We begin this unit by examining the structural features that enable the heart to perform so reliably. We will then consider the physiological mechanisms that regulate cardiac activity to meet changing circumstances.
Blood flows through a network of blood vessels that extend between the heart and peripheral tissues. Those blood vessels can be subdivided into a pulmonary circuit, which carries blood to and from the gas exchange surfaces of the lungs, and a systemic circuit, which transports blood to and from the rest of the body. Each circuit begins and ends at the heart, and blood travels through these circuits in sequence. For example, blood returning to the heart from the systemic circuit must complete the pulmonary .circuit before reentering the systemic circuit.
Arteries, or efferent vessels, carry blood away from the heart; veins, or afferent vessels, return blood to the heart. Capillaries are small, thin-walled vessels between the smallest arteries and veins. Capillaries are called exchange vessels, because their thin walls permit the exchange of nutrients, dissolved gases, and waste products between the blood and surrounding tissues.
Despite its impressive workload, the heart is a small organ, roughly the size of a clenched fist. The heart contains four muscular chambers, two associated with each circuit. The right atrium receives blood from the systemic circuit and passes it to the right ventricle (little belly).
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The right ventricle discharges blood into the pulmonary circuit. The left atrium collects blood from the pulmonary circuit and empties it into the left ventricle. Contraction of the left ventricle ejects blood into the systemic circuit. When the heart beats, the atria contract first, followed by the ventricles. The two ventricles contract at the same time and eject equal volumes of blood into the pulmonary and systemic circuits.
I ANATOMY OF THE HEART
The heart is located near the anterior chest wall, directly posterior to the sternum. A midsagittal section through the trunk would not divide the heart into two equal halves because the heart (1) lies slightly to the left of the midline, (2) sits at an angle to the longitudinal axis of the body, and (3) is rotated toward the left side. The heart is surrounded by the pericardium.
The serous membrane lining the pericardial cavity is called the pericardium. To visualize the relationship between the heart and the pericardial cavity, imagine pushing your fist toward the center of a large balloon. The pericardium can be subdivided into the visceral pericardium and the parietal pericardium and the fibrous pericardium. The visceral pericardium, or epicardium, covers the outer surface of the heart; the parietal pericardium lines the inner surface of the pericardial sac, which surrounds the heart. The pericardial sac is formed from the fibrous pericardium which is relatively inelastic and is slightly bigger than the heart The space between the opposing parietal and visceral surfaces is the pericardial cavity. This cavity normally contains 10–20 ml of pericardial fluid secreted by the pericardial membranes. Pericardial fluid acts as a lubricant, reducing friction between the opposing surfaces as the heart beats.
Superficial Anatomy of the Heart
The four cardiac chambers can easily be identified in a superficial view of the heart. The two atria have relatively thin muscular walls, and they are highly expandable. This expandable extension of an atrium is called an auricle (auris, ear), because it reminded early anatomists of the external ear, or an atrial appendage.
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Internal Anatomy and Organization
The right atrium communicates with the right ventricle, and the left atrium with the left ventricle. The two atria are separated by the interatrial septum (septum, wall), and the two ventricles are separated by the much thicker interventricular septum. Each septum is a muscular partition with valves of fibrous tissue that extend into the openings between the atria and ventricles. These valves permit blood flow in one direction only: from the atria into the ventricles.
The Right Atrium
The right atrium receives blood from the systemic circuit through three, the coronary sinus, the superior vena cava (plural, venae cavae) and the inferior vena cava. The superior vena cava delivers blood to the right atrium from the head, neck, upper limbs, and chest. The inferior vena cava carries blood to the right atrium from the rest of the trunk, the viscera, and the lower limbs. The cardiac veins of the heart return blood via the coronary sinus.
The Right Ventricle
Blood travels from the right atrium into the right ventricle through a broad opening bounded by three fibrous flaps. These flaps, or cusps, are part of the right atrioventricular (AV) valve, also known as the tricuspid valve (tri, three) valve. The free edge of each cusp is attached to tendinous connective tissue fibers called the chordae tendinae (tendinous cords).
These fibers originate at the papillary muscles, conical muscular projections that arise from the inner surface of the right ventricle. The valve closes when the right ventricle contracts, preventing the backflow of blood into the right atrium.
The pulmonary semilunar valve consists of three semilunar (half-moon shaped) cusps of thick connective tissue. Blood flowing from the right ventricle passes through this valve to enter the pulmonary trunk, the start of the pulmonary circuit. The arrangement of cusps prevents backflow as the right ventricle relaxes.
Once within the pulmonary trunk, blood flows into the left pulmonary arteries and the right pulmonary arteries. These vessels branch repeatedly within the lungs before supplying the capillaries where gas exchange occurs.
The Left Atrium
From the respiratory capillaries, blood collects into small veins that ultimately unite to form the four pulmonary veins. The posterior wall of the left atrium receives blood from two left and two right pulmonary veins. Like the right atrium, the left atrium has an auricle and a valve, the left atrioventricular (AV) valve, or bicuspid valve. As the name bicuspid implies, the left AV valve contains a pair, not a trio, of cusps. Clinicians often use the term mitral (mitre, a bishop’s hat) when referring to this valve. The left AV valve permits the flow of blood from the left atrium into the left ventricle.
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The Left Ventricle
The right and left ventricles contain equal amounts of blood, but the left ventricle is much larger than the right because it has thicker walls. The thick, muscular wall enables the left ventricle to develop pressure sufficient to push blood through the large systemic circuit; the right ventricle needs to pump blood, at lower pressure, only about 15 cm (6 in.) to and from the lungs. The internal organization of the left ventricle resembles that of the right ventricle.
Blood leaves the left ventricle by passing through the aortic semilunar valve into the ascending aorta. The arrangement of cusps in the aortic semilunar valve is the same as that in the pulmonary semilunar valve. Once the blood has been pumped out of the heart and into the systemic circuit, the aortic semilunar valve prevents backflow into the left ventricle. From the ascending aorta, blood flows on through the aortic arch and into the descending aorta.
The Atrioventricular Valves
The atrioventricular valves prevent the backflow of blood from the ventricles to the atria when the ventricles are contracting. The chordae tendineae and papillary muscles play an important role in the normal function of the AV valves. During the period known as ventricular diastole, the ventricles are relaxed. As each relaxed ventricle fills with blood, the chordae tendineae are loose and the AV valves offer no resistance to the flow of blood from the atria to the ventricles. The ventricles contract during the period of ventricular systole. As the ventricles begin to contract, blood moving back toward the atria swings the cusps together, closing the valves. At the same time, the contraction of the papillary muscles tenses the chordae tendineae and stops the cusps before they swing into the atria.
The Semilunar Valves
The pulmonary and aortic semilunar valves prevent the backflow of blood from the pulmonary trunk and aorta into the right and left ventricles. The semilunar valves do not require muscular braces because the arterial walls do not contract, and the relative positions of the cusps are stable.
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The Heart Wall
A section through the wall of the heart reveals three distinct layers: (1) an outer epicardium, (2) a middle myocardium, and (3) an inner endocardium.
• The epicardium is the visceral pericardium that covers the outer surface of the heart.
• The myocardium, or muscular wall of the heart, forms both atria and ventricles. The myocardium contains cardiac muscle tissue, blood vessels, and nerves. The myocardium consists of concentric layers of cardiac muscle tissue.
• The inner surfaces of the heart, including those of the heart valves, are covered by the endocardium (endo-, inside).
The endocardium is simple squamous epithelium that is continuous with the endothelium of the attached blood vessels.
Cardiac Muscle Tissue
Cardiac muscle cells are interconnected by intercalated discs. These discs convey the force of contraction from cell to cell and propagate action potentials.
The Blood Supply to the Heart
The heart works continuously, and cardiac muscle cells require reliable supplies of oxygen and nutrients. The coronary circulation supplies blood to the muscles of the heart. During maximum exertion, the oxygen demand rises considerably, and the blood flow to the heart may increase to nine times that of resting levels. The coronary circulation includes an extensive network of coronary blood vessels.
The Coronary Arteries
The left and right coronary arteries originate at the base of the ascending aorta. Blood pressure here is the highest in the systemic circuit, and this pressure ensures a continuous flow of blood to meet the demands of active cardiac muscle tissue.
The Cardiac Veins
The great cardiac vein begins on the anterior surface of the ventricles, along the interventricular sulcus. This vein drains blood from the region supplied by the anterior interventricular branch of the left coronary artery. The great cardiac vein reaches the level of the atria and then curves around the left side of the heart within the coronary sulcus. The vein empties into the coronary sinus.
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Innervation of the Heart
The sympathetic and parasympathetic divisions of the ANS provide innervation to the heart through the cardiac plexus. Postganglionic sympathetic neurons are located in the cervical and upper thoracic ganglia. The vagus nerve (N X) carries parasympathetic preganglionic fibers to small ganglia in the cardiac plexus. Both ANS divisions innervate the SA and AV nodes and the atrial muscle cells.
II THE HEARTBEAT
In a single heartbeat, the entire heart—atria and ventricles—contracts in a coordinated manner so that blood flows in the right direction at the proper time. Each time your heart beats, the contractions of individual cardiac muscle cells within the atria and ventricles must occur in a specific sequence. Two types of cardiac muscle cells are involved in a normal heartbeat: (1) contractile cells that produce the powerful contractions that propel blood, and (2) specialized muscle cells of the conducting system that control and coordinate the activities of the contractile cells
The Conducting System
In contrast to skeletal muscle, cardiac muscle tissue contracts on its own, in the absence of neural or hormonal stimulation. This property is called automaticity, or autorhythmicity. The cells responsible for initiating and distributing the stimulus to contract are part of the conducting system of the heart. The conducting system is a network of specialized cardiac muscle cells that initiates and distributes electrical impulses. The actual contraction lags behind the passage of an electrical impulse, as excitation-contraction coupling occurs and cross-bridge interactions take place.
The conducting system includes:
• The sinoatrial (SA) node, located in the wall of the right atrium.
• The atrioventricular (AV) node, located at the junction between the atria and ventricles.
• Conducting cells, which interconnect the two nodes and distribute the contractile stimulus throughout the myocardium. Conducting cells in the atria are found in the internodal pathways, which distribute the contractile stimulus to atrial muscle cells as the impulse travels from the SA node to the AV node. The ventricular conducting cells include those in the AV bundle and the bundle branches as well as the Purkinje fibers, which distribute the stimulus to the ventricular myocardium.
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We shall now trace the path of an impulse from its initiation at the SA node and consider its effects on the surrounding myocardium.
The Sinoatrial (SA) Node
The SA node is embedded in the posterior wall of the right atrium, near the entrance of the superior vena cava. The SA node contains pacemaker cells, which establish the heart rate. As a result, the SA node is also known as the cardiac pacemaker or the natural pacemaker. It is connected to the larger AV node by the internodal pathways in the atrial walls. Along the way, the conducting cells pass the contractile stimulus to cardiac muscle cells of the right and left atria. The action potential then spreads across the atrial surfaces through cell-to-cell contact.
The Atrioventricular (AV) Node
The relatively large AV node sits within the floor of the right atrium near the opening of the coronary sinus. The rate of propagation slows as the impulse leaves the internodal pathways and enters the AV node, because the nodal cells are smaller in diameter than the conducting cells. In addition, the connections between nodal cells are less efficient than those between conducting cells at relaying the impulse from one cell to another. The delay at the AV node is important because the atria must contract before the ventricles; once the ventricles start contracting, the AV valves close. If all the chambers contracted at the same time, the AV valves would prevent blood flow from the atria and into the ventricles. Because there is a delay at the AV node, the atrial myocardium completes its contraction before ventricular contraction begins.
The AV Bundle, Bundle Branches, and Purkinje Fibers
The connection between the AV node and the AV bundle, or bundle of His, is the only electrical connection between the atria and the ventricles. Once an impulse enters the AV bundle, it travels to the interventricular septum and enters the right and left ventricles. The left bundle branch, which supplies the relatively massive left ventricle, is much larger than the right bundle branch. Both bundle branches extend toward the apex of the heart, turn, and fan out beneath the endocardial surface. As the branches diverge, they conduct the impulse to and to papillary muscles.
The Purkinje fibers conduct action potentials very rapidly, as fast as small myelinated axons. The atria have now completed their contractions, and ventricular contraction can safely occur. Because the bundle branches deliver the impulse across the moderator band to the papillary muscles directly rather than via Purkinje fibers, the papillary muscles begin contracting before the rest of the ventricular musculature. This contraction applies tension to the chordae tendineae, bracing the AV valves. By limiting the movement of the cusps, tension in the papillary muscles prevents the backflow of blood into the atria when the ventricles contract. Ventricular contraction proceeds in a wave that begins at the apex and spreads toward the base.
The electrical events occurring in the heart are powerful enough to be detected by electrodes on the body surface. A recording of these electrical activities constitutes an electrocardiogram, also called an ECG or EKG. Each time the heart beats, a wave of depolarization radiates through the atria, reaches the AV node, travels down the interventricular septum to the apex, turns, and spreads through the ventricular myocardium toward the base.
By comparing the information obtained from electrodes placed at different locations, a clinician can monitor the electrical activity of the heart, which is directly related to the performance of specific nodal, conducting, and contractile components. An ECG will reveal an abnormal pattern of impulse conduction. The appearance of the ECG recording varies with the placement of the monitoring electrodes, or leads. Note the following ECG features:
• The small P wave accompanies the depolarization of the atria. The atria begin contracting about 100 msec after the start of the P wave.
• The QRS complex appears as the ventricles depolarize. This is a relatively strong electrical signal, because the mass of the ventricular muscle is much larger than that of the atria. The ventricles begin contracting shortly after the peak of the R wave.
• The smaller T wave indicates ventricular repolarization. You do not see a deflection corresponding to atrial repolarization, because it occurs while the ventricles are depolarizing and it’s electrical events are masked by the QRS complex.
The Cardiac Cycle
The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle. The cardiac cycle therefore includes alternating periods of contraction and relaxation. For any one chamber in the heart, the cardiac cycle can be divided into two phases: (1) systole and (2) diastole. During systole, or contraction, the chamber contracts and pushes blood into an adjacent chamber or into an arterial trunk. Systole is followed by the second phase, diastole, or relaxation. During diastole, the chamber fills with blood and prepares for the next cardiac cycle.
In the course of the cardiac cycle, the pressure within each chamber rises in systole and falls in diastole. Valves between adjacent chambers help ensure that blood flows in the desired direction, but the mere presence of valves is not enough. Blood will flow from one chamber to another only if the pressure in the first chamber exceeds that in the second. This basic principle governs the movement of blood between atria/ventricles, between ventricles and arterial trunks, and between the major veins and the atria.
The correct pressure relationships are dependent on the careful timing of contractions. The elaborate pace-making and conducting systems normally provide the required spacing between atrial and ventricular systoles. As a result, atrial systole and ventricular systole do not occur at the same time, and atrial diastole and ventricular diastole differ in duration.
There are two audible heart sounds. These sounds accompany the action of your heart valves. The first heart sound, known as “lubb” lasts a little longer than the second and marks the start of ventricular contraction, is produced as the AV valves close. The second heart sound, “dupp” occurs at the beginning of ventricular filling, when the semilunar valves close.
The term cardiodynamics refers to the movements and forces generated during cardiac contractions. Each time the heart beats, the two ventricles eject equal amounts of blood. Stroke volume, the amount of blood pumped by each ventricle/beat, is the most important factor in an examination of a single cardiac cycle. When considering cardiac function over time, physicians generally are most interested in the cardiac output (CO), the amount of blood pumped by each ventricle/minute. The cardiac output provides a useful indication of ventricular efficiency over time.
We can calculate it by multiplying the average stroke volume (SV) by the heart rate (HR):
= SV x HR
cardiac stroke heart
output volume rate
(ml/min) (ml/beat) (beats/min)
Cardiac output is precisely adjusted so that peripheral tissues receive an adequate circulatory supply under a variety of conditions. When necessary, stroke volume in a normal heart can almost double, and the heart rate can increase by 250 percent. In most healthy people, increasing both the stroke volume and the heart rate, as during heavy exercise, can raise the cardiac output by 300-500 percent. Trained athletes exercising at maximal levels may increase cardiac output by nearly 700 percent, to 40 l/min. The difference between resting and maximal cardiac output is the cardiac reserve.
Factors Controlling Stroke Volume
Filling time depends entirely on the heart rate. The faster the heart rate, the shorter the available filling time.
Venous return changes in response to alterations in cardiac output, blood volume, patterns of peripheral circulation, skeletal muscle activity, and other factors that affect the rate of blood flow through the venae cavae.
The Frank-Starling Principle. The relationship between the amount of ventricular stretching and contractile force means that within normal physiological limits, increasing the stretching of the cardiac muscle fibers prior to contraction results in a corresponding increase in the stroke volume. This general rule of “more in = more out” was first proposed by Ernest H. Starling on the basis of an analysis of research performed by Otto Frank. The relationship is therefore known as the Frank-Starling principle, or Starling’s law of the heart.
Autonomic Activity. Autonomic activity alters the degree of contraction and changes the SV in the following ways:
• Sympathetic stimulation causes the release of norepinephrine (NE) by postganglionic fibers and the secretion of epinephrine (E) and NE by the adrenal medullae. In addition to their effects on heart rate, discussed below, these compounds stimulate cardiac muscle cell metabolism and increase the force and degree of contraction.
• Parasympathetic stimulation from the vagus nerve has the primary effect of releasing acetylcholine (ACh) is at the membrane surface, where it produces hyperpolarization and inhibition. The primary result is a decrease in heart rate through effects on the SA and AV nodes. There is also a reduction in the force of cardiac contractions.
Hormones. Several hormones, including the following, affect the contractility of the heart:
• Epinephrine and norepinephrine increase contractility.
• Glucagon decreases contractility.
• Thyroid hormones increase contractility. Thyroid gland disorders that result in hyper-secretion of thyroid hormones commonly produce excessive stimulation of cardiac muscle tissue, leading to cardiac arrhythmias.
Factors Affecting the Heart Rate
A number of clinical problems are the result of abnormal pacemaker function. Bradycardia (bradys, slow) is the term used to indicate a heart rate that is slower than normal, whereas tachycardia (tachys, swift) indicates a faster-than-normal heart rate. These are relative terms, and in clinical practice the definitions vary with the normal resting heart rate of the individual.
Changes in Body Temperature
Temperature changes affect metabolic operations throughout the body. For example, a reduction in temperature lowers the heart rate and reduces the strength of cardiac contractions. An elevated body temperature accelerates the heart rate and the contractile force. That is one reason why your heart seems to race and pound whenever you have a fever.
Cardiac Output and Heart Rate
Cardiac output cannot increase indefinitely, primarily because the available filling time becomes shorter and shorter as the heart rate increases. At heart rates up to 160-180 bpm, the combination of an increased rate of venous return and increased contractility compensates for the reduction in filling time. Over this range, cardiac output and heart rate increase together. But if the heart rate continues to climb, the stroke volume begins to drop, and cardiac output first plateaus and then declines.