compres- 22.2c Internal Heart Anatomy: Chambers and Valves sion necessary to pump large volumes of blood out of the heart. Figure 22.6 depicts the internal anatomy and structural orga- The internal surface of the heart and the external surfaces of nization of the four heart chambers: the right atrium, right ven- the heart valves are covered by endocardium (en-dō -kar ́dē -ŭm; tricle, left atrium, and left ventricle. Each of these chambers plays endon = within). The endocardium is composed of a simple squa- a role in the continuous process of blood circulation. Important to mous epithelium, called an endothelium, and a layer of areolar their function are valves, epithelium-lined dense connective tis- connective tissue. sue cusps that permit the passage of blood in one direction and

Simple squamous epithelium Figure 22.4 Epicardium Areolar connective Organization of the Heart Wall. (visceral layer of tissue and fat serous pericardium) 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). Myocardium (cardiac muscle)

Areolar connective tissue Endothelium Endocardium

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Aortic arch Superior vena cava Ligamentum Ascending aorta arteriosum Left pulmonary artery Branches of the Pulmonary trunk right pulmonary artery Left pulmonary veins Right pulmonary Auricle of left atrium veins Left coronary artery Auricle of right atrium Circumflex artery

Right atrium Great cardiac vein Right coronary artery In anterior (in coronary sulcus) Anterior interventricular interventricular sulcus artery Marginal artery

Right ventricle Left ventricle Small cardiac vein

Inferior vena cava Apex of heart

Descending aorta

Aortic arch

Branches of the Ligamentum arteriosum right pulmonary artery

Ascending aorta Left pulmonary vein Right pulmonary Pulmonary trunk vein Auricle of left atrium Left coronary artery

Auricle of right atrium

Right atrium

Right coronary artery Anterior (in coronary sulcus) interventricular In anterior artery interventricular Marginal artery sulcus

Right ventricle Left 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)

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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 In posterior Middle cardiac vein interventricular sulcus

Apex of heart 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 In posterior Middle cardiac vein Apex of heart 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.

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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 Left atrioventricular valve Opening for coronary sinus Pulmonary semilunar Right atrium valve Opening for inferior vena cava Trabeculae carneae Right atrioventricular valve Interventricular septum Chordae tendineae Left ventricle Papillary muscle Right ventricle Septomarginal trabecula 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 Pulmonary semilunar valve vena cava

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.

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Table 22.1 Heart Valves Heart Valve Location Structure Function Right atrioventricular valve Between right atrium and right Three triangular-shaped cusps of Prevents backfl ow of blood into right ventricle dense connective tissue covered by atrium when ventricles contract endothelium; chordae tendineae attached to free edges Pulmonary semilunar valve Between right ventricle and Three semilunar cusps of dense Prevents backfl ow of blood into right pulmonary trunk connective tissue covered by ventricle when ventricles relax endothelium; no chordae tendineae Left atrioventricular valve Between left atrium and left ventricle Two triangular-shaped cusps of Prevents backfl ow of blood into left dense connective tissue covered by atrium when ventricles contract endothelium; chordae tendineae attached to free edges Aortic semilunar valve Between left ventricle and ascending Three semilunar cusps of dense Prevents backfl ow of blood into left aorta connective tissue covered by ventricle when ventricles relax endothelium; no chordae tendineae

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 Posterior semilunar valves close; producing a “dupp” sound. (See Clinical View, “Heart Sounds,” on page 666 for a more detailed description.) Fibrous Skeleton Right Left atrioventricular atrioventricular The fibrous skeleton of the heart is located between the atria and valve valve the ventricles, and is formed from dense regular connective tissue (figure 22.7). The fibrous skeleton performs the following functions: Aortic semilunar valve Fibrous ■ Separates the atria and ventricles. Openings to coronary skeleton ■ Anchors heart valves by forming supportive rings at their arteries attachment points. Pulmonary semilunar valve ■ Provides electrical insulation between atria and ventricles. This insulation ensures that muscle impulses are not spread Anterior randomly throughout the heart, and thus prevents all of the heart chambers from beating at the same time. Figure 22.7 ■ Provides a rigid framework for the attachment of cardiac Fibrous Skeleton of the Heart. Four thick regions of strong dense muscle tissue. regular connective tissue encircle the four heart valves and the origins of the pulmonary trunk and the aorta. This fibrous skeleton isolates Right Atrium the atria from the ventricles (so muscle impulses won’t randomly pass The right atrium receives venous blood from the systemic circula- between them), stabilizes the heart valves, and provides an attachment tion and the heart muscle itself. Three major vessels empty into site for cardiac muscle. This is a superior view of a transverse section. the right atrium: (1) The superior vena cava drains blood from the head, neck, upper limbs, and superior regions of the trunk; (2) the it has three triangular cusps). Deoxygenated venous blood flows inferior vena cava drains blood from the lower limbs and trunk; from the right atrium, through the right atrioventricular opening and (3) the coronary sinus drains blood from the heart wall. when the valve is open, into the right ventricle. The right AV valve The interatrial (in-ter-ā tŕē -ăl) septum forms a thin wall is forced closed when the right ventricle begins to contract, pre- between the right and left atria. The posterior atrial wall is smooth, venting blood from flowing back into the right atrium. but the auricle and anterior wall exhibit obvious muscular ridges, called pectinate (pek ti-ń ā t; teeth of a comb) muscles. The struc- Right Ventricle tural differences in the anterior and posterior walls occur because The right ventricle receives deoxygenated venous blood from the two walls formed from separate structures during embryonic the right atrium. An interventricular septum forms a thick wall development. Inspection of the interatrial septum reveals an oval between the right and left ventricles. The internal wall surface depression called the fossa ovalis, also called the oval fossa. It of each ventricle displays characteristic large, smooth, irregular occupies the former location of the fetal foramen ovale, which muscular ridges, called the trabeculae carneae (tră-bek ́ū-lē ; trabs = shunted blood from the right atrium to the left atrium during fetal beam; kar ńē -ē; carne = flesh) (see figure 22.6). life, as described later in this chapter. The right ventricle typically has three cone-shaped, muscular Separating the right atrium from the right ventricle is the projections called papillary (pap ́i-lăr-ē; papilla = nipple) muscles, right atrioventricular opening. This opening is covered by a right which anchor numerous thin strands of collagen fibers called atrioventricular (AV) valve (also called the tricuspid valve, since chordae tendineae (kō r ́dē ten ́di-nē -ē). The chordae tendineae

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CLINICAL VIEW Valve Defects and completely. A stenotic valve is narrowed and presents resistance to the Their Effects on Circulation flow of blood, so that output from the affected chamber decreases. Often the affected chamber hypertrophies and dilates—both conditions Structural damage to the heart valves can impair blood circulation and that may have deleterious consequences. Heart function may become lead to serious health problems. Damage may result from developmental so reduced that the rest of the body cannot receive adequate blood abnormalities, infection, hypertension, or other cardiovascular problems. flow. A primary cause of valvular stenosis is rheumatic heart disease. Valvular insufficiency, also termed valvular incompetence, occurs when Rheumatic (roo-mat ́ik) heart disease may follow a streptococ- one or more of the cardiac valves leaks (called “regurgitant flow”) because cal infection of the throat. It results when antibodies produced to the valve cusps do not close tightly enough. Inflammation or disease may kill the bacteria cross-react with the body’s own connective tissue, cause the free edges of the valve cusps to become scarred and constricted, thereby initiating an autoimmune disease. All parts of the heart are allowing blood to regurgitate back through the valve. As the heart works subject to injury, but the endocardium, the valve cusps, and the left to overcome the effect of the backflow, blood must be forced through the AV valve are typically most affected. Significantly scarred and narrow valve openings and this effort may cause heart enlargement. As a result, valves must be surgically repaired or replaced. Patients with a history the heart must work harder to circulate the normal amount of blood. of rheumatic heart disease must take antibiotics before undergoing dental or medical procedures that are likely to introduce bacteria into Valvular stenosis (ste-no¯ ́sis; narrowing) is scarring of the valve the bloodstream. 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 when the valve is open, into the left ventricle. The left AV valve is the valve from everting and flipping into the atrium when the right forced closed when the left ventricle begins to contract, preventing ventricle is contracting. A muscle bundle called the septomarginal blood backflow into the left atrium. trabecula (see figure 22.6), or moderator band, connects the base of the anterior papillary muscle to the interventricular septum. At Left Ventricle its superior end, the right ventricle narrows into a smooth-walled, The left ventricular wall is typically three times thicker than the conical region called the conus arteriosus. Beyond the conus arte- right ventricular wall (figure 22.8). The left ventricle requires riosus is the pulmonary semilunar valve, which marks the end thicker walls to generate enough pressure to force the oxygenated of the right ventricle and the entrance into the pulmonary trunk. blood that has returned to the heart from the lungs into the aorta and The pulmonary trunk divides shortly into right and left pulmonary then through the entire systemic circulation. (The right ventricle, in arteries, which carry deoxygenated blood to the lungs. Semilunar valves are located within the walls of both ventri- cles immediately before the connection of the ventricle to the pul- monary 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 block- ing blood backflow. Left Atrium Left ventricular Once gas exchange occurs in the lungs, the oxygenated blood wall travels through the pulmonary veins to the left atrium (see figure 22.6). The smooth posterior wall of the left atrium contains open- ings for approximately four pulmonary veins. Sometimes two of these vessels fuse prior to reaching the left atrium, thus decreas- Right ventricular wall ing 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 Figure 22.8 atrioventricular (AV) valve (also called the bicuspid valve, since it Comparison of Right and Left Ventricular Wall Thickness. has two triangular cusps). This valve is also sometimes called the The wall of the left ventricle is about three times thicker than that of the mitral (mı̄ tŕăl) valve, because the two triangular cusps resemble right ventricle, because the left ventricle must generate a force sufficient a miter (the headpiece worn by a bishop). Oxygenated blood flows to push blood through the systemic circulation to capillary beds. from the left atrium, through the left atrioventricular opening

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CLINICAL VIEW Heart Sounds 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 By using a stethoscope, a physician can discern four normal heart sounds a misshapen valve. Sometimes heart murmurs are of little consequence, but with each contraction: the two familiar sounds referred to as “lubb-dupp” all of them need to be evaluated to rule out a more serious heart problem. 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. 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 Pulmonary semilunar valve intercostal space to the right of the sternum. Aortic semilunar valve Left atrioventricular valve ■ 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 Right atrioventricular valve 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 Actual location of heart valve Area where valve sound is best heard heard before, during, or after normal heart sounds. A heart murmur is Heart valve locations and auscultation (listening) sites.

contrast, merely has to pump blood to the nearby lungs.) The tra- The right coronary artery typically branches into the right beculae carneae in the left ventricle are more prominent than in the marginal artery, which supplies the right border of the heart, right ventricle. Typically, two large papillary muscles project from and the posterior interventricular artery, which supplies the the ventricle’s inner wall and anchor the chordae tendineae that posterior surface of both the left and right ventricles. The left attach to the cusps of the left AV valve. At the superior end of the coronary artery typically branches into the anterior interven- ventricular cavity, the aortic semilunar valve marks the end of tricular artery (also called the left anterior descending artery), the left ventricle and the entrance into the aorta (see figure 22.7). which supplies the anterior surface of both ventricles and most of the interventricular septum, and the circumflex (ser ́kum- ̆ WHATW DID YOU LEARN? fleks; around) artery, which supplies the left atrium and ven- tricle. This arterial pattern can vary greatly among individuals. ●5 Where is the coronary sulcus located? For example, some people may have a posterior interventricular ●6 What are two similarities and two differences between the right artery that is a branch of the left coronary artery. Knowledge of and left ventricles? this variation is essential when treating individuals for coronary ●7 What is the composition and function of the chordae tendineae? 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 22.3 Coronary Circulation anastomoses, functionally they act like end arteries, which have Learning Objective: no anastomoses and are the “end of the line” when it comes to arterial blood flow. The anastomoses allow the coronary arter- 1. Identify and describe the location, origins, and branches of ies to shunt a tiny amount of blood from one artery to another. the coronary blood vessels. However, if one of the arteries becomes blocked, as happens with Left and right coronary arteries travel within the coronary coronary artery disease, these anastomoses are too tiny to shunt sulcus of the heart to supply the heart wall (figure 22.9a). These sufficient blood from one artery to the other. For example, if a arteries are the only branches of the ascending aorta. The open- branch of the left coronary artery becomes blocked, the right ings for these arteries are located in the wall of the ascending aorta coronary artery cannot shunt enough blood to the part of the immediately superior to the aortic semilunar valve. heart supplied by this branch. As a result, the part of the heart

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Aortic arch

Superior vena cava Pulmonary trunk

Left coronary artery Aortic semilunar valve Left atrium

Right atrium Circumflex artery Anterior Right coronary artery interventricular Branches of left artery coronary artery Posterior Branches of right interventricular artery 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 the coronary sulcus. The coronary sinus drains directly into the right dies due to lack of blood flow to the tissue. atrium of the heart. Venous return occurs through one of several cardiac veins Because the ventricular myocardium is compressed during (figure 22.9b). The great cardiac vein runs alongside the anterior contraction, most coronary flow occurs during ventricular interventricular artery; the middle cardiac vein runs alongside the relaxation. Normally, flow is evenly distributed throughout the posterior interventricular artery; and the small cardiac vein travels thickness of the myocardium. Under certain circumstances, how- close to the right marginal artery. These cardiac veins all drain into ever, coronary flow may be reduced, especially to the regions the coronary sinus, a large vein that lies in the posterior aspect of immediately external to the endocardium. Situations related to

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CLINICAL VIEW:IEW: In Depth Angina Pectoris and the exertion, and normal blood flow to the heart is restored. Although Myocardial Infarction many people are successfully treated for years with medications that cause temporary vascular dilation, such as nitroglycerine, the prognosis and long- The most common cause of death in the United States is coronary athero- term therapy for angina depend on the severity of the vascular narrowing. ́ ́ sclerosis (ath er-o¯-skler-o¯ sis; athere = gruel, sclerosis = hardness), or Myocardial infarction (in-fark sh˘́un) (MI), commonly called a heart coronary heart disease (see Clinical View, “Atherosclerosis,” in chapter 23). attack, is a potentially fatal condition resulting from sudden and complete This condition is characterized by narrowing of the coronary arteries occlusion (blockage) of a coronary artery. A region of the myocardium that reduces blood flow to the myocardium and gives rise to chest pain. is deprived of oxygen, and some of this tissue may die (necrose). The Coronary atherosclerosis can lead to either angina pectoris or myocardial symptoms of MI are often different for men and women. Most men experi- infarction. encing an MI report a sudden, excruciating, and crushing substernal chest Angina pectoris (an ́jı¯-na˘ , an-ji ́na) is not a disease, it is actually a pain, and marked sweating, Many women, however, have relatively little symptom of coronary artery disease caused by narrowing or blockage of chest pain. When they experience pain, they describe it as an “aching,” coronary arteries. Generally it results from strenuous activity, when work- “tightness,” or “pressure,” rather than pain, and the main locations are load demands on the heart exceed the ability of the narrowed coronary in the back and high chest. In addition, women more often experience vessels to supply blood. The pain from angina is typically referred along shortness of breath, but because they do not exhibit the traditional or the sympathetic pathways (T1–T5 spinal cord segments), so an individual classic symptoms that some doctors expect, it is likely for a female to may experience pain in the chest region or down the left arm, where the T1 be sent home from the ER with an incorrect diagnosis of heartburn or dermatome is located. The pain diminishes shortly after the person stops anxiety, rather than MI.

inadequate coronary blood flow include tachycardia (tak ́i-kar ́dē -ă; cells that usually house one or two central nuclei and numerous tachys = quick), an increased heart rate that shortens diastole, and mitochondria for ATP supply (figure 22.10) . Cardiac muscle cells hypotension (low blood pressure), which reduces the ability of are arranged in spiral bundles and wrapped around and between blood to flow through the ventricular myocardium. the heart chambers. Cardiac muscle tissue exhibits some charac- teristics that are similar to those of skeletal muscle and others that are different (table 22.2) . For example, the cells in both tissue WHAT DO YOU THINK? types are striated, with extensive capillary networks that sup- ●2 Do the coronary arteries fill with blood when the ventricles ply needed nutrients and oxygen. However, cardiac and smooth contract or when they relax? muscle differ in the following ways: ■ The sarcoplasmic reticulum in cardiac muscle is less WHATW DID YOU LEARN? extensive and not as organized as in skeletal muscle. ●8 Why are coronary arteries considered functional end arteries? ■ Cardiac muscle has no terminal cisternae, while skeletal muscle does.

Table 22.2 Comparison of Cardiac 22.4 How the Heart Beats: Electrical and Skeletal Muscle Properties of Cardiac Tissue Cardiac Muscle Skeletal Muscle Learning Objectives: Cells are short and branching Cells are long and cylindrical 1. Distinguish between, and compare, cardiac muscle and One or two nuclei in the center of Multiple nuclei at the periphery of skeletal muscle. the cell the cell 2. Trace the conduction of muscle impulses along muscle fibers. Cells joined by intercellular Cells do not have specialized 3. Describe autorhymicity and the heart’s conducting system. junctions in intercalated discs intercellular junctions The efficient pumping of blood through the heart and blood Functional contractile unit is the Functional contractile unit is the sarcomere sarcomere vessels requires precisely coordinated contractions of the heart chambers. These contractions are made possible by the proper- T-tubules overlie Z discs T-tubules overlie A band/I band junctions ties of cardiac muscle tissue and by specialized cells in the heart, known collectively as its conducting system. Composed of thick and thin Composed of thick and thin fi laments fi laments Contains sarcoplasmic reticulum Contains sarcoplasmic reticulum 22.4a Characteristics of Cardiac Muscle Tissue but less than in skeletal muscle but more than in cardiac muscle The myocardium is composed of cardiac muscle, which was More mitochondria than in skeletal Fewer mitochondria than in cardiac described briefly in chapters 4 (see table 4.14) and 10 (see table muscle muscle 10.6). Recall that cardiac muscle cells are relatively short, branched

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Openings of Intercalated disc transverse tubules 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 Z disc H zone (c) Longitudinal section of cardiac muscle M line I band 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 to those in skeletal muscle, allowing for both the delayed endoplasmic reticulum (SER) and transverse tubules onset and prolonged contraction of cardiac muscle tissue. (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 22.4b Contraction of Heart Muscle skeletal muscle. Cardiac muscle cells contract as a single unit because muscle ■ The T-tubules in cardiac muscle have a less extensive impulses (changes in voltage potential across the sarcolemma distribution and a reduced association with SER compared [see chapter 10]) are distributed immediately and simultaneously

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Superior vena cava Right atrium Left atrium Sinoatrial node (pacemaker) Internodal Internodal pathway pathway Atrioventricular node Atrioventricular node Atrioventricular bundle Interventricular (bundle of His) septum Atrioventricular bundle Right bundle Purkinje fibers

Purkinje fibers Left bundles

1 Muscle impulse is generated at the sinoatrial node. It spreads throughout the atria and 2 Atrioventricular node cells delay the to the atrioventricular node by the internodal pathway. muscle impulse as it passes to the atrioventricular bundle.

Atrioventricular bundle Interventricular septum

Left and right bundle branches Purkinje fibers

3 The atrioventricular bundle (bundle 4 Within the interventricular septum, the 5 The muscle impulse is delivered to Purkinje of His) conducts the muscle impulse right and left bundles split from the fibers in each ventricle and distributed into the interventricular septum. atrioventricular bundle. 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. 22.4c The Heart’s Conducting System Neighboring cardiac muscle cells in the walls of heart chambers The heart exhibits autorhythmicity, meaning that the heart itself have formed specialized cell–cell contacts called intercalated (not external nerves) is responsible for initiating the heartbeat. discs (in-ter ́kă-lā -ted disk), which electrically and mechani- Certain cardiac muscle cells are specialized to initiate and conduct cally link the cells together and permit the immediate passage of muscle impulses to the contractile muscle cells of the myocardium. muscle impulses (figure 22.10). Collectively, these specialized cells are called the heart’s conducting Within the intercalated discs, gap junctions increase the system (figure 22.11). flow of ions between the cells as the muscle impulse moves The heartbeat is initiated by the specialized cardiac muscle along the sarcolemma. The gap junctions of intercalated cells of the sinoatrial (SA) node, which are located in the posterior discs provide a low-resistance pathway across the membranes wall of the right atrium, adjacent to the entrance of the superior of adjoining cardiac muscle cells, allowing the unrestricted vena cava. The cells of the SA node act as the pacemaker, the passage of ions required for the synchronous beating of cardiac rhythmic center that establishes the pace for cardiac activity. muscle cells. Numerous desmosomes (see figure 4.1) within Under the influence of parasympathetic innervation, SA node cells the intercalated discs prevent cardiac muscle cells from pull- initiate impulses 70 to 80 times per minute. ing apart. Therefore, cardiac muscle cells function as a single, The muscle impulse travels from the SA node to the atrioven- coordinated unit; the precisely timed stimulation and response tricular (AV) node. The AV node is located in the floor of the right by cardiac muscle cells of both the atria and the ventricles are atrium between the right AV valve and the opening for the coronary dependent upon these structural features. sinus. The AV node normally slows conduction of the impulse as it

mck78097_ch22_656-682.indd 670 2/14/11 4:29 PM mck78097_ch22_656-682.indd 671 larger than other cardiac muscle cells. Muscle impulse impulse Muscle cells. muscle cardiac other than larger are fibers Purkinje ventricles. of the walls the through extend and Purkinje called fibers toconduction impulse the conduct bundles These ventricular septum before dividing into inter- the into AV extends the and from node impulse muscle the bundle. the system, conduction of the part next the fibrous skeleton that allows the the in AV opening an is node There ventricles. the to the and from communicate atria the atria the between insulates skeleton with fibrous the that Recall ventricles ventricles. of the contraction and activation to between adelay providing ventricles, tothe prevent atria the from travels random muscle impulses from spreading ECG diogram an as charted and collected are signals electrical The chest. the on locations separate six and ankles, wrist, the at skin—usually physical examination using monitoring attached to electrodes the aroutine during detected be can heart the within currents Electrical CLINICAL VIEW the Purkinje fibers in the ventricular myocardium. ventricular inthe fibers Purkinje the (4) and apex, stimulates heart the to septum (potential) interventricular the and voltage in AV node the change through AV (3) node, the to passes of atria the of both through wave the records (2) node, SA inthe radiates It (1) that originates sarcolemma the across cells. myocardial by generated impulses muscle all of tracing acomposite ECGprovides An heart. the of activity electrical the of assessment comprehensive anaccurate, provide collectively they or EKG. This bundle, also known as the the as known also bundle, This (p ŭ (e r-kin ¯ -lek-tro When readings from the different electrodes are compared, compared, are electrodes different the from readings When ́ j ē ¯ -kar ) cells -1 +1 Millivolts ́ de 0 ¯ -o¯ -gram; that begin within the apex of the heart The Electrocardiogram 1 gramma P wave The eventsofasinglecardiac cycleasrecorded onanelectrocardiogram. 2 left QRS complex

bundle of His, of bundle drawing), also called an an called also = drawing), atrioventricular (AV) atrioventricular and right bundles. Q R S conduction 3 electrocar- receives receives T wave 0.8 second 3. The 3. The 2. The 1. The heart: the of regions specific within repolarization and depolarization of indicators are waves These figure). (see baseline the above aTwave and baseline, the (R) above deflection alarge has and baseline the from deflection downward small (Q) (S) with ends and begins that complex aQRS baseline, the above aPwave deflections: principal three has cycle heart one for ECG tracing typical A electrocardiogram. an in mirrored is system conducting the of parts various the through transmission impulse of progress The throughout the ventricular myocardium. immediately spreads impulse the and cells, of the size large the with rapid, consistent extremely is fibers Purkinje the along ● ● ● 11 10 9 repolarization. of the ventricles. this repolarization signal is masked however, by the repolarize; electrical atria the activity Simultaneously, ventricles. of the atria. the of cells the depolarizes node the heart? term the by meant is What sinus opening? coronary the and rightAV the valve between right atrium, of the floor inthe located is system conducting of the component Which skeletal muscle? from differs muscle cardiac inwhich ways ofthe three are What WHAT DID YOU LEARN? WHATW DID is a small, rounded peak that denotes ventricular is a rounded ventricular small, that peak T wave denotes complex QRS SA inthe originating impulse the when isgenerated P wave Chapter Twenty-Two identifies the beginning of depolarization depolarization of beginning the identifies autorhythmic when used to describe Heart 2/14/11 4:29 PM 671 672 Chapter Twenty-Two Heart

CLINICAL VIEW Cardiac Arrhythmia initiated within the AV node or the ventricular conduction system. All of us experience an occasional PVC, and they are Cardiac arrhythmia (a˘-rith mé¯ -a˘ ; a = not, rhythmos = rhythm), also called not detrimental unless they occur in great numbers. Most PVCs dysrhythmia, is any abnormality in the rate, regularity, or sequence of go unnoticed, although occasionally one is perceived as the the cardiac cycle. Several common arrhythmias have been described: heart “skipping a beat” and then “jumping” in the chest.

■ Atrial flutter occurs when the atria attempt to beat at a rate A more serious arrhythmia is ventricular fibrillation, a rapid, of 200 to 400 times per minute, and as a consequence literally repetitious movement of the ventricular muscle that replaces normal bombard the AV node with muscle impulses. Abnormal muscle contraction. This is a life-threatening condition caused by scattered impulses flow continuously through the atrial conduction impulses originating at different times and places throughout the system, thus stimulating the atrial musculature and AV node entire myocardium. Because the contractions of a heart in fibrillation over and over. This condition may persist for years, and are uncoordinated, the heart does not pump blood, and blood circula- frequently degenerates into atrial fibrillation. tion stops. This cessation of cardiac activity is called cardiac arrest. ■ Atrial fibrillation (fı¯-bri-la¯ ́shu˘n) differs from atrial flutter Fibrillation almost certainly results in death unless the normal rhythmic in that the muscle impulses are significantly more chaotic, contractions of the heart are promptly restored. To restore normal heart leading to an irregular heart rate. The ventricles respond by contractions, medical personnel apply a strong electrical shock to the increasing and decreasing contraction activities, which may skin of the chest using paddle electrodes. The electrical current passes lead to serious disturbances in the cardiac rhythm. through the chest wall to completely and immediately depolarize the ■ Premature ventricular contractions (PVCs) often result from entire myocardium. This procedure is analogous to pushing the reset stress, stimulants such as caffeine, or sleep deprivation. They button on a computer—and as in rebooting the computer, the hope is occur either singly or in rapid bursts due to abnormal impulses that when the heart begins to function again, it will work as intended.

parasympathetic components, collectively referred to as the coro- 22.5 Innervation of the Heart nary plexus. The innervation by autonomic centers in the brain- Learning Objective: stem doesn’t initiate a heartbeat, but it can increase or decrease the rate of the heartbeat. 1. Describe and explain how the sympathetic and Sympathetic innervation arises from the T1–T5 segments of parasympathetic divisions of the autonomic nervous system the spinal cord. Preganglionic axons enter the sympathetic trunk regulate heart rate. and ascend into the thoracic and cervical portions, where they syn- The heart is innervated by the autonomic nervous system apse on ganglionic neurons. Postganglionic axons project from the (figure 22.12). This innervation consists of sympathetic and superior, middle, and inferior cervical ganglia and the T1–T5 ganglia,

Autonomic centers Parasympathetic

Vagal nucleus

Medulla oblongata

Sympathetic

Cervical sympathetic Vagus nerve (CN X) Figure 22.12 ganglion Autonomic Innervation of the Heart. Spinal cord The amount of blood pumped from the Sympathetic heart and the heart rate are modified by preganglionic axon Parasympathetic autonomic centers in the brainstem. preganglionic axon (Left) Sympathetic stimulation is Sympathetic carried through the cardiac nerves. postganglionic axon (Right) Parasympathetic stimulation is carried by vagus nerves. Cardiac nerve

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and travel directly to the heart through cardiac nerves. Sympathetic AV valves (figure 22.13b). Therefore, for the last half of the cardiac innervation increases the rate and the force of heart contractions. cycle, all four chambers are in diastole together. Parasympathetic innervation comes from the medulla oblon- gata via the left and right vagus nerves (CN X). As the vagus nerves WHAT DO YOU THINK? descend into the thoracic cavity, they give off branches that supply the heart. Parasympathetic innervation decreases the heart rate, but ●3 What could happen if the AV valves were to evert into the atria? generally tends to have no effect on the force of contractions. Although there is a particularly rich sympathetic and para- 22.6a Steps in the Cardiac Cycle sympathetic innervation of the SA and AV nodes, the working myo- cardial cells are also supplied by both types of autonomic axons. 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 Study Tip! is a brief contraction of the atrial myocardium initiated by the heart pacemaker. Contraction of the atria finishes filling the Here is one way to help you remember the effects of sympathetic ventricles through the open AV valves while the ventricles are and parasympathetic innervation on the heart: in diastole. The semilunar valves remain closed. Sympathetic innervation Speeds the heart rate. 2. Early ventricular systole is the beginning of the ventricular Parasympathetic innervation does the oPposite (decreases the heart contraction. The atria remain in diastole. The AV valves rate). are forced closed (producing the “lubb” sound), and the semilunar valves remain closed. 3. Late ventricular systole occurs later in the ventricular WHATW DID YOU LEARN? contraction. The atria remain in diastole, and the AV valves remain closed. Pressure on blood in the ventricles forces ●12 How does the sympathetic division of the autonomic nervous the semilunar valves to open, and blood is ejected into the system affect the heart rate? arterial trunks. 4. Early ventricular diastole is the start of ventricular relaxation. The atria remain in diastole. The semilunar 22.6 Tying It All Together: valves close to prevent blood backflow into the ventricles The Cardiac Cycle (producing the “dupp” sound). The AV valves remain closed. Learning Objectives: 5. Late ventricular diastole is a continuation of ventricular relaxation and an important time for ventricular filling. The 1. Identify and describe the events in the cardiac cycle. atria remain in diastole. The AV valve opens, and passive 2. Trace the pattern of blood flow through the heart. filling of the ventricle from the atria begins and continues A cardiac cycle is the time from the start of one heartbeat to as most of the ventricular filling occurs. The semilunar the initiation of the next. During a single cardiac cycle, all cham- valves remain closed. bers within the heart experience alternate periods of contraction and relaxation. The contraction of a heart chamber is called systole 22.6b Summary of Blood Flow During the (sis t́ō -lē). During this period, the contraction of the myocardium Cardiac Cycle forces blood either into another chamber (from atrium to ventricle) As the heart chambers cyclically contract and relax, pressure on or into a blood vessel (from a ventricle into the attached large the blood in the chambers alternately increases and decreases. artery). The relaxation phase of a heart chamber is termed dias- Table 22.3 illustrates the flow of blood through the four heart tole (dı̄ -as t́ō -lē; dilation). During this period between contraction chambers during the cardiac cycle. As you learn this sequence, phases, the myocardium of each chamber relaxes, and the chamber keep the following general principles in mind: fills with blood. At the beginning of the cardiac cycle, the left and right ■ Blood flows from veins into the atria under low pressure. atria contract simultaneously. When the atria contract (atrial ■ Blood only passes from the atria into the ventricles if systole), blood is forced into the ventricles through the open AV the AV valves are open. Most of the ventricular filling is valves. During this time, blood is still returning to the atria in the passive (about 70%), occurring when both chambers are superior vena cava, inferior vena cava, and coronary sinus (right relaxing (in diastole) and the atrial pressure is greater than atrium) and pulmonary veins (left atrium). After the atria begin the ventricular pressure. Filling of the final 30% of the to relax (atrial diastole), left and right ventricular contraction ventricles occurs when the atria contract (in systole). (ventricular systole) occurs (figure 22.13a). Thus, only two of ■ Ventricular contraction (systole) increases pressure on the the four chambers (either the atria or the ventricles) contract at blood within the ventricles. When ventricular pressure rises the same time. When the ventricles contract, the atrioventricular significantly, the AV valves close, and the semilunar valves openings close as blood pushes against the cusps of the AV valves, are forced open due to increased pressure in the ventricles, and their edges meet to form a seal. Papillary muscles and the allowing blood to enter the large arterial trunks. chordae tendineae prevent these valve cusps from everting. The semilunar valves are forced open, and blood enters the pulmonary WHATW DID YOU LEARN? trunk and the aorta. When the ventricles are relaxing during the cardiac cycle (ventricular diastole), most of the blood flows pas- ●13 Distinguish between systole and diastole. sively from the relaxing atria into the ventricles through the open ●14 What events occur in the ventricles during late ventricular diastole?

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(a) Ventricular Systole (Contraction)

Aortic arch Blood flow into ascending aorta Ascending Pulmonary aorta trunk Blood flow into Blood flow into right atrium pulmonary trunk Right Left atrium atrium

Ventricular contraction pushes Ventricles contract, forcing blood against the open AV semilunar valves to open and valves, causing them to close. blood to enter the pulmonary Contracting papillary muscles trunk and the ascending aorta. and the chordae tendineae prevent valve flaps from everting into atria.

Atrioventricular valves closed Semilunar valves open Right ventricle Left ventricle

Cusp of Cusp of atrioventricular semilunar valve 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 Traansversensverse sectsectiion

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.

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(b) Ventricular Diastole (Relaxation)

Aortic arch

Blood flow into Blood flow into right atrium left ventricle

Right Left atrium atrium During ventricular relaxation, some blood in the ascending aorta and pulmonary trunk Blood flow into flows back toward the right ventricle ventricles, filling the semilunar valve cusps and forcing them Ventricles relax and fill with to close. blood both passively and then by atrial contraction as AV valves remain open.

Atrioventricular valves open Semilunar valves closed Right ventricle Left ventricle Atrium

Cusp of Blood atrioventricular valve Cusps of Chordae semilunar tendineae valve

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

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Atria contract Atria relax Atria relax

Semilunar valves open

AV All AV valves valves valves open closed closed

Ventricles relax Ventricles contract Ventricles contract 1 Atrial systole 2 Early ventricular systole 3 Late ventricular systole Atria contract; AV valves are open, Atria relax; ventricles begin to contract; Atria continue to relax; ventricles contract; semilunar valves are closed AV valves are forced closed (lubb AV valves remain closed; semilunar sound); semilunar valves still closed valves are forced open

Phase Atrial Early Late Early Late systole ventricular ventricular ventricular ventricular Structure systole systole diastole diastole Atria Contract Relax Relax

Ventricles Relax Contract Relax

AV valves Open Closed Open Semilunar valves Closed Open Closed Time (seconds) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Atria relax Atria relax

Semilunar valves closed

AV All valves valves open closed

Ventricles relax Ventricles relax

5 Late ventricular diastole 4 Early ventricular diastole Atria and ventricles relax; atria continue Atria and ventricles relax; AV valves passively filling with blood; AV valves remain closed and semilunar valves close open and ventricles begin to passively fill; (dupp sound); atria continue passively semilunar valves remain closed 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.

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Table 22.3 Blood Flow Through the Heart

Superior Right Pulmonary Pulmonary Systemic veins and inferior Right atrioventricular Right semilunar trunk and venae cavae atrium valve ventricle valve arteries

Gas and nutrient exchange Gas exchange in peripheral in the lungs tissues

Aortic Left Systemic Pulmonary Aorta semilunar atrioventricular arteries Left Left veins valve ventricle valve atrium

Chamber of the Heart Receives Blood From Sends Blood To Valves Through Which Blood Flows Right atrium Superior vena cava, inferior vena Right ventricle Right AV valve cava, coronary sinus Right ventricle Right atrium Pulmonary trunk (blood enters Pulmonary semilunar valve vessels of pulmonary circulation) Left atrium Pulmonary veins Left ventricle Left AV valve Left ventricle Left atrium Ascending aorta (blood enters Aortic semilunar valve vessels of systemic circulation)

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. 1. Trace and describe the formation of postnatal heart structures from the primitive heart tube. A healthy heart is capable of quickly and efficiently altering both the heartbeat rate and the volume of blood pumped during Development of the heart commences in the third week, either an increase or decrease in activity. Consuming a diet low in when the embryo becomes too large to receive its nutrients saturated fats, abstaining from smoking, and exercising regularly through diffusion alone. At this time, the embryo needs its own help maintain a strong, vigorous heart. However, the decreased blood supply, heart, and blood vessels for transporting oxygen and flexibility and elasticity of connective tissue that occur with aging nutrients through its growing body. The steps involved in heart can cause the heart valves to become slightly inflexible. As a result, development are complex, because the heart must begin working a heart murmur may develop, and blood flow through the heart before its development is complete. may be altered. Decreased conducting system efficiency reduces By day 19 (middle of week 3), two heart tubes (or endocardial the heart’s ability to pump the extra blood needed during stress and tubes) form from mesoderm in the embryo. By day 21, these paired exercise. In addition, the muscular wall of the ventricle increases tubes fuse, forming a single primitive heart tube (figure 22.15). This in thickness when high blood pressure causes the ventricles to tube develops the following named expansions that ultimately give work harder to pump blood into the arterial trunks. Consequently, rise to postnatal heart structures (listed from inferior to superior): the ventricular myocardium undergoes hypertrophy. Hypertrophy sinus venosus, primitive atrium, primitive ventricle, and bulbus of the heart may have different causes (such as hypertension cordis (bŭl ́bŭs kō r dis).́ The sinus venosus and primitive atrium form or narrowing of vessels connected to the heart), but cardiac parts of the left and right atria. The primitive ventricle forms most of muscle cells always thicken and lose the normal arrangement of the left ventricle. The bulbus cordis may be further subdivided into a formed cell bundles. Thus, although the heart is enlarged, it works trabeculated part of the right ventricle, which forms most of the right less efficiently. ventricle; the conus cordis, which forms the outflow tracts for the ventricles; and the truncus arteriosus, which forms the ascending WHATW DID YOU LEARN? aorta and pulmonary trunk. Table 22.4 lists these primitive heart tube components and the structures they develop into. ●15 What are some age-related changes that result in altered heart By day 22, the primitive heart begins to beat and begins its functions? process of bending and folding. The heart folding is complete by

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Aortic arch 1 Aortic arch 2

Truncus arteriosus

Truncus arteriosus

Bulbus cordis Bulbus cordis Fusing paired Primitive heart tubes ventricle form a Primitive primitive Primitive ventricle atrium heart tube Sinus venosus 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 Septum secundum Sinus venosus Superior vena cava, coronary sinus, Blood flow smooth posterior wall of right atrium Right atrium Septum primum Primitive atrium Anterior muscular portions of left and right atria Left atrium Primitive ventricle Most of left ventricle Foramen ovale Bulbus cordis Endocardial Trabeculated part of right Most of right ventricle cushion ventricle Left ventricle Conus cordis Outfl ow tracts from ventricles to aorta and Right ventricle pulmonary trunk Interventricular septum Truncus arteriosus Ascending aorta, pulmonary trunk

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 Early week 7 (43 days) heart tube is S-shaped (figure 22.15c). The next major steps in heart development occur during Figure 22.16 weeks 5–8, when the single heart tube becomes partitioned into Interatrial Septum. The interatrial septum is formed by two overlapping septa (primum and secundum). The foramen ovale is a four chambers (two atria and two ventricles), and the main vessels passageway that detours blood away from the pulmonary circulation entering and leaving the heart form. The common atrium is sub- into the systemic circulation prior to birth. divided 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

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not functional, much of the blood is shunted away from the lungs aorticopulmonary septum, which is a spiral-shaped mass that and to the rest of the body. Since it is the right side of the heart that also subdivides the truncus arteriosus into the pulmonary trunk sends blood to the lungs, blood is shunted from the right atrium to and the ascending aorta. the left atrium by traveling through the foramen ovale and push- Many congenital heart malformations result from incomplete ing the septum primum to the left. Blood cannot flow back from or faulty development during these early weeks. For example, in the left atrium to the right, because the septum primum’s move- an atrial septal defect the postnatal heart still has an opening ment to the right is stopped when it comes against the septum between the left and right atria. Thus, blood from the left atrium secundum. Thus, the septum primum acts as a unidirectional (the higher-pressure system) is shunted to the right atrium (the flutter valve. When the baby is born and the lungs are fully func- lower-pressure system). This can lead to enlargement of the right tional, the blood from the left atrium pushes the septum primum side of the heart. Ventricular septal defects can occur if the inter- and secundum together, creating a closed interatrial septum. The ventricular septum is incompletely formed. A common malforma- only remnant of the embryonic opening is an oval-shaped depres- tion called tetralogy of Fallot occurs when the aorticopulmonary sion in the interatrial septum called the fossa (fos á;̆ trench) ovalis. septum divides the truncus arteriosus unevenly. As a result, the Left and right ventricles are partitioned by an interven- patient has a ventricular septal defect, a very narrow pulmonary tricular septum that grows superiorly from the floor of the ven- trunk (pulmonary stenosis), an aorta that overlaps both the left tricles. The AV valves, papillary muscles, and chordae tendineae and right ventricles, and an enlarged right ventricle (right ven- all form from portions of the ventricular walls as well. The tricular hypertrophy). A good knowledge of heart development is superior part of the interventricular septum develops from the essential in understanding these congenital heart malformations.

Clinical Terms

bradycardia (brad-ē-kar d́ē- ă; bradys = slow) Slowing of the microorganisms), mycotic (due to infection by fungi), heartbeat, usually described as less than 50 beats per minute. and rheumatic (due to endocardial involvement as part of cardiomyopathy (kar ́dē -ō -mı̄ -op ́ă-thē ) Another term for rheumatic heart disease). disease of the myocardium; causes vary and include ischemia (is-kē ́mē -ă; ischo = to keep back) Inadequate blood thickening of the ventricular septum (hypertrophy), flow to a structure caused by obstruction of the blood secondary disease of the myocardium, or sometimes a supply, usually due to arterial narrowing or disruption of disease of unknown cause. blood flow. endocarditis (en ́dō -kar-dı̄ ́tis) Inflammation of the myocarditis (mı̄ ́ō-kar-d ı̄ ́tis) Inflammation of the muscular walls endocardium. Types include bacterial (caused by of the heart. This uncommon disorder is caused by viral, the direct invasion of bacteria), chorditis (affecting bacterial, or parasitic infections, exposure to chemicals, or the chordae tendineae), infectious (caused by allergic reactions to certain medications.

Chapter Summary

22.1 Overview of ■ Arteries carry blood away from the heart, and veins return blood to the heart. the Cardiovascular ■ Heart functions include one-directional blood flow through the cardiovascular system, coordinated side-by-side pumps, System 657 and generation of pressure to drive blood through blood vessels. 22.1a Pulmonary and Systemic Circulations 657 ■ 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 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.

22.2 Anatomy of ■ The heart is a small, cone-shaped organ. the Heart 660 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.

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Chapter Summary (continued) 22.2 Anatomy of 22.2c Internal Heart Anatomy: Chambers and Valves 660 the Heart 660 ■ Dense regular connective tissue forms the fibrous skeleton of the heart that separates the atria and the ventricles. This (continued) 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.

22.3 Coronary ■ The major coronary artery branches are the anterior interventricular and circumflex arteries from the left coronary artery, Circulation 666 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. 22.4 How the Heart 22.4a Characteristics of Cardiac Muscle Tissue 668 Beats: Electrical ■ Cardiac muscle cells are small and branched. They form sheets of cardiac muscle tissue arranged into spiral bundles Properties of wrapped around and between the heart chambers. Cardiac Tissue 668 ■ 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 ■ Sympathetic and parasympathetic innervation of the heart have opposing influences on heart rate. Autonomic the Heart 672 innervation increases or decreases the rate of the heartbeat, but does not initiate it.

22.6 Tying It All ■ A cardiac cycle is the period of time from the start of one heartbeat to the beginning of the next. Together: The ■ All chambers within the heart experience alternate periods of contraction (systole) and relaxation (diastole) during a Cardiac Cycle 673 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 ■ Decreased flexibility of heart structures as we age reduces the efficiency of cardiac function. Increased blood pressure Heart 677 results in hypertrophy of the myocardium.

22.8 Development ■ Development of the heart commences in the third week; two heart tubes form and fuse to become a single primitive of the Heart 677 heart tube. ■ The primitive heart tube develops expansions that later form postnatal heart structures.

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Challenge Yourself

Matching ______4. Which of the following is the correct circulatory Match each numbered item with the most closely related lettered sequence for blood to pass through part of the heart? → → → item. a. R. atrium right AV valve R. ventricle pulmonary semilunar valve ______1. right marginal artery a. veins that carry blood to b. R. atrium → left AV valve → R. ventricle → right atrium ______2. pulmonary veins pulmonary semilunar valve → → → b. period of relaxation c. L. atrium right AV valve L. ventricle ______3. left AV valve aortic semilunar valve c. contains three cusps; also → → → ______4. diastole cells d. L. atrium left AV valve L. ventricle known as tricuspid valve pulmonary semilunar valve ______5. SA node d. specialized junction between ______5. The pericardial cavity is located between the ______6. venae cavae cardiac muscle cells a. fibrous pericardium and the parietal layer of the serous pericardium. ______7. intercalated disc e. veins that carry blood to b. parietal and visceral layers of the serous left atrium ______8. right AV valve pericardium. f. period of contraction c. visceral layer of the serous pericardium and the ______9. circumflex artery epicardium. g. branch of right coronary ______10. systole d. myocardium and the visceral layer of the serous artery pericardium. h. origin of heartbeat ______6. In the developing heart, the atria form from the i. branch of left coronary primitive atrium and the artery a. sinus venosus. b. bulbus cordis. j. also known as bicuspid or c. primitive ventricle. mitral valve d. conus cordis. Multiple Choice ______7. The irregular muscular ridges in the ventricular walls are the Select the best answer from the four choices provided. a. papillary muscles. ______1. Muscle impulses are spread rapidly between cardiac b. trabeculae carneae. muscle cells by c. chordae tendineae. a. sarcomeres. d. moderator bands. b. intercalated discs. ______8. Sympathetic innervation of cardiac muscle c. chemical neurotransmitters. originates from d. AV valves. a. CN X (vagus nerve). ______2. Venous blood from the heart wall enters the right b. L1–L2 segments of the spinal cord. atrium through the c. the AV node. a. superior vena cava. d. T1–T5 segments of the spinal cord. b. coronary sinus. ______9. When the ventricles contract, all of the following c. inferior vena cava. occur except d. pulmonary veins. a. closing of the AV valves. ______3. How is blood prevented from flowing into the right b. blood ejecting into the pulmonary trunk and aorta. ventricle from the pulmonary trunk? c. closing of the semilunar valves. a. closing of the right AV valves d. opening of the semilunar valves. b. opening of the pulmonary semilunar valve ______10. The thickest part of the heart wall is the c. contraction of the right atrium a. pericardium. d. closing of the pulmonary semilunar valve b. epicardium. c. myocardium. d. endocardium.

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Content Review 10. What are the phases of the cardiac cycle? Describe each phase with respect to which heart chambers are in systole 1. What are the differences between the pulmonary and or diastole, which valves are open or closed, and blood flow systemic circulations? into or out of the chambers. 2. What chamber walls primarily form the anterior side of the heart? What chamber walls form the posterior side of the heart? Developing Critical Reasoning 3. Compare the structure, location, and function of the parietal 1. It was the end of the semester, and Huang had begun to and visceral layers of the serous pericardium. prepare for his final examinations. Unfortunately, he still 4. Where is the fibrous skeleton of the heart located? What are had to work his full-time job. In order to find sufficient its functions? time to study, he stayed up late and drank large amounts 5. Why are the chordae tendineae required for the proper of coffee to stay alert. One evening during a very late study functioning of the AV valves? session, Huang felt a pounding in his chest and thought he was having a heart attack. His roommate took Huang to the 6. Explain why the walls of the atria are thinner than those of emergency room. After an examination and interview by the the ventricles, and why the walls of the right ventricle are physician, Huang was told that he probably had a cardiac relatively thin when compared to the walls of the left ventricle. arrhythmia. What was the most probable cause of the 7. Identify and compare the branches of the right and left arrhythmia? coronary arteries. In general, what portions of the heart does 2. Josephine is a 55-year-old overweight woman who has a each branch supply? poor diet and does not exercise. One day while walking 8. Compare cardiac and skeletal muscle. In what ways briskly, she experienced pain in her chest and down her are these muscle types similar? In which ways are they left arm. Her doctor told her she was experiencing angina different? due to heart problems. Josephine asks you to explain what 9. Describe the functional differences in the effects of the causes angina, and why she was feeling pain in her arm sympathetic and parasympathetic divisions of the autonomic even though the problem was with her heart. What do you nervous system on the activity of cardiac muscle. tell her?

Answers to “What Do You Think?”

1. The pulmonary arteries carry blood low in oxygen to the 3. If the AV valves were to evert into the atria, some of the lungs because the lungs are responsible for replenishing blood from the ventricles would be pushed into the atria, the oxygen supplies in the blood. After deoxygenated blood resulting in inefficient pumping of blood. When the heart travels to the lungs, the pulmonary capillaries are involved has to work harder to pump the blood out of the heart, in gas exchange—that is, carbon dioxide is removed from clinical problems result (see Clinical View: “Valve Defects the blood, and oxygen enters the blood. Then the pulmonary and Their Effects on Circulation” for further information). veins carry the newly oxygenated blood back to the heart. 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.

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