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Chapter 17 / The Cardiovascular System: The Heart I. INTRODUCTION A. The heart is the center of the cardiovascular system. This chapter explores the design of the heart and the unique properties of cardiac muscle that permit a lifetime of pumping with never a minute's rest. B. The study of the normal heart and diseases associated with it is known as cardiology. II. OVERVIEW OF THE CIRCULATION A. Once blood has been oxygenated in the lungs it passes through a series of channels via the heart and body before returning to the heart, all together called the systemic circulation ,which includes; 1. pulmonary veins, 2. left atrium and ventricle, 3. aorta and progressively smaller arterial branches, 4. capillaries (where exchange with the tissues occurs), 5. venules, 6. progressively larger veins, and 7. vena cava. B. The deoxygenated blood containing high levels of carbon dioxide from the tissues returned to the right heart passes through the pulmonary circulation, which includes; 1. the right atrium and ventricle, 2. pulmonary artery, and 3. arteries and capillaries of the lungs. III. LOCATION AND SIZE OF THE HEART A. The heart is situated between the lungs in the mediastinum. B. About two-thirds of its mass is to the left of the midline. C. The heart is about 12 cm long, 9 cm wide, and 6 cm thick. IV. PERICARDIUM A. The heart is enclosed and held in place by the pericardium. 1. The pericardium consists of an outer fibrous pericardium and an inner serous pericardium. 2. The serous pericardium is composed of a parietal layer and a visceral layer. 3. Between the parietal and visceral layers of the serous pericardium is the pericardial cavity, a potential space filled with pericardial fluid that reduces friction between the two membranes. B. An inflammation of the pericardium is known as pericarditis. Associated bleeding into the pericardial cavity compresses the heart (cardiac tamponade) and is potentially lethal. V. HEART WALL A. The wall of the heart has three layers: epicardium, myocardium, and endocardiurn B. The epicardium consists of mesothelium and connective tissue, the myocardium is composed of cardiac muscle tissue, and the endocardiurn consists of endothelium and connective tissue. VI. CHAMBERS OF THE HEART A. The chambers of the heart include two upper atria and two lower ventricles. B. An interatrial septum separates the atria; an interventricular septum separates the ventricles. VII. VALVES OF THE HEART A. Valves, composed of dense connective tissue covered by endothelium, prevent backflow of blood in the heart. 1. Atrioventricular (AV) valves, between the atria and their ventricles, are the tricuspid valve on the right side of the heart and the bicuspid (mitral) valve on the left. 2. The chordae tendineae and their papillary muscles keep the flaps of the valves pointing in the direction of the blood flow and stop blood from backing into the atria. 3. Semilunar valves prevent blood from flowing back into the heart as it leaves the heart for the lungs (pulmonary semilunar valve) or for the rest of the body (aortic semilunar valve). B. Rheumatic fever is precipitated by infection with group A, alphahemolytic strains of Streptococcus pyogenes bacteria. The ultimate result is damage to valves of the heart, most commonly the bicuspid and aortic semilunar valves. VIII. BLOOD FLOW THROUGH THE PULMONARY AND SYSTEMIC CIRCULATIONS A. Blood flows through the heart from the superior and inferior venae cavae and the coronary sinus to the right atrium, through the tricuspid valve to the right ventricle, through the pulmonary trunk and pulmonary arteries to the lungs, through the pulmonary veins into the left atrium, through the bicuspid valve to the left ventricle, and out through the aorta. B. Divisions of the aorta are the ascending aorta, arch of the aorta, thoracic aorta, and abdominal aorta. C. Once blood has been oxygenated in the lungs it passes through a series of channels via the heart and body before returning to the heart, all together called the systemic circulation, which includes; 1. pulmonary veins, 2. left atrium and ventricle, 3. aorta and progressively smaller arterial branches, 4. capillaries (where exchange with the tissues occurs), 5. venules, 6. progressively larger veins, and 7. vena cava. D. The deoxygenated blood containing high levels of carbon dioxide from the tissues returned to the right heart passes through the pulmonary circulation, which includes; 1. the right atrium and ventricle, 2. pulmonary artery, and 3. arteries and capillaries of the lungs. IX. HEART BLOOD SUPPLY A. The flow of blood through the many vessels that pierce the myocardium of the heart is called the coronary (cardiac) circulation; it delivers oxygenated blood and nutrients to the myocardiurn and removes carbon dioxide and wastes from it. B. The principal arteries, branching from the ascending aorta and carrying oxygenated blood, are the right and left coronary arteries; deoxygenated blood returns to the right atrium primarily via the principal vein, the coronary sinus. C. Most heart problems result from faulty coronary circulation due to blood clots, fatty atherosclerotic plaques, or spasms of the smooth muscle in coronary artery walls. Complications of this system include angina pectoris (severe pain that accompanies reduced blood flow, or ischernia, to the myocardium) and myocardial infarction (MI, or heart attack, in which there is death of an area of the myocardium due to an interruption of the blood supply; it may result from a thrombus or embolus). D. Whenever a disease or injury deprives a tissue of oxygen, reestablishing the blood flow (reperfusion) may damage the tissue further due to the formation of oxygen free radicals; these can destabilize the molecular structure of proteins, neurotransmitters, nucleic acids, and phospholipids of plasma membranes. X. CONDUCTION SYSTEM AND PACEMAKER A. The conduction system consists of tissue specialized for generation and conduction of spontaneous action potentials that stimulate the cardiac muscle fibers (cells) to contract. B. Components of this system are the sinoatrial (SA) node (pacemaker), atrioventricular (AV) node, atrioventricular (AV) bundle (bundle of His), right and left bundle branches, and the conduction myofibers (Purkinje fibers) C. Signals from the autonomic nervous system and hormones, such as epinephrine, do modify the heartbeat (in terms of rate and strength of contraction), but they do not establish the fundamental rhythm. D. An artificial pacemaker may be used to restore cardiac rhythm due to disruption of some component of the conduction system. XI. PHYSIOLOGY OF CARDIAC MUSCLE CONTRACTION A. An impulse in a ventricular contractile fiber is characterized by rapid depolarization, plateau, and repolarization. B. The refractory period of a cardiac muscle fiber (the time interval when a second contraction cannot be triggered) is longer than the contraction itself. XII. ELECTROCARDIOGRAM A. Impulse conduction through the heart generates electrical currents that can be detected at the surface of the body. A recording of the electrical changes that accompany each cardiac cycle (heartbeat) is called an electrocardiogram (ECG or EKG). B. A normal ECG consists of a P wave (atrial depolarization-spread of impulse from SA node over atria), QRS complex (ventricular depolarization-spread of impulse through ventricles), and T wave (ventricular repolarization). C. The P-Q (PR) interval represents the conduction time from the beginning of atrial excitation to the beginning of ventricular excitation. D. The S-T segment represents the time when ventricular contractile fibers are fully depolarized, during the plateau phase of the impulse. E. The ECG is invaluable in diagnosing abnormal cardiac rhythms and conduction patterns and following the course of recovery from a heart attack. XIII. CARDIAC CYCLE A. A cardiac cycle consists of the systole (contraction) and diastole (relaxation) of both atria, rapidly followed by the systole and diastole of both ventricles. B. The phases of the cardiac cycle are (1) the relaxation (or quiescent) period, (2) ventricular filling, and (3) ventricular systole. C. With an average heart rate of 75 beats/riiin, a complete cardiac cycle requires 0.8 sec. D. The act of listening to sounds within the body is called auscultation, and it is usually done with a stethoscope. The sound of a heartbeat comes primarily from the turbulence in blood flow caused by the closure of the valves, not from the contraction of the heart muscle. 1. The first heart sound (S1 --lubb) is created by blood turbulence associated with the closing of the atrioventricular valves soon after ventricular systole begins. 2. The second heart sound (S2--dupp) represents the closing of the semilunar valves close to the end of the ventricular systole. 3. A heart murmur is an abnormal sound that consists of a flow noise that is heard before, between, or after the lubb-dupp or that may mask the normal sounds entirely. Some murmurs are caused by turbulent blood flow around valves due to abnormal anatomy or increased volume of flow. a. Not all murmurs are abnormal or symptomatic, but most indicate a valve disorder. b. Among the valvular disorders that may contribute to murmurs are mitral stenosis, mitral insufficiency, aortic stenosis, aortic insufficiency, and mitral valve prolapse (MVP). XIV. CARDIAC OUTPUT A. Since the body's need for oxygen varies with the level of activity, the heart's ability to discharge oxygen-carrying blood must also be variable. Body cells need specific amounts of blood each minute to maintain health and life. B. Cardiac output (CO) is the amount of blood ejected by the left ventricle (or right ventricle) into the aorta (or pulmonary trunk) per minute. It is calculated as follows: CO = stroke volume x beats per minute. 1. Stroke volume (SV) is the amount of blood ejected by a ventricle during each systole. a. Stroke volume depends on how much blood enters a ventricle during diastole, that is, the stretch on the heart before it contracts (end-diastolic volume, or EDV, also called preload), and how much blood is left in a ventricle following its systole (endsystolic volume, or ESV). EDV averages 120-130 ml and SV is about 70 ml; ESV, then, is approximately 50-60 ml. b. Stroke volume also is related to contractility (forcefulness of contraction at any given time) and afterload (pressure that must be exceeded before ventricular ejection can begin); factors that increase the stroke volume or heart rate tend to increase CO and vice versa. c. Cardiac reserve is the ratio between the maximum cardiac output a person can achieve and the cardiac output at rest. d. According to the Frank-Starling law of the heart, a greater preload (stretch) on cardiac muscle fibers just before they contract increases their force of contraction during systole. e. Congestive heart failure (CHF) results when the heart cannot supply the oxygen demands of the body; it is characterized by diminished blood flow to the various tissues of the body and by accumulation of excess blood in the various organs because the heart is unable to pump out the blood returned to it by the great veins. Causes include chronic hypertension and myocardial infarction (heart attack). 2. Cardiac output depends on heart rate as well as stroke volume. Changing heart rate is the body's principal mechanism of short-term control over cardiac output and blood pressure; several factors contribute to regulation of heart rate. a. Nervous system control of the cardiovascular system stems from the cardiovascular center in the medulla 1) Sympathetic impulses increase heart rate and force of contraction; parasympathetic impulses decrease heart rate. 2) Baroreceptors (pressure receptors) are nerve cells that respond to changes in blood pressure and relay the information to the cardiovascular center; important baroreceptors are located in the arch of the aorta and carotid arteries. b. Heart rate is also affected by hormones (epinephrine, norepinephrine, thyroid hormones), ions (Na+, K+, Ca2+), age, gender, physical fitness, and temperature. C. Help for Failing Hearts 1. Although a variety of drugs are helpful in the earlier stages of heart disease, at some point they are no longer effective because there is too little functional cardiac muscle left. 2. Researchers are investigating a wide variety of devices and techniques to aid a failing heart, including heart transplants, artificial hearts, and cardiac assist devices. XV. RISK FACTORS IN HEART DISEASE A. Risk factors in heart disease that can be modified include high blood cholesterol level, high blood pressure, cigarette smoking, obesity, and lack of regular exercise. B. Other factors include diabetes mellitus; genetic predisposition; male gender; high blood levels of fibrinogen, renin, and uric acid; and left ventricular hypertrophy. XVI. PLASMA LIPIDS AND HEART DISEASE A. A strong risk factor for developing heart disease is high blood cholesterol level. The reason is that high blood cholesterol promotes growth of fatty plaques that build up in the walls of arteries. B. Most lipids are transported in the blood in combination with proteins as lipoproteins. 1. Three classes of lipoproteins are called low-density lipoproteins (LDLs), high density lipoproteins (HDLs), and very low-density lipoproteins (VLDLs). 2. Whereas HDLs remove excess cholesterol from circulation, high levels of LDLs are associated with the formation of fatty plaques in arteries. VLDLs also contribute to increased fatty plaque formation. C. There are two sources of cholesterol in the body: some is present in foods we ingest, but most is synthesized by the liver. D. For adults, desirable levels of blood cholesterol are TC (total cholesterol) under 200 mg/dl, LDL under 130 mg/dl, and HDL over 40 mg/dl; normally, triglycerides are in the range of 10- 190 mg/dl. E. Among the therapies used to reduce blood cholesterol level are exercise, diet, and drugs. XVII. EXERCISE AND THE HEART A. Sustained exercise increases oxygen demand in muscles. B. Among the benefits of aerobic exercise (any activity that works large body muscles for at least 20 minutes, preferably 3-5 times per week) are increased cardiac output, increased HDL and decreased triglycerides, improved lung function, decreased blood pressure, and weight control. XVIII. DEVELOPMENTAL ANATOMY OF THE HEART A. The heart develops from mesoderm before the end of the third week of gestation. B. The endothelial tubes develop into the four-chambered heart and great vessels of the heart. XIX. DISORDERS: HOMEOSTATIC IMBALANCES A. Coronary artery disease (CAD), or coronary heart disease (CHD), is a condition in which the heart muscle receives an inadequate amount of blood due to obstruction of its blood supply. It is the leading cause of death in the United States each year. The principal causes of narrowing include atherosclerosis, coronary artery spasm, or a clot in a coronary artery. 1. Atherosclerosis is a process in which smooth muscle cells proliferate and fatty substances, especially cholesterol and triglycerides (neutral fats), accumulate in the walls of medium-sized and large arteries in response to certain stimuli, such as endothelial damage. 2. Treatment of coronary artery disease varies with the nature and urgency of symptoms. Among the treatment options are drug therapy (nitroglycerin, beta blockers, and thrombolytic agents) and various surgical and nonsurgical procedures. Examples include coronary artery bypass grafting (CABG), percutaneous transluminal coronary angioplasty (PTCA), percutaneous balloon valvuloplasty, laser angioplasty, balloon-laser welding, and catheter arthrectomy. 3. Coronary artery spasm is a condition in which the smooth muscle of a coronary artery undergoes a sudden contraction, resulting in narrowing of a blood vessel. B. Congenital Defects 1. A congenital defect is a defect that exists at birth, and usually before birth. 2. Congenital defects of the heart include coarctation of the aorta , patent ductus arteriosus, septal defects (interatrial or interventricular; valvular stenosis, and tetralogy of Fallot. a. Some congenital defects are not serious or remain asymptomatic; others heal themselves. b. A few congenital defects are life threatening and must be corrected surgically. Fortunately, surgical techniques are highly refined for most of the defects listed. C. Arrhythmias 1. An arrhythmia is an abnormality or irregularity in the heart rhythm resulting from a disturbance in the conduction system of the heart, due either to faulty production of electrical impulses or to poor conduction of impulses as they pass through the system. 2. Examples of arrhythmias include heart block (most commonly atrioventricular block, flutter, fibrillation (atrial and ventricular), and premature ventricular contraction (PVC). a. Some are caused by the effects of such factors as caffeine, nicotine, alcohol, anxiety, certain drugs, hyperthyroidism, potassium deficiency, and certain heart diseases. b. Most can be controlled if detected and treated early enough.