Cardiovascular Physiology Primer HBY501 PDF

CARDIOVASCULAR PHYSIOLOGY HBY 501 PHYSIOLOGY Roger H. Cameron, Ph.D. Department of Physiology and Biophysics Stony Brook University Table of Contents CARDIOVASCULAR PHYSIOLOGY ............................................................................. 1 CHAPTER 1: ORGANIZATION OF THE CARDIOVAS...

CARDIOVASCULAR PHYSIOLOGY HBY 501 PHYSIOLOGY Roger H. Cameron, Ph.D. Department of Physiology and Biophysics Stony Brook University Table of Contents CARDIOVASCULAR PHYSIOLOGY ............................................................................. 1 CHAPTER 1: ORGANIZATION OF THE CARDIOVASCULAR SYSTEM ............ 4 I. Overview to Cardiovascular Function: .................................................................... 4 II. Circulation of Blood ............................................................................................... 7 III. Classification of Blood Vessels ............................................................................ 8 IV. Sample Test Questions (see Appendix for Explained Answers) ........................ 11 CHAPTER 2: THE HEART AS A PUMP .................................................................. 12 I. Structure and Function of the Contractile Apparatus .......................................... 12 II. Contractile Properties of Isolated Cardiac Muscle Fibers .................................. 16 III. The Heart – Lung Preparation........................................................................... 19 IV. Sample Test Questions (see Appendix for Explained Answers) ........................ 23 CHAPTER 3: ELECTROPHYSIOLOGY OF THE HEART ..................................... 25 I. Overview to Electrical Activity within the Heart ................................................. 25 II. The Maximum Diastolic Potential ....................................................................... 27 III. The Ionic Basis of the Cardiac Action Potential: Contractile Cells................... 29 IV. The Ionic Basis of the Cardiac Action Potential: Conducting Cells ................. 33 V. Electrical Excitation of the Heart ......................................................................... 36 VI. Sample Test Questions (see Appendix for Explained Answers) ........................ 38 CHAPTER 4: THE ELECTROCARDIOGRAM ........................................................ 39 I. Extracellular Recordings: ..................................................................................... 39 II. The Normal ECG ................................................................................................. 41 III. The Effect of Altered Serum Electrolyte Concentrations on the ECG ............... 48 IV. Sample Test Questions (see Appendix for Explained Answers) ........................ 51 CHAPTER 5: THE CARDIAC CYCLE ..................................................................... 52 I. The Cardiac Cycle ................................................................................................ 52 II. Pressure – Volume Loops .................................................................................. 56 III. Laplace’s Law ..................................................................................................... 57 IV. Estimating Cardiac Output: The Fick Principle.................................................. 58 V. Sample Test Questions (see Appendix for Explained Answers) ......................... 59 CHAPTER 6: REGULATION OF CARDIAC OUTPUT........................................... 62 I. Cardiac Output ...................................................................................................... 62 II. Nervous Control of Heart Rate........................................................................... 63 III. Intrinsic Regulation of Myocardial Function .................................................... 66 IV. Extrinsic Regulation of Myocardial Function .................................................... 68 V. Exercise ............................................................................................................... 70 VI. Sample Test Questions (see Appendix for Explained Answers) ........................ 71 CHAPTER 7: HEMODYNAMIC PRINCIPLES IN THE PERIPHERAL VASCULAR SYSTEM ................................................................................................ 73 I. General Hydraulic Principles................................................................................ 73 II. The Nature of Blood Flow ................................................................................... 75 III. Vascular Resistance ............................................................................................ 81 2 IV. Determinants of Arterial Blood Pressure ............................................................ 84 V. Hemodynamics in Veins: .................................................................................... 85 VI. Sample Test Questions (see Appendix for Explained Answers) ........................ 85 CHAPTER 8: MICROCIRCULATION AND LYMPH ............................................. 88 I. Morphology of the Microcirculation .................................................................... 88 II. Transcapillary Transport of Solutes ..................................................................... 91 III. Transcapillary Transport of Fluids...................................................................... 95 IV. Sample Test Questions (see Appendix for Explained Answers) ...................... 102 CHAPTER 9: COORDINATED CARDIOVASCULAR RESPONSES .................. 103 I. Overview to Cardiovascular Reflexes ................................................................ 103 II. Response to a Change in Posture ....................................................................... 104 III. The Valsalva Maneuver .................................................................................... 105 IV. Exercise............................................................................................................. 106 V. Shock and Hemorrhage ..................................................................................... 109 VI. Sample Test Questions (see Appendix for Explained Answers) ...................... 114 APPENDIX: EXPLAINED ANSWERS TO SAMPLE TEST QUESTIONS ........... 115 I. Chapter 1............................................................................................................. 115 II. Chapter 2 ............................................................................................................ 116 III. Chapter 3 ........................................................................................................... 117 IV. Chapter 4 ........................................................................................................... 119 V. Chapter 5 ............................................................................................................ 120 VI. Chapter 6 ........................................................................................................... 123 VII. Chapter 7 ......................................................................................................... 124 VIII. Chapter 8 ........................................................................................................ 126 IX. Chapter 9 ........................................................................................................... 127 3 CHAPTER 1: ORGANIZATION OF THE CARDIOVASCULAR SYSTEM I. Overview to Cardiovascular Function: 1. Functions of the Cardiovascular System: The primary function of the cardiovascular system is to provide a rapid transport system that distributes a number of substances throughout the body for use by our cells. For example, oxygen derived from the lungs and nutrients derived from the gastrointestinal tract are transported to the various cells and tissues of the body within the blood stream. Similarly, the waste products produced at the tissue level are also conveyed to the appropriate excretory organs via the blood stream. The cardiovascular system has an important role in endocrine function as well, serving as the pathway by which hormones travel from their sites of origin to find their appropriate target cells. And finally, the cardiovascular system is largely responsible for the process of thermoregulation due to the fact that heat from the body core can be either delivered or shunted away from the skin to meet changes in our temperature balance. Implicit in this description of cardiovascular function are two basic facts. First, the fundamental means of transport of substances between cells and the blood stream is diffusion, and secondly, the need for a cardiovascular system is based on certain fundamental limitations involved with the process of diffusion. Recall that diffusion is a passive process in which individual molecules move by random thermal motion. While each movement is random in direction, net movement of a given molecule occurs when there is a concentration gradient for that solute. With regard to the rate of diffusion, the Einstein (1905) equation shows that the time (t) it takes a molecule to move a distance x in one direction in free solution is proportional to the square of the distance: t= x2 2D (Equation 1-1) The diffusion coefficient (D) of a given solute depends on the size of the diffusing molecule and viscosity of the medium but for most small solutes in dilute aqueous solutions, D is on the order of 10-5 cm2/sec. A few simple calculations demonstrate that while the time needed for solutes to diffuse over short distances is quite short, larger distances require diffusion times that are not compatible with life (Table 1-1). DISTANCE 0.1 m 1.0 m 10.0 m 100 m 1 mm 1 cm TIME 5 sec 0.5 msec 50 msec 5 sec 8.3 min 13.8 hr 4 Figure 1-1: Overview of the cardiovascular system illustrating the roles of diffusion and convective transport in oxygen transport. A particularly relevant example is the ventricular wall of the heart, typically on the order of 1 cm in thickness. If the outer layers of the heart were solely dependent on receiving nutrients diffusing from the ventricular lumen, it would take such a nutrient approximately 14 hours to diffuse from the lumen to the outer layers of the epicardium. Clearly, for distances much greater than 100 m, a faster transport system is needed. This fast transport system is provided by the cardiovascular system. Consider for example the transportation of oxygen through the body (Fig. 1-1). At both the interface with the environment (lungs) and between blood vessels and cells, the time needed for O 2 molecules to diffuse the appropriate distances is in the millisecond range (see Table 1-1). But for the O2 molecule to travel the approximately 1meter distance to reach a capillary in the foot using only diffusion would require a time period approaching 16 years obviously, an untenable situation. Instead, O2 and a host of other molecules are transported throughout the body via a process called convection (or bulk flow) in which they are swept along in a stream of pumped fluid. Of course, this process requires energy, in this case supplied by the heart, but convective transport shortens the transit time from years down to much less than a minute to reach even the most distant sites in the body. 2. Role of the Heart: As shown in Fig. 1-1, the heart can be considered as two intermittent pumps arranged in series. The right ventricle pumps blood through the lungs to the left atrium (the pulmonary circulation) while the left ventricle simultaneously pumps blood throughout the rest of the body returning it to the right atrium (the systemic circulation). The blood follows a circular path because of the presence of one-way valves (i.e., the tricuspid, mitral, aortic semilunar, and pulmonary semilunar valves). • The Cardiac Cycle: The time period in which the heart is contracting is called systole while the period in which the heart is relaxed is referred to as diastole. The sequence of events in which the atria contract first (atrial systole) followed by the ventricles (ventricular systole) results in a cycle of pressure and volume changes, collectively referred to as the cardiac cycle. • Cardiac Output: The volume of blood that leaves the ventricle each minute is called the cardiac output. It can be determined by multiplying two quantities: the amount of blood that leaves the ventricle each beat (called the stroke volume) 5 multiplied by the number of heart beats per minute (called heart rate). In a normal, resting 70 kg individual, cardiac output is typically 5 L/min, but can quadruple during heavy exercise. However, in the failing heart, cardiac output is reduced, both at rest and during exertion. Accordingly, cardiac output represents a useful clinical parameter for characterizing relative cardiac function. 3. Role of Blood Vessels: Blood vessels serve as more than simple conduits for blood flow by virtue of the fact that they possess smooth muscle within their walls which can contract in a regulated faction. Because this smooth muscle is circumferentially (or helically) arranged about the vessel lumen, contraction of the individual smooth muscle cells results in an overall constriction of the vessel – a process called vasoconstriction. By contrast, when smooth muscle cells relax, the inherent blood pressure forces exerted onto a relaxed vessel wall result in dilation of that vessel – a process called vasodilation. Several aspects of these processes should be noted. 1. Vessel Tone: In general, the degree of active tension exerted by smooth muscle within the walls of blood vessels is called vessel tone. Some vessels, however, retain a degree of vessel tone even in the absence of any excitatory stimulation (e.g., in the denervated state), and this is referred to as basal tone. A moment’s reflection will show that any vessel whose function involves any significant degree of vasodilation must have a relatively high basal tone because vasodilation is, after all, simply a reduction in vessel tone. Accordingly, tissues that are capable of significantly increasing their local blood flow (e.g., skeletal muscle, salivary glands, etc.) typically have high basal tone. The relative degree of basal tone of any vessel is determined by a balance between the cellular mechanisms that produce vasoconstriction and those that produce smooth muscle relaxation. 2. Regulation of Vascular Smooth Muscle: Vessel tone can be altered by a variety of substances, conveniently distinguished into two classes called intrinsic mechanisms and extrinsic mechanisms. Intrinsic mechanisms involve local processes that take place solely within the organ or tissue in question and typically involve alterations that better match blood flow to changes in metabolic needs. By contrast, extrinsic mechanisms involve the autonomic nervous system and circulating endocrine secretions and often (although not always) affect the peripheral vasculature system as a whole. It should be emphasized, however, that the relative importance of intrinsic and extrinsic mechanisms vary among different organs and tissues. The blood supply to the heart and brain, for example, are more highly dependent on intrinsic regulation whereas the blood supply to skin and the GI system are more dependent on their innervation and circulating substances. 3. Changes in Vessel Dimension Affect Blood Pressure and Flow: Changes in vessel dimension have two basic effects within the cardiovascular system: a) Changes in Local Blood Flow: The fundamental relationship between flow, pressure and vascular resistance states that: Flow = ∆𝑃 𝑅 (Equation 1-2) 6 where: flow is given in units of volume/time; ∆P refers to the pressure gradient driving flow; and R is the vascular resistance. Thus, when a blood vessel constricts so that its vascular resistance increases (details provided in Chapter 7), blood flow through that vessel will decrease. Conversely, vasodilation produces reductions in vascular resistance and an increase in local blood flow. b) Changes in Arterial Blood Pressure: In Chapter 7 we will also learn that mean arterial pressure is the product of cardiac output and total peripheral resistance. Thus, when blood vessels constrict, the resultant increase in total peripheral resistance serves as a means to keep blood pressure from falling even under circumstances in which cardiac output is compromised. II. Circulation of Blood 1. Arrangement of the Circulation: The blood is compelled to follow a circular path due to the presence of one-way valves in the heart and veins. Fig. 1-2 shows the basic arrangement of the circulation. Pulmonary Circulation: Deoxygenated blood returning from the systemic circuit enters the right atrium which then fills the right ventricle. From here blood is pumped to the lungs via pulmonary arteries which pass to pulmonary capillaries where gas exchange takes place. Oxygenated blood is returned to the heart in the pulmonary veins. Systemic Circulation: Blood within the left atrium fills the left ventricle which then simultaneously pumps an equal volume of blood (i.e., cardiac output) to the rest of the body via the aorta. Note the parallel arrangement of arteries that supply the various organs. In most cases, there is a single capillary bed situated between the artery and vein which then returns blood to the right atrium. In some cases however, there are two systemic capillary beds arranged in series with a portal vessel (in this case, the portal vein in the gut, or the efferent arteriole in the kidney) in between. Figure 1-2: Arrangement of the Circulation 7 2. Distribution of Cardiac Output: As illustrated above, the entire output of the right ventricle passes exclusively to the lungs. The output of the left ventricle passes out the aorta where it then distributes to all of the other tissues of the body including the heart itself (via coronary arteries) and even the larger airways located within the lung (via bronchial arteries). As a rule of thumb, the left ventricular cardiac output distributes to the peripheral tissues and organs in rough proportion to their metabolic demands, yet there are some notable exceptions. Figure 1-3 compares the distribution of left ventricular output (a) with O2 consumption of these same tissues and organs at rest (b). While the relative proportion of cardiac output is similar to metabolic need in some tissues (e.g., resting skeletal muscle), the blood flow to other organs greatly exceeds their individual metabolic needs. For example, the kidneys receive 20% of cardiac output yet account for only 6% of O2 consumption – a fact that relates to their function of filtering blood. But for this to occur, other organs must be under-supplied, and it is surprising to find that the heart and brain are two such examples. However, these organs feature functional specializations that allow them to compensate for this seemingly dangerous arrangement. Finally, it should be emphasized that these proportions are not fixed, but can be rapidly adjusted to meet some demand. Perhaps the best example is strenuous exercise in which the proportion of cardiac output to skeletal muscle increases to 80% or more. Figure 1-3: Distribution of cardiac output and O2 consumption at rest III. Classification of Blood Vessels 1. Structure and Dimensions of Blood Vessels: Both the aorta and pulmonary artery progressively divide into smaller muscular arteries, ultimately giving rise to high-resistance vessels called arterioles. Fig. 14 shows how the wall thickness varies along this branching in the systemic circuit. Figure 1-4: Dimensions of Various Blood Vessels 8 Arterioles branch into large numbers of thin walled capillaries which proceed to converge to form veins. The smallest veins are called venules which become larger and become known simply as veins. The largest veins in the systemic circuit are the superior and inferior vena cava. Note that because of the increase in vessel number, the total cross-sectional area of the vascular system increases despite the progressive decrease in vessel dimensions (Table 1.2). This fact has important implications in determining the velocity of blood flow as will be shown in Chapter 7. Also note that the volume of blood is not distributed equally, but is found mostly in the veins at any one instant in time. Most blood vessels are structured along a similar plan consisting of three concentric layers called the tunica intima, tunica media, and tunica adventitia (Fig. 1-5). Figure 1-5: General Structure of Blood Vessels 9 • • • Tunica Intima: is the innermost layer and is present in all blood vessels. In the smallest vessels, this layer consists only of a single layer of endothelial cells, but in larger vessels, there is a progressively thick layer of subjacent connective tissue. The tunica intima is quite delicate, but has important functional properties in terms of secreting vasoactive substances. Tunica Media: is the middle layer and consists of smooth muscle cells which supply mechanical strength and contractile force, thereby altering the vessel dimensions. These vascular smooth muscle cells are typically concentrically arranged and are embedded in an extracellular matrix consisting of collagen and elastic fibers that varies in extent in different vessels. In certain vessels, elastic fibers are concentrated at the inner junction with the tunica intima (internal elastic lamina) and the outer junction with the tunica adventitia (external elastic lamina). Tunica Adventitia: is the outermost layer and consists of connective tissue. Typically, this connective tissue layer blends in with the surrounding tissues of the body so that the exact outer boundary is somewhat indistinct. In the largest blood vessels, one can find small adventitial blood vessels (vasa vasorum) and nerves (nervi vasorum). Finally, large veins are characterized by having longitudinally oriented smooth muscle in this layer. 2. Functional Classification of Blood Vessels: By comparing the detailed structure of the three tunics among various blood vessels, it becomes clear that each vessel possesses certain structural features enabling it to serve some function in addition to simply serving as a conduit for blood. As a result, it is possible to categorize blood vessels into functional classes: • Elastic Arteries: The largest arteries such as the pulmonary arteries, aorta, and its largest branches are capable of great degrees of distension and elastic recoil due to the numerous elastic fibers present in the tunica media. As a result, they are able to expand as they receive the ventricular stroke volume and then recoil during diastole, thereby converting the intermittent ejection of blood into continuous flow in the more distal vessels. • Muscular Arteries: represent the next class of arteries characterized by a relatively thick tunica media containing a high proportion of smooth muscle cells but less elastic fibers compared to elastic arteries. For example, while smooth muscle comprises 25% and elastic fibers 40% of the wall composition in elastic arteries, muscular arteries contain 60% smooth muscle and only 10% elastic fibers. These vessels serve as stiff conduits that are not very prone to collapse. The abundant smooth muscle content is under autonomic control and can contract or relax controlling blood flow in such organs as the brain and skeletal muscle. • Resistance Vessels: As we will see in Chapter 7, the main resistance to blood flow takes place in the smallest arteries (100-500 m diameter) and arterioles (<100 m diameter). These two vessel types have a rich innervation which when activated result in profound alterations in blood flow through a particular organ or tissue. For example, when the smooth muscle in these vessels relaxes (vasodilation), the resistance decreases and blood flow increases. The opposite occurs during vasoconstriction. 10 • • Exchange Vessels: Capillaries and post capillary venules have walls consisting of only a single layer of endothelial cells which facilitates the transfer of materials back and forth between blood and interstitial fluids. As we will also describe in Chapter 7, because of their number, the velocity of blood flow is lowest in capillaries thereby maximizing time for substances to exchange. Capacitance Vessels: As indicated above, most of the circulating volume at any one time is found in the veins. Venules and veins differ little in terms of their basic structure but are named as such based on their size. In contrast to arteries, the predominant layer in a vein is the tunica adventitia and the main tissue type present is connective tissue. As a result of the limited amount of smooth muscle and the fact that the wall thickness/lumen ratio is so large, veins have the property that they are very distensible or prone to collapse depending on changes in blood pressure. Thus because of their larger size and ability to distend, they act as a reservoir of blood. Further the smooth muscle that they do possess is also under autonomic control and under times of physiological stress, they can contract thereby displacing blood forward into the heart and arteries. IV. Sample Test Questions (see Appendix for Explained Answers) 1. Blood enters the systemic circulation from which of the following heart chambers? a. the right atrium b. the right ventricle c. the left atrium d. the left ventricle 2. After passing through systemic capillaries, blood returns to the heart by passing first to which of the following heart chambers? a. the right atrium b. the right ventricle c. the left atrium d. the left ventricle 3. Mean arterial pressure (MAP) can be computed as the product of: a. stroke volume and heart rate b. cardiac output and heart rate a. cardiac output and total peripheral resistance b. stroke volume and total peripheral resistance 4. Within the systemic circulation, total cross-sectional area is highest within: a. elastic arteries b. arterioles c. capillaries d. veins 11 CHAPTER 2: THE HEART AS A PUMP I. Structure and Function of the Contractile Apparatus Figure 2-1: Structure of a Working Cardiac Myocyte 1. Ultrastructure of Cardiac Myocytes: Working cardiac myocytes are typically about 10-20 m in diameter and 50-100 m in length with a single, centrally placed nucleus (Fig. 2-1). Among the features to note are: • Cell Arrangement: Unlike skeletal muscle cells, cardiac myocytes are branched and join to adjacent cells in an end-to-end fashion by means of structures called intercalated discs. Each intercalated disc has a characteristic stepped appearance with two distinct regions (Fig. 2-2). In the portions of the intercalated disc that are perpendicular to the long axis of the cell, cell adhesion junctions such as desmosomes are found. By contrast in the portions of the intercalated disc that are parallel to the long axis, there are gap junctions, which allow for the spread of ionic currents from one cell to the next. As a result, the myocardium is able to function as a syncytium allowing for the spread of action potentials. Figure 2-2: Ultrastructure of Two Cardiac Myocytes 12 Figure 2-3: Organization of the Sarcoplasmic Reticulum and T-tubule system • • • • T tubules: Like skeletal muscle myocytes, the sarcolemma is invaginated, forming a series of T tubules that serve the purpose of transmitting action potentials into the cell interior allowing for uniformity of myofibrillar contraction. Myofibrils: The contractile apparatus of cardiac myocytes is organized in a similar manner as skeletal muscle cells: the contractile units are found within myofibrils, each consisting of smaller units called sarcomeres. The sarcomeres are the fundamental contractile unit, defined as the region between two adjacent Z lines. The sarcomeres contain thick and thin filaments with a similar protein composition relative to skeletal muscle, and are organized in register so that the myofibrils have a banded pattern consisting of A-bands and I-bands. Furthermore, the individual myofibrils within a cell also line up in register giving the cardiac myocyte its striated appearance. One notable exception with skeletal muscle, however, involves the presence of a very large protein called titin. This protein, which has a molecular weight of ~2500 kD, extends from Z-line to Mline, and imparts limitations on the degree to which cardiac muscle fibers can stretch. Sarcoplasmic Reticulum (SR): Like skeletal muscle the sarcoplasmic reticulum of cardiac myoctyes is quite elaborate, and serves as a store for intracellular Ca2+. The SR is able to maintain high concentrations of Ca2+ within its lumen due to the presence of the Ca2+ binding protein, calsequestrin, and the membrane of the SR is enriched with Ca2+ ATPase proteins that function in association with a protein called phospholamban, the functional details of which are described below. Energy Metabolism: Like type I (slow oxidative) skeletal muscle fibers, cardiac myocytes possess a high oxidative capacity reflected by a high density of mitochondria which can occupy up to a third of cell volume. Relying on oxidative phosphorylation, cardiac function is directly dependent on an adequate O2 supply, in turn dependent on an adequate coronary blood flow. Estimates of intracellular O2 content indicate that sarcoplasmic PO2 is quite low (i.e., < 20 mmHg), such that there is a substantial O2 gradient directed into the cell. Also relevant is the fact that cardiac myocytes possess myoglobin, an O 2-binding protein that serves as an O2 store. Myoglobin is ~50% saturated with O2 at a PO2 of 5 mmHg. 13 Figure 2-4: Excitation-Contraction Coupling within Cardiac Myocytes 2. Excitation-Contraction Coupling: Like skeletal muscle, the link between electrical excitation and contraction is provided by Ca2+ ions. Following the arrival of the action potential, the intracellular calcium concentration ([Ca2+]i) rises from about 0.1 M to about 2.0 M within only a few msec. The mechanisms by which this increase in [Ca2+]i occur are shown in Fig. 2-4. • Contraction: As the action potential propagates along the surface of the cardiac myocyte, it passes down T tubules into the interior of the cell. It is during the plateau phase of the action potential that Ca2+ permeability is high resulting in a Ca2+ influx. At rest (i.e., low sarcoplasmic Ca2+), the Ca2+ release channels within the SR are closed. But as sarcoplasmic Ca2+ levels rise as a result of the inward Ca2+ current, these release channels open resulting in Ca2+ release from the SR (Ca2+- induced Ca2+ release). Thus, while the amount of Ca2+ that enters the cell from interstitial fluids is not enough to support contraction, it instead serves as a stimulus for Ca2+ release from the SR. The Ca2+ now in the sarcoplasm binds to troponin C, and the resulting complex interacts with tropomyosin causing it to unblock the myosin binding sites on the actin filaments. The result is cross-bridge cycling and contraction. It has been estimated that ~80-90% of the increase in [Ca2+]i results from Ca2+ release from the SR, and ~10-20% from extracellular Ca2+. • Relaxation: Relaxation of the cardiac myocyte involves reducing the intracellular Ca2+ concentration back to resting levels which is accomplished in several ways, the most important of which is that it is pumped back into the SR. In this case, the rise in [Ca2+]i that occurs during contraction also stimulates Ca2+ ATPase proteins located on the SR. Other mechanisms involve extruding Ca2+ from the 14 cell using the Ca2+/Na+ exchanger located within the sarcolemma which removes 1 Ca2+ ion from the cell in exchange for 3 Na+ ions that enter. The sarcolemma also contains a Ca2+ ATPase which can also remove Ca2+ ions from the sarcoplasm. Typically, ~80-90% of the Ca2+ ions are pumped back to the SR while the remainder are expelled by sarcolemma transporters. 3. Positive Inotropic Agents: Studies on membrane-permeabilized cardiac myocytes indicate that a systolic [Ca2+]i of 2.0 M result in only partial activation of the contractile machinery. In other words, at these concentrations, only a portion of the potential crossbridges are activated, meaning that a further increase in Ca2+ beyond 2.0 M will result in more cross-bridge formation and thus more force. Any agent that is able to increase systolic [Ca2+]i beyond normal and thus increase myocardial contractile force is referred to as a positive inotropic agent. Conversely, any substance which reduces systolic [Ca2+]i and thus force is considered to be a negative inotropic agent. In general, positive inotropy is accomplished in three ways: • An increase in Extracellular Ca2+: The importance of extracellular Ca2+ was first established inadvertently by Sidney Ringer (1883) who discovered that after substituting NaCl and KCl bathing solutions made with London tap water with solutions made with distilled water, the isolated hearts under study quickly weakened and failed. Ringer was able to surmise that the tap water contained substantial amounts of Ca2+, which was thus required for contraction. Subsequent work has confirmed this finding as well as the finding that a modest increase in extracellular Ca2+ is correlated with an increase in contractile force. However, very high extracellular Ca2+ can result in cardiac arrest in systole. • Activation of 1 Receptors by Catecholamines: Activation of 1 receptors on cardiac myocytes produces an increase in cAMP which in turn activates protein kinase A (PKA). PKA in turn affects cardiac contraction in three ways. 1. PKA phosphorylates sarcolemma Ca2+ channels resulting in an increase in the size of the inward Ca2+ current. The resulting increase in [Ca2+]i not only causes a relatively greater Ca2+ induced Ca2+ release, but also increases the size of Ca2+ stores within the SR. 2. PKA increases Ca2+ uptake into the SR by phosphorylating phospholamban. Phospholamban is an integral membrane protein located within the SR membrane. In the non-phosporylated state, this protein has an inhibitory influence on the SR Ca2+ ATPase, but phosphorylation of this protein removes the inhibition resulting in a relatively higher activity of the SR Ca2+ ATPase. 3. PKA phosphorylates troponin I which in turn inhibits the binding of Ca2+ to tropinin C, thus allowing tropomyosin to cover the myosin binding sites on actin. Thus, note that of the various effects of 1 activation, catecholamines act to both increase the force of contraction as well as to increase the rate of relaxation. It should also be noted that caffeine at therapeutic concentrations also serves as a positive inotropic agent through its inhibitory effects on phosphodiesterases, thus raising cAMP levels. 15 • Decreasing the Na+ gradient through Cardiac Glycosides: For centuries, it has been known that the leaves of the foxglove (Digitalis) plant were an effective treatment for heart failure. Digoxin is one example of a group of compounds known as cardiac glycosides that exert their effects by partial inhibition of the Na+/K+ ATPase. As a result, the Na+ gradient across the sarcolemma is reduced which in turn decreases the activity of the Na+/Ca2+ exchanger such that intracellular Ca2+ concentrations increase. 4. Negative Inotropic Agents: With the preceding discussion in mind, it is not difficult to realize that any agent that results in a reduction in intracellular Ca2+ will reduce the force of contraction. Thus 1 antagonists ( blockers) and Ca2+ channel blockers represent negative inotropic agents. Other factors that can reduce the strength of contraction include a reduction in extracellular Ca2+ (which can arrest the heart in diastole), and an increase in the Na+ gradient across the sarcolemma. Finally, ryanodine antagonizes Ca2+ release from the SR and thus represents a negative inotropic agent. II. Contractile Properties of Isolated Cardiac Muscle Fibers 1. Isometric Contraction: Much useful information can be obtained regarding cardiac contraction by considering the contractile properties of an isolated strip of myocardium, typically involving a strip of papillary muscle. First consider the experimental arrangement (Fig. 2-5a): A strip of papillary muscle is placed within an apparatus such as the one shown here (Fig. 2-5a). Prior to activation, the relaxed muscle is first stretched to a known length by attaching a small weight to one end, which is referred to as the preload. The preload is then clamped into place. Next the muscle is stimulated, and the resulting tension is measured by means of a force transducer. Because the muscle cannot shorten, this arrangement results in an isometric contraction – which we will consider analogous to a portion of the cardiac cycle called isovolumetric contraction. Figure 2-5a: Experimental Apparatus By repeating the experiment for different resting lengths, we can produce the following type of graph (Fig 2-5b). From these data it is apparent that as the resting length of the papillary muscle is increased, the peak tension that can be produced increases. When applied to the heart in vivo, the conclusion is that stretching the relaxed myocardium prior to contraction (i.e., an increase in preload) results in an increase in the force of contraction. Figure 2-5b: Length-Tension Relationships for Isolated Papillary Muscle 16 2. Isotonic Contraction: Using the same experimental apparatus (Fig. 2-5a), it is possible to produce an isotonic contraction by allowing the muscle to lift a weight (called the afterload). Consider what happens (Fig. 2-6): In this case, our muscle is prestretched by some preload (point A). If we now stimulate, we find that initially there is no external shortening of the muscle because of the need to first stretch the series elastic element (EE). By point C, the force developed by the contractile element (CE) has come to equal the load (i.e., the afterload) which can now be lifted under constant tension. Figure 2-6: Sequence of events during an Isotonic contraction. Note that as the afterload is increased, both the amount of shortening as well as the velocity are decreased (Fig. 2-7). Yet, by comparing isotonic contractions conducted using two resting lengths, pre-stretching the muscle fiber to longer initial lengths results in a greater amount of shortening and a higher velocity per given afterload compared to a shorter initial length. When applied to the in vivo heart, the implications of these data are two-fold: 1) An increase in afterload ( = arterial blood pressure) will reduce both the amount of shortening and the velocity of contraction. 2) But by increasing preload, both the amount of shortening and velocity are increased. Thus, again the energy of contraction is a function of the resting fiber length. Figure 2-7: Results of Isotonic Muscle Contractions 3. The Length-Tension Relationship: The length-tension relationship is somewhat different in cardiac muscle as compared to skeletal muscle. To illustrate, consider an experiment in which a strip of cardiac muscle is stretched to various lengths, stimulated, and the resulting force measured. But unlike the typical protocol involving skeletal muscle, the experiment with cardiac muscle is done with two bathing solutions: one containing a physiological concentration of Ca2+; and the other with a higher concentration. 17 Consider first the curve obtained under physiological conditions (labeled normal Ca2+). As these data show, the force of contraction increases with sarcomere length reaching a maximum at about 2.2-2.3 m. Beyond 2.3 m, force drops off (data not shown), but it is difficult to stretch sarcomeres to this point due to the stiffness of the series elastic element. Thus. because sarcomere lengths rarely reach this maximum value in the in vivo heart, the heart is considered to be working on the “ascending portion” of the curve. Figure 2-8: Relation between Force and Sarcomere Length Based on these data, it would appear that there are two contributing factors to the lengthtension relationship: • Thick and Thin Filament Geometry: Recall from our discussion of skeletal muscle, that sarcomere length affects the overlap of thick and thin filaments which in turn affects possible cross-bridge interactions – and the same considerations hold here as well. For example, at sarcomere lengths of less than 2.0 m, actin filaments overlap, and at sarcomere lengths of less than 1.6 m, the thick filaments interfere with the Z lines. When these mechanical interferences are reduced by stretch, the force of contraction increases. • Altered Ca2+ Sensitivity: In Fig. 2-8, it is apparent that at a physiological [Ca2+]i, only a fraction of potential cross-bridges are activated based on the observation that force increases (per given sarcomere length) in the presence of a saturating concentration of Ca2+. Yet note that the slope of the physiological curve is steeper and approaches that of the saturating curve – indicating that the fraction of potential cross-bridges activated by physiological concentrations of Ca2+ increases with stretch – a phenomenon sometimes called length-dependent activation. It would appear that this phenomenon is due to an increase in the sensitivity of the contractile proteins to Ca2+ upon stretch (Fig. 2-9). When sarcomeres are stretched, the curve is shifted to the left, such that the Ca2+ concentration needed to produce 50% of maximum tension ([Ca2+]50%) is decreased with stretch. The mechanism by which this phenomenon occurs is not known. Figure 2-9: Length Dependence of Ca2+ Sensitivity 18 III. The Heart – Lung Preparation 1. The Heart-Lung Preparation: While information obtained from studying the mechanics of isolated cardiac fibers is certainly useful to understanding how the heart functions as a pump, it has also been necessary to use experimental systems involving isolated organ preparations. Two of the most famous groups of experiments using organ preps involve the work of Frank and Starling – the results of which have led to a fundamental length – tension relationship named after these two physiologists, the Frank-Starling Law of the Heart. 2. The Experiments of Otto Frank: One of the first, great advances in understanding length-tension relationships in intact hearts was accomplished well over a century ago by the German physiologist, Otto Frank, who studied the effect of diastolic stretch on the ventricular contraction of the frog heart. Figure 2-10: The Effect of Changes in Diastolic Volume on Systolic Pressure (Frank, 1895) In his experiments, the aorta was ligated so that contraction became essentially isovolumetric, and different amounts of fluid were injected to stretch the wall. Frank then measured the systolic pressure achieved by the ventricle as a function of different filling pressures (Fig. 2-10). The results of these experiments produced a family of curves in which ventricular volume is increased from 1 to 4 (arbitrary units). By plotting peak systolic pressure as well as diastolic pressure as a function of volume, it is possible to construct separate systolic and diastolic curves. • Diastolic Relationship: The curve indicated by the open circles represents the passive pressure-volume relationship. Note that at high ventricular volumes, the pressure increases quickly due to an increase in the stiffness upon ventricular distension. • Systolic Relationship: As we have seen with isolated papillary muscles, the peak pressure increases with stretch. • The pressure actively generated by the ventricle can be determined by taking the difference in the two curves. Again, the conclusion is that the energy of contraction depends on the diastolic distension. 19 3. The Experiments of Ernest Starling: The British physiologist, Ernest Starling and colleagues followed up these experiments some years later using a canine heart-lung preparation perfused with warm oxygenated blood (Fig. 2-11). Among the different variables: 1) By adjusting the height of the venous reservoir, Starling et al. were able to alter central venous pressure (CVP). While technically the pressure within the great veins, it is essentially equivalent to the EDP of the right ventricle, and thus equivalent to the filling pressure. 2) The aortic pressure was held constant by the “Starling Resistor” – a device that provided for variable vascular resistance so that arterial pressure could be maintained. 3) The combined stroke volumes of the two ventricles were determined by a bell cardiometer – an inverted glass bell attached to the atrioventricular groove by a rubber diaphragm. Beat to beat volume changes were recording on a rotating smoked drum. Fig. 2-11: Isolated Canine Heart-Lung Preparation of Starling et al. By using isolated hearts, Starling and his colleagues were able to study the heart free from any nervous and hormonal influences. Among their findings (Fig. 2-12): These data are a reproduction of the original smoked drum records of Starling et al. Note that the volume has an inverted scale (in mL) such that the diastolic volume is at the bottom and end-systolic volume is at the top. Stroke volume is the distance from the top to the bottom of the trace. Also plotted are CVP which was increased from 9 cmH2O (period A) to 14 cmH2O (period B) and aortic pressure which was kept approximately constant throughout the experiment. From these data, note there was a significant (64%) increase in stroke volume that occurred in response to an increase in filling pressure. Figure 2-12: The results of Patterson, Piper, and Starling, 1914) 4. Ventricular Function Curves: By varying filling pressure in a series of steps, it is possible to plot the relationship between stroke volume and filling pressure in a type of curve called a ventricular function curve, or a Starling curve (Fig. 2-13). 20 In the isolated dog heart, the curves for the two ventricles have a similar shape except that the left ventricle (LV) has slightly higher filling pressures – presumably because the wall is thicker and less distensible in comparison to the right ventricle (RV). As a result, the EDP has to be 4-5 mmHg higher to produce the same cardiac output as that produced by the RV. In the over-distended heart, the stroke volume declines because of AV valve leakage and decreased curvature of the ventricular wall impairs the conversion of tension into pressure (LaPlace’s Law). Figure 2-13: Ventricular Function Curves for the Isolated Dog Heart While we have restricted our definition of ventricular function curves to those plots of stroke volume, there are actually a host of different kinds of plots referred to as ventricular function curves. For example, consider Fig. 2-14. In these curves, the x-axis is often CVP, EDP, or EDV: all indications of resting fiber length. The y-axis can be stoke volume (provided that arterial pressure is held constant), or better yet, stroke work, which is equal to the stroke volume multiplied by mean arterial pressure. In any event, these curves demonstrate the observation that the greater the stretch of the ventricle in diastole, the greater the stoke work achieved in systole – a deduction that is referred to as Starling’s Law (or the Frank-Starling Law of the Heart) in honor of these early physiologists. Figure 2-14: Ventricular Function Curves 21 5. The Effect of Arterial Pressure on Stroke Volume: Thus far, most of our observations have been obtained in the presence of a constant arterial pressure, but arterial pressure (representing afterload) has a profound influence on stroke volume, which can be divided into direct and indirect effects. • Direct Effects: It stands to reason that a high arterial pressure will oppose the ejection of blood from the ventricle and thus decrease stroke volume. For example, consider a typical mechanical pump (Fig. 2-15). If you increase the pressure at the outlet of a pump, you will decrease the outflow – a relationship we can refer to as the pump function curve. When applied to the heart, the same applies in that if we keep EDP constant and raise arterial pressure, the SV will decrease (e.g., point A → point B). In other words, an increase in arterial pressure will increase the proportion of energy that must be spent during the isovolumetric contraction phase. Figure 2-15: Pump Function Curves • Secondary Effects: However, if we allow EDP to vary, other effects will be seen. For example, the initial decrease in SV will cause EDV and thus EDP to increase. This in turn (through Frank-Starling mechanisms) will cause stroke volume to increase (e.g., point B → point C). These findings were actually documented by Starling et al. (Fig. 2-16). Here, arterial pressure was increased which caused SV to decrease initially (see solid arrow), but with a continuous inflow, there was a rise in venous filling pressure (V.P.) over the next few seconds, such that with the increased ventricular stretch, SV was restored. Fig. 2-16: Effects of Increased Arterial Pressure on Stroke Volume (Piper, et al., 1914) 22 • Summary: The effects of increased arterial pressure (afterload) on stroke volume are complex, but can be subdivided into three basic effects (Fig. 2-17). 1. Effect 1: The increase in afterload will initially result in a decrease in stroke volume in accordance with pump function curves. 2. Effect 2: The decrease in stroke volume will result in an increase in EDV which in turn will cause increase ventricular stretch and a restoration of stroke volume. 3. Effect 3: Baroreceptor Reflex: The increase in arterial pressure will stimulate cardiovascular centers within the brainstem to decrease the sympathetic output to the heart which will decrease the contractile strength of the heart thereby reducing stroke volume. Figure 2-17: Summary of Effects of an Increased Arterial Pressure on Stroke Volume. In the intact heart, the actual effect will depend on the interplay of these three effects. IV. Sample Test Questions (see Appendix for Explained Answers) 5. Which of the following events will occur following stimulation of β1 receptors on the surface of working ventricular myocytes? a. increased Ca2+ conductance via L-type Ca2+ channels b. increased pump rate of Ca2+ back into the SR via the Ca2+ ATPase c. increased rate of phosphorylation of troponin I d. all of the above 23 6. Which of the following agents is most likely to increase contractility within the left ventricle? a. a Ca2+ channel blocker b. a drug that partially inhibits the Na+/K+ pump c. a β1 antagonist d. none of the above 7. Which of the following agents is most likely to decrease contractility within the left ventricle? a. Dobutamine (a β1 agonist) b. Digitalis (a cardiac glycoside that partially inhibits the Na+/K+ pump) c. Verapamil (a Ca2+ channel blocker) d. Caffeine (a phosphodiesterase inhibitor) 8. Which of the following quantities pertaining to the left ventricle provides the most accurate estimation of preload of the left ventricle? a. peak pressure b. end diastolic volume (EDV) c. stroke volume d. cardiac output 9. Which portion of the cardiac cycle includes isometric contraction of ventricular myocytes? a. ventricular filling b. isovolumetric contraction c. ventricular ejection d. isovolumetric relaxation 10. Which portion of the cardiac cycle features isotonic contraction of the left ventricle? a. isovolumetric relaxation b. isovolumetric contraction c. ventricular ejection d. diastasis 24 CHAPTER 3: ELECTROPHYSIOLOGY OF THE HEART I. Overview to Electrical Activity within the Heart The heart beat is initiated within the heart itself, normally arising from a group of modified cardiac muscle cells that possess the property of being able to spontaneously generate action potentials. These pacemaker cells, in turn are coupled to other cardiac muscle cells through gap junctions, which allow ionic currents to spread from cell to cell, thus propagating through the heart. In some cases, the action potentials spread to normal working cardiac myocytes (cells designed to generate force), but in other cases, the action potential is transferred to cells similar in nature to pacemaker cells which are referred to in general as conducting cardiac myocytes. Together these conducting cells form a preferential path by which the electrical impulse is carried through the heart. This preferred path, referred to as the conduction system of the heart, is important in several respects. First, because the cardiac action potential is the trigger for EC coupling, it is imperative that propagation be organized so as to allow the atria to contract first, followed by the ventricles. It is also important that there be one path of excitation in order to allow the sequential activation of specific regions of the heart so as to produce a coordinated contraction. It is the conduction system of the heart which makes this possible. Any alterations in the normal spatial or temporal sequence of propagation is called an arrhythmia, which has important functional consequences since the result is often a pattern of uncoordinated contraction, limiting the hearts ability to generate a sufficient cardiac output. In describing the conduction system of the heart and the different cell types which comprise it, it is important to recognize that action potentials take different forms as they propagate between different cell types. It is thus the purpose of this chapter to describe the ionic basis behind the different cardiac action potentials observed, and the factors that affect the ability to generate and conduct these action potentials through the heart. 1. The Conduction System of the Heart: Sinoatrial (SA) Node: The heart beat is normally initiated within the SA node, a structure located at the junction of the superior vena cava and right atrium, and consisting of small conducting cells embedded in a mass of connective tissue. The conducting cells of the SA node have unstable resting potentials, which cause them to spontaneously Figure 3-1: The Conduction System of the Heart 25 • • • • generate action potentials at a rate of between 60-100 beats per minute. The fact that they are coupled to other adjacent cardiac myocytes allows the action potential to spread from cell to cell, thereby propagating throughout the right atrium and eventually into the left atrium. Whether there are established conduction paths through the atria or between the SA and AV nodes remains unclear. However, the action potential eventually passes down the inter-atrial septum and arrives at the AV node. Atrioventricular (AV) Node: consists of a small number of conducting cells embedded in a mass of connective tissue located in the lower, posterior region of the inter-atrial septum. A characteristic feature of the AV node (and SA node for that matter) is the slow conduction, itself a property of their small diameters and the relatively low density of gap junctions between interconnecting cells. For example, typical conduction velocities within the SA and AV nodes are only about 0.05 m/sec which are slow in comparison to working atrial (0.1 m/sec) and ventricular myoctes (0.2 m/sec). This resulting “AV delay” allows the atria to have sufficient time to depolarize before the action potential enters the ventricles. Bundle of His: Recall that atria and ventricles are electrically isolated by virtue of the fibrous skeleton the heart – in this case the paired annuli fibrosi and membranous IV septum. As a result, the Bundle of His and its descending branches represent the only path by which the electrical impulse can travel between atria and ventricles. As a structure, the Bundle of His consists of a bundle of fast-conducting cells that leave the AV node, travel across the annulus fibrosus to enter the membranous IV septum. At this point, it divides into a number of bundle branches which descend down on either side of the muscular IV septum. Purkinje cells: Bundle branches terminate into cells that were first described by the Hungarian histologist, Jan Purkinje. Purkinje cells are large diameter conducting cells that form a layer just below the endocardium of the IV septum. They conduct at high velocity (~2.0 m/sec), which gives them the ability to distribute the electrical signal rapidly throughout the ventricles allowing the ventricles to depolarize within ~ 0.1 sec. Pacing of the Heart: Under normal circumstances, cells of the SA node pace the heart, but in theory any conducing cell (or in certain pathophysiological states, non-conducting cells) can pace the heart. The reason the SA node is the normal pacemaker has to do with the fact that these cells spontaneously generate action potentials at the fastest rate. However, if the SA node is rendered dysfunctional, cells of AV node can take over as pacemaker, but their maximum firing rate is only about 50 beats/min. If these cells are no longer able to pace the heart, then cells of the His-Purkinje cells can take over, but their firing rates are even slower, ~30 beats/min. The point is that there is a system in place which allows the heart to continue to demonstrate automaticity, even when normal pacemakers are blocked. 26 2. Heterogeneity among Cardiac Action Potentials: Figure 3-2 depicts a number of action potentials recorded from different cell types in the heart: Figure 3-2: Action Potentials recorded at five sites along the spread of excitation. Note the timing of the upstroke of each action potential on the x-axis, reflecting the temporal sequence of activation through the different regions of the heart. The waveform of these action potentials represents the sum of a number of different ionic currents; how these currents differ among the various cell types in the heart is described below. II. The Maximum Diastolic Potential 1. The Concept of Maximum Diastolic Potential: A comparison of the resting potentials depicted in Fig. 3-2 reveals two basic differences among the cells shown. First, the resting potentials of atrial, ventricular, and Purkinje fibers are at considerably more negative potentials than those observed in SA and AV nodal cells (i.e., ~ -80 mV compared to ~ -60 mV in the nodal cells). The second observation has to do with the fact that the two nodal cells types (as well as Purkinje fibers) do not exhibit a stable resting potential in the intervals between successive action potentials, giving rise to the concept of a maximum diastolic potential, or MDP, defined as the most negative potential the cell achieves following the repolarization phase of the action potential. By convention, all “resting potentials” of cardiac myocytes, even those recorded from atrial and ventricular working myoctes, are referred to as the MDP. 2. The Relationship between MDP and the Equilibrium Potential of K +: Recall that the resting potential of a cell, as a first approximation, represents a weighted average of 27 the equilibrium potentials for K+ and Na+, the weighting depending on the relative resting ionic conductances of these two ions. Using the Nernst equation and typical intracellular and extracellular concentrations to calculate equilibrium potentials, it is obvious that the MDP of cardiac myocytes is much closer to EK than ENa, a reflection of the fact that the resting K+ conductance is larger by at least an order of magnitude than the resting Na+ conductance. As a consequence, it is not surprising to observe that the MDP depends heavily on changes in EK. Fig. 3-3 shows how the MDP of atrial, ventricular, and Purkinje fibers varies with changes in the extracellular [K+]. According to the Nernst equation, as extracellular [K+] increases, EK moves to progressively more positive voltages (dotted line), and for serum K+ levels above 5 mM, there is good agreement between EK and MDP. However, as serum K+ levels are lowered, there is increasing departure such that MDP is at a more positive po