The Cardiovascular System: The Heart
In this chapter, you will learn that
18The heart pumps blood through the pulmonary and systemic circuits
starting with then looking at
18.1 Anatomy of the heart Physiology of the heart
then asking starting with
18.5 Electrical events 18.2 Why does the 18.3 What path does 18.4 How do cardiac heart have valves? blood take through muscle fibers differ which cause the heart? from skeletal muscle fibers? 18.6 Mechanical events
then exploring
18.7 How is pumping regulated?
and finally, exploring
Developmental Aspects of the Heart
CAREER CONNECTION
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Our ceaselessly beating heart has intrigued people for Capillary beds centuries. The ancient Greeks believed the heart was the seat of lungs where of intelligence. Others thought it was the source of emotions. gas exchange occurs While these ideas have proved false, we do know that emo- tions affect heart rate. When your heart pounds or skips a beat, you become acutely aware of how much you depend on this dynamic organ for your very life. Despite its vital importance, the heart does not work alone. Indeed, it is only part of the cardiovascular system, which Pulmonary Circuit includes the miles of blood vessels that run through your body. Day and night, tissue cells take in nutrients and oxygen and Pulmonary arteries Pulmonary excrete wastes. Cells can make such exchanges only with veins their immediate environment, so some means of changing and renewing that environment is necessary to ensure a con- Aorta and branches tinual supply of nutrients and prevent a buildup of wastes. The Venae cavae cardiovascular system provides the transport system “hardware” that keeps blood continuously circulating to fulfill this critical Left homeostatic need. atrium
Left 18.1 The heart has four chambers Right ventricle atrium and pumps blood through the Heart Right pulmonary and systemic circuits ventricle
Learning Outcomes Systemic Circuit NN Describe the size, shape, location, and orientation of the heart in the thorax. NN Name the coverings of the heart. NN Describe the structure and function of each of the three layers of the heart wall. NN Describe the structure and functions of the four heart chambers. Name each chamber and provide the name and general route of its associated great vessel(s). Capillary beds of all body tissues The Pulmonary and Systemic Circuits where gas Stripped of its romantic cloak, the heart is no more than the Oxygen-rich, exchange CO -poor blood occurs transport system pump, and the blood vessels are the delivery 2 1718 routes. In fact, the heart is actually two pumps side by side Oxygen-poor, CO -rich blood (Figure 18.1). 2
●● The right side of the heart receives oxygen-poor blood from Figure 18.1 The systemic and pulmonary circuits. The right body tissues and then pumps this blood to the lungs to pick side of the heart pumps blood through the pulmonary circuit and up oxygen and dispel carbon dioxide. The blood vessels that the left side of the heart pumps blood through the systemic circuit. carry blood to and from the lungs form the pulmonary circuit The arrows indicate the direction of blood flow. For simplicity, the (pulmo 5 lung). actual number of two pulmonary arteries and four pulmonary veins has been reduced to one each. ●● The left side of the heart receives the oxygenated blood returning from the lungs and pumps this blood throughout the body to supply oxygen and nutrients to body tissues. The blood vessels that carry blood to and from all body tissues ●● The left atrium receives blood returning from the pulmonary form the systemic circuit. circuit. The heart has two receiving chambers: The heart also has two main pumping chambers:
●● ●● The right atrium receives blood returning from the systemic The right ventricle pumps blood into the pulmonary circuit. circuit. ●● The left ventricle pumps blood into the systemic circuit.
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Mediastinum Midsternal line
2nd rib Heart
Sternum Left lung
Diaphragm Location of apical Body of T8 impulse vertebra
Posterior
(a) (b)
Superior Aorta vena cava Parietal pleura (cut)
Pulmonary trunk Left lung
Pericardium (cut)
Apex of heart Diaphragm
1718
(c)
Figure 18.2 Location of the heart in the mediastinum. (a) Relationship of the heart to the sternum, ribs, and diaphragm in a person who is lying down (the heart is slightly inferior to this position in a standing person). (b) Inferior view of a cross section showing the heart’s relative position in the thorax. (c) Relationship of the heart and great vessels to the lungs.
Size, Location, and Orientation superior surface of the diaphragm, the heart lies anterior to the of the Heart vertebral column and posterior to the sternum. Approximately two-thirds of its mass lies to the left of the midsternal line; the The modest size and weight of the heart belie its incredible balance projects to the right. The lungs flank the heart laterally strength and endurance. About the size of a fist, the hollow, and partially obscure it (Figure 18.2b, c). cone-shaped heart has a mass of 250 to 350 grams—less than a The broad, flat base, or posterior surface, of the heart is pound (Figure 18.2). about 9 cm (3.5 in) wide and directed toward the right shoul- Snugly enclosed within the mediastinum (me0de-ah- der. Its apex points inferiorly toward the left hip. If you press sti9num), the medial cavity of the thorax, the heart extends your fingers between the fifth and sixth ribs just below the left obliquely for 12 to 14 cm (about 5 inches) from the second rib nipple, you can easily feel the apical impulse caused by your to the fifth intercostal space (Figure 18.2a). As it rests on the beating heart’s apex where it touches the chest wall.
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Pulmonary trunk
Fibrous pericardium Pericardium Parietal layer of serous pericardium
Myocardium Pericardial cavity
Epicardium (visceral layer of serous pericardium) Heart Myocardium wall
Endocardium
Heart chamber
Figure 18.3 The layers of the pericardium and of the heart wall.
Coverings of the Heart Layers of the Heart Wall The heart is enclosed in a double-walled sac called the The heart wall, richly supplied with blood vessels, is composed pericardium (per0ĭ-kar9de-um; peri 5 around, cardi 5 heart) of three layers: the epicardium, myocardium, and endocardium (Figure 18.3). The loosely fitting superficial part of this sac is (Figure 18.3). the fibrous pericardium. This tough, dense connective tissue As we have noted, the superficial epicardium is the visceral layer (1) protects the heart, (2) anchors it to surrounding struc- layer of the serous pericardium. It is often infiltrated with fat, tures, and (3) prevents overfilling of the heart with blood. especially in older people. Deep to the fibrous pericardium is the serous pericardium, The middle layer, the myocardium (“muscle heart”), is a thin, slippery, two-layer serous membrane that forms a closed composed mainly of cardiac muscle and forms the bulk of the sac around the heart ( Figure 1.11, p. 19). Its parietal layer heart. This is the layer that contracts. In the myocardium, the lines the internal surface of the fibrous pericardium. At the branching cardiac muscle cells are tethered to one another by superior margin of the heart, the parietal layer attaches to the crisscrossing connective tissue fibers and arranged in spiral large arteries exiting the heart, and then turns inferiorly and or circular bundles (Figure 18.4). These interlacing bundles continues over the external heart surface as the visceral layer, effectively link all parts of the heart together. also called the epicardium (“upon the heart”), which is an inte- gral part of the heart wall. Between the parietal and visceral layers is the slitlike peri- cardial cavity, which contains a film of serous fluid. The 1718 serous membranes, lubricated by the fluid, glide smoothly past one another, allowing the mobile heart to work in a relatively friction-free environment.
HOMEOSTATIC CLINICAL IMBALANCE 18.1 Cardiac Pericarditis, inflammation of the pericardium, roughens the muscle serous membrane surfaces. Consequently, as the beating heart bundles rubs against its pericardial sac, it creates a creaking sound (pericardial friction rub) that can be heard with a stethoscope. Pericarditis is characterized by pain deep to the sternum. Over time, it may lead to adhesions in which the visceral and parietal pericardia stick together and impede heart activity. In severe cases, large amounts of inflammatory fluid seep into the pericardial cavity. This excess fluid compresses the heart and limits its ability to pump blood, a condition called cardiac tamponade (tam0pŏ-nād9), literally, “heart plug.” Physicians treat cardiac tamponade by inserting a syringe into the pericardial Figure 18.4 The circular and spiral arrangement of cardiac cavity and draining off the excess fluid. muscle bundles in the myocardium of the heart.
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The connective tissue fibers form a dense network, the Functionally, the atria are receiving chambers for blood fibrous cardiac skeleton, that reinforces the myocardium inter- returning to the heart from the circulation (atrium 5 entryway). nally and anchors the cardiac muscle fibers. This network of The atria are relatively small, thin-walled chambers because collagen and elastic fibers is thicker in some areas than others. they need to contract only minimally to push blood “downstairs” For example, it constructs ropelike rings that provide additional into the ventricles. They contribute little to the propulsive pumping support where the great vessels issue from the heart and around activity of the heart. the heart valves (see Figure 18.6a, p. 678). Without this sup- Blood enters the right atrium via three veins (Figure 18.5c–e): port, the vessels and valves might eventually become stretched ●● The superior vena cava returns blood from body regions because of the continuous stress of blood pulsing through them. superior to the diaphragm. Additionally, because connective tissue is not electrically excit- able, the cardiac skeleton allows action potentials to spread ●● The inferior vena cava returns blood from body areas below only via specific pathways in the heart. the diaphragm. The third layer of the heart wall, the endocardium (“inside ●● The coronary sinus collects blood draining from the myo- the heart”), is a glistening white sheet of endothelium (squa- cardium. mous epithelium; p. 121) resting on a thin connective tissue Four pulmonary veins enter the left atrium, which makes layer. Located on the inner myocardial surface, it lines the heart up most of the heart’s base. These veins, which transport blood chambers and covers the fibrous skeleton of the valves. The from the lungs back to the heart, are best seen in a posterior endocardium is continuous with the endothelial linings of the view (Figure 18.5d). blood vessels leaving and entering the heart. Ventricles: The Discharging Chambers Chambers and Associated Great Vessels Together the ventricles (ventr 5 underside) make up most of The heart has four chambers (Figure 18.5e on p. 677)—two the volume of the heart. As already mentioned, the right ven- superior atria (a9tre-ah) and two inferior ventricles (ven9trĭ- tricle forms most of the heart’s anterior surface and the left klz). The internal partition that divides the heart longitudinally ventricle dominates its posteroinferior surface. Irregular ridges is called the interatrial septum where it separates the atria, and of muscle called trabeculae carneae (trah-bek9u-le kar9ne-e; the interventricular septum where it separates the ventricles. “crossbars of flesh”) mark the internal walls of the ventricular The right ventricle forms most of the anterior surface of the chambers. Other muscle bundles, the papillary muscles, which heart. The left ventricle dominates the inferoposterior aspect of play a role in valve function, project into the ventricular cavity the heart and forms the heart apex. (Figure 18.5e). Two grooves visible on the heart surface indicate the bound- The ventricles are the discharging chambers, the actual aries of its four chambers and carry the blood vessels supply- pumps of the heart. Their walls are much more massive than the ing the myocardium. The coronary sulcus (Figure 18.5b, d), or atrial walls, reflecting the difference in function between the atrioventricular groove, encircles the junction of the atria and atria and ventricles (Figure 18.5e and f). When the ventricles ventricles like a crown (corona 5 crown). The anterior inter- contract, they propel blood out of the heart into the circulation. ventricular sulcus, cradling the anterior interventricular artery, The right ventricle pumps blood into the pulmonary trunk, marks the anterior position of the septum separating the right which routes the blood to the lungs where gas exchange occurs. and left ventricles. It continues as the posterior interventricu- The left ventricle ejects blood into the aorta (a-or9tah), the larg- 1718 lar sulcus, which provides a similar landmark on the heart’s est artery in the body. posteroinferior surface. Check Your Understanding Atria: The Receiving Chambers 1. The heart is in the mediastinum. Just what is the mediastinum? Except for small, wrinkled, protruding appendages called auri- 2. From inside to outside, list the layers of the heart wall and the cles (or9ĭ-klz; auricle 5 little ear), which increase the atrial vol- coverings of the heart. ume somewhat, the right and left atria are remarkably free of 3. What is the purpose of distinguishing surface features. Internally, the right atrium has the serous fluid inside the two basic parts (Figure 18.5c): a smooth-walled posterior part pericardial cavity? a and an anterior portion in which bundles of muscle tissue form 4. In the frontal section of the ridges in the walls. These muscle bundles are called pectinate heart at right, name the b c muscles because they look like the teeth of a comb (pectin 5 structures indicated by a–g. d comb). The posterior and anterior regions of the right atrium Which heart chamber receives are separated by a C-shaped ridge called the crista terminalis oxygen-poor blood from veins? Which chamber pumps blood e (“terminal crest”). f into the systemic circulation? In contrast, the left atrium is mostly smooth and pectinate Which blood vessel transports g muscles are found only in the auricle. The interatrial septum blood toward the lungs? bears a shallow depression, the fossa ovalis (o-vă9lis) (Figure For answers, see Answers Appendix. 18.5c, e), that marks the spot where an opening, the foramen ovale, existed in the fetal heart (see p. 700). (Text continues on p. 679.)
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Left subclavian artery Left common carotid artery
Brachiocephalic trunk Aortic arch Ligamentum arteriosum
Ascending aorta Pulmonary trunk
Auricle of left atrium Right atrium
Right coronary artery Anterior interventricular artery (in coronary sulcus) (in anterior interventricular sulcus)
Additional branch off left Right ventricle coronary artery, normal variation
Left ventricle
Apex of heart (left ventricle) (a) Anterior view (pericardium removed)
Left common carotid Brachiocephalic trunk artery Left subclavian artery
Superior vena cava Aortic arch Ligamentum arteriosum Right pulmonary artery Left pulmonary artery Ascending aorta Left pulmonary veins Pulmonary trunk
Auricle of left atrium 1718 Right pulmonary veins Circumflex artery Right atrium Left coronary artery Right coronary artery (in coronary sulcus) (in coronary sulcus)
Anterior cardiac vein Left ventricle Right ventricle
Right marginal artery Great cardiac vein Anterior interventricular Small cardiac vein artery (in anterior interventricular sulcus) Inferior vena cava
Apex
(b) Anterior view
Figure 18.5 Gross anatomy of the heart. In diagrammatic views, vessels transporting oxygen-rich blood are red; those transporting oxygen-poor blood are blue.
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Superior vena cava Ascending aorta
Pectinate muscles
Crista terminalis
Interatrial septum
Fossa ovalis
Opening of inferior vena cava (cut) Myocardium of Opening of coronary sinus left ventricle
Interventricular sulcus
(c) Internal aspect of the right atrium, anterior view
Aorta Superior vena cava
Right pulmonary artery Left pulmonary artery Right pulmonary veins Left pulmonary veins
Auricle of left atrium Right atrium
1718 Left atrium Inferior vena cava Great cardiac vein Coronary sinus
Posterior vein of left ventricle Right coronary artery (in coronary sulcus)
Left ventricle Posterior interventricular artery (in posterior interventricular sulcus)
Middle cardiac vein
Right ventricle
Apex
(d) Posterior view
Figure 18.5 (continued) Gross anatomy of the heart. In (c), the anterior wall of the atrium has been opened and folded superiorly.
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Aorta
Left pulmonary artery Superior vena cava Left atrium Right pulmonary artery
Left pulmonary veins Pulmonary trunk Right atrium
Mitral (bicuspid) valve Right pulmonary veins
Fossa ovalis Aortic valve Pectinate muscles Pulmonary valve Tricuspid valve
Right ventricle Left ventricle Chordae tendineae Papillary muscle Interventricular septum Trabeculae carneae Epicardium Inferior vena cava Myocardium Endocardium
(e) Frontal section
Left subclavian artery Left common carotid artery Brachiocephalic trunk Aortic arch
Ligamentum arteriosum Superior vena cava Pulmonary trunk 1718 Pulmonary valve
Pulmonary vein Right auricle
Left auricle
Tricuspid valve Chordae tendineae of mitral valve
Chordae tendineae Papillary muscle
Papillary muscle
Myocardium of left ventricle Myocardium of right ventricle Trabeculae carneae Interventricular septum
(f) Internal aspect of ventricles; dissection of view similar to (e)
Figure 18.5 (continued)
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Pulmonary valve Aortic valve
Area of cutaway
Mitral valve Tricuspid valve
Myocardium
Mitral (left atrioventricular) valve
Tricuspid (right atrioventricular) valve
Aortic valve
Pulmonary valve
Cardiac skeleton Anterior (a) (b)
Right Left atrium atrium
Tricuspid 1718 valve Chordae Mitral valve tendineae Myocardium Myocardium of left of right ventricle ventricle
Papillary muscles Interventricular septum Chordae tendineae attached Papillary to tricuspid valve flap muscle (c) (d)
Figure 18.6 Heart valves. (a) Superior view of the two sets of heart valves (atria removed). The paired atrioventricular valves are located between atria and Practice Human Cadaver lab questions: ventricles; the two semilunar valves are located at the junction of the ventricles and > Study Area > Lab Tools > PAL the arteries issuing from them. (b) Photograph of the heart valves, superior view. (c) Photograph of the tricuspid valve. This bottom-to-top view shows the valve as seen from the right ventricle. (d) Frontal section of the heart.
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●● The left AV valve, with two cusps, is called the mitral valve 18.2 Heart valves make blood flow (mi9tral) because it resembles the two-sided bishop’s miter in one direction (tall, pointed hat). It is sometimes called the bicuspid valve. Learning Outcome Attached to each AV valve flap are tiny white collagen cords called chordae tendineae (kor9de ten0dĭ9ne-e; “tendinous NN Name the heart valves and describe their location, cords”), “heart strings” which anchor the cusps to the papillary function, and mechanism of operation. muscles protruding from the ventricular walls (Figure 18.6c, d). Blood flows through the heart in one direction: from atria to When the heart is completely relaxed, the AV valve flaps ventricles and out the great arteries leaving the superior aspect hang limply into the ventricular chambers below. During this of the heart. Four valves enforce this one-way traffic (Figure time, blood flows into the atria and then through the open AV 18.5e and Figure 18.6). They open and close in response to valves into the ventricles (Figure 18.7a). When the ventricles differences in blood pressure on their two sides. contract, compressing the blood in their chambers, the intraven- tricular pressure rises, forcing the blood superiorly against the Atrioventricular (AV) Valves valve flaps. As a result, the flap edges meet, closing the valve The two atrioventricular (AV) valves, one located at each (Figure 18.7b). atrial-ventricular junction, prevent backflow into the atria when The chordae tendineae and the papillary muscles act as the ventricles contract. tethers that anchor the valve flaps in their closed position. If the cusps were not anchored, they would be blown upward ●● The right AV valve, the tricuspid valve (tri-kus9pid), has (everted) into the atria, in the same way an umbrella is blown three flexible cusps (flaps of endocardium reinforced by con- inside out by a gusty wind. The papillary muscles contract with nective tissue cores). the other ventricular musculature so that they take up the slack on the chordae tendineae as the full force of ventricular contrac- tion hurls the blood against the AV valve flaps.
Direction of 1 Blood returning to the heart blood flow fills atria, pressing against the AV valves. The increased Atrium pressure forces AV valves open. Cusp of 2 As ventricles fill, AV valve flaps atrioventricular hang limply into ventricles. valve (open)
Chordae 3 Atria contract, forcing additional tendineae blood into ventricles. Papillary Ventricle muscle 1718
(a) AV valves open; atrial pressure greater than ventricular pressure
Atrium
Cusps of 1 Ventricles contract, forcing atrioventricular blood against AV valve cusps. valve (closed)
2 AV valves close. Blood in 3 Papillary muscles contract ventricle and chordae tendineae tighten, preventing valve flaps from everting into atria.
(b) AV valves closed; atrial pressure less than ventricular pressure
Figure 18.7 The function of the atrioventricular (AV) valves.
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HOMEOSTATIC Semilunar (SL) Valves CLINICAL IMBALANCE 18.2 The aortic and pulmonary (semilunar, SL) valves guard the bases of the large arteries issuing from the ventricles (aorta and Heart valves are simple devices, and the heart—like any pulmonary trunk, respectively) and prevent backflow into the mechanical pump—can function with “leaky” valves as long as associated ventricles. Each SL valve is fashioned from three the impairment is not too great. However, severe valve deformi- pocketlike cusps, each shaped roughly like a crescent moon ties can seriously hamper cardiac function. (semilunar 5 half-moon). An incompetent, or insufficient, valve forces the heart to Like the AV valves, the SL valves open and close in response repump the same blood over and over because the valve does not to differences in pressure. When the ventricles contract and close properly and blood backflows. In valvular stenosis (“nar- intraventricular pressure rises above the pressure in the aorta rowing”), the valve flaps become stiff and constrict the opening. and pulmonary trunk, the SL valves are forced open and their [Stenosis is typically due to calcium salt deposits or scar tissue cusps flatten against the arterial walls as blood rushes past that forms following endocarditis (see Related Clinical Terms, them (Figure 18.8a). When the ventricles relax, and the blood p. 702).] The constricted opening compels the heart to con- flows backward toward the heart, it fills the cusps and closes tract more forcibly than normal. Both types of valve problems the valves (Figure 18.8b). increase the heart’s workload and may weaken the heart severely We complete the valve story by noting what seems to be an over time. The mitral and aortic valves are most often affected. important omission—there are no valves guarding the entrances of The faulty valve can be replaced with a mechanical valve, a the venae cavae and pulmonary veins into the right and left atria, pig or cow heart valve chemically treated to prevent rejection, respectively. Small amounts of blood do spurt back into these ves- or cryopreserved valves from human cadavers. Heart valves sels during atrial contraction, but backflow is minimal because of tissue-engineered from a patient’s own cells grown on a biode- the inertia of the blood and because as it contracts, the atrial myo- gradable scaffold are being developed. cardium compresses (and collapses) these venous entry points. Check Your Understanding 5. What is the function of the papillary muscles and chordae tendineae? 6. Name the valve that has just two cusps. Aorta For answers, see Answers Appendix. Pulmonary trunk
As ventricles 18.3 Blood flows from atrium to contract and intraventricular ventricle, and then to either the lungs pressure rises, blood is pushed or the rest of the body up against semilunar valves, Learning Outcomes forcing them NN Trace the pathway of blood through the heart. open. NN Name the major branches and describe the distribution of 1718 the coronary arteries. (a) Semilunar valves open Having covered the basic anatomy of the heart, we can now follow the path that blood takes through the heart and its associated circuits. Focus on Blood Flow through the Heart (Focus Figure 18.1) follows a single “spurt” of blood as it passes through all four chambers of the heart and both blood As ventricles circuits in its ever-repeating journey. relax and As you work your way through this figure, keep in mind that intraventricular the left side of the heart is the systemic circuit pump and the pressure falls, blood flows right side is the pulmonary circuit pump. Notice how unique back from the pulmonary circuit is. Elsewhere in the body, veins carry arteries, filling relatively oxygen-poor blood to the heart, and arteries transport the cusps of semilunar valves oxygen-rich blood from the heart. The opposite oxygenation and forcing conditions exist in veins and arteries of the pulmonary circuit. them to close. Equal volumes of blood are pumped to the pulmonary and systemic circuits at any moment, but the two ventricles have (b) Semilunar valves closed very unequal workloads. The pulmonary circuit, served by the right ventricle, is a short, low-pressure circulation. In contrast, Figure 18.8 The function of the semilunar (SL) valves. the systemic circuit, associated with the left ventricle, takes a
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The heart is a double pump, each side supplying its own circuit.
Oxygen-poor blood Oxygen-rich blood
Tricuspid Pulmonary Superior vena cava (SVC) Right valve Right semilunar valve Pulmonary Inferior vena cava (IVC) atrium ventricle trunk Coronary sinus
Pulmonary arteries SVC Coronary sinus Pulmonary trunk
Right Tricuspid atrium valve Pulmonary semilunar valve Right IVC ventricle
Oxygen-poor blood is carried Oxygen-poor blood in two pulmonary arteries to To lungs To heart returns from the body the lungs (pulmonary circuit) tissues back to the heart. to be oxygenated.
Systemic Pulmonary capillaries capillaries
Oxygen-rich blood is Oxygen-rich blood returns To body delivered to the body to the heart via the four To heart tissues (systemic circuit). pulmonary veins.
Aorta Pulmonary veins
Aortic Left semilunar Mitral atrium valve valve Left ventricle
Aortic Mitral semilunar valve Left valve Left Four Aorta ventricle atrium pulmonary veins
681
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