Heart Anatomy PDF

Chapter 19: The Cardiovascular System: The Heart Suggested Lecture Outline I. INTRODUCTION A. The cardiovascular system consists of the blood, heart, and blood vessels. B. The heart is the pump that beats about 100,000 times every day to circulate blood through an estim...

Chapter 19: The Cardiovascular System: The Heart Suggested Lecture Outline I. INTRODUCTION A. The cardiovascular system consists of the blood, heart, and blood vessels. B. The heart is the pump that beats about 100,000 times every day to circulate blood through an estimated 60,000 miles of blood vessels. II. THE HEART IS LOCATED IN THE MEDIASTINUM AND HAS A MUSCULAR WALL COVERED BY PERICARDIUM. A. Location of the Heart 1. The heart is situated between the lungs in the mediastinum with about two-thirds of its mass to the left of the midline 2. The apex is inferior and the base is superior. B. Pericardium 1. The heart is enclosed and held in place by the pericardium. a. The pericardium consists of an outer fibrous pericardium (dense irregular connective tissue) and an inner serous pericardium (epicardium) b. The serous pericardium is composed of a parietal layer and a visceral layer c. 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. C. Layers of the Heart Wall 1. The wall of the heart has three layers: epicardium, myocardium, and endocardium CHAPTER 19( HEART) DR PARIA PARTO 1  2. The epicardium consists of mesothelium and connective tissue and contains blood and lymphatic vessels that supply the myocardium. The myocardium is composed of cardiac muscle which is organized in bundles that swirl diagonally around the heart. The endocardium consists of a thin layer of endothelium and connective tissue that provides a smooth lining for the heart chambers and also covers the valves. 3. Clinical Connection: CPR Standard CPR uses both cardiac compressions and mouth-to-mouth respiration. Recently, hands-only CPR has become the preferred method for adults. CPR works because the heart is located between two rigid structures; the sternum and the vertebral column, so chest pressure can be used to force blood out of the heart. Hands-only CPR boosts the survival rate from 18% to 34% compared to the traditional method or none at all. Also this method is more likely to be performed by bystanders since they do not have to worry about contracting contagious diseases. III. THE HEART HAS FOUR CHAMBERS, TWO UPPER ATRIA AND TWO LOWER VENTRICLES. A. The chambers of the heart include two upper atria and two lower ventricles (Figure 19.3). The atria receive blood from veins while the ventricles eject blood into arteries. B. On the surface of the heart are the auricles and sulci 1. The auricles are small pouches on the anterior surface of each atrium that slightly increase the capacity of each atrium. 2. The sulci are grooves that contain blood vessels and fat and separate the chambers. They are the coronary sulcus, anterior interventricular sulcus, and posterior interventricular sulcus. CHAPTER 19( HEART) DR PARIA PARTO 2 C. Right Atrium 1. The right atrium receives blood from the superior and inferior vena cava and the coronary sinus. The anterior wall of the right atrium contains muscular ridges called pectinate muscles which also extend into the auricle. 2. In the septum separating the right and left atria is an oval depression, the fossa ovalis, which is the remnant of the foramen ovale. 3. Blood passes from the right atrium into the right ventricle through the tricuspid valve. D. Right Ventricle 1. The right ventricle forms most of the anterior surface of the heart. 2. Trabeculae carneae are raided bundles of myocardium. 3. The cusps of the tricuspid valves are connected to chordae tendineae that in turn are connected to papillary muscles. 4. Blood passes from the right ventricle to the pulmonary trunk via the pulmonary valve. E. Left Atrium 1. The left atrium receives blood from the pulmonary veins. 2. Its anterior wall is smooth because the pectinate muscles are within the auricle. Blood passes from the left atrium to the left ventricle through the bicuspid (mitral) valve. F. Left Ventricle 1. The left ventricle is the thickest chamber and forms the apex of the heart. 2. The left ventricle also has trabeculae carneae and chordae tendineae 3. Blood passes from the left ventricle through the aortic valve into the ascending aorta. Some of the blood from the aorta flows into coronary arteries while the remainder passes through the arch of the aorta and descending aorta. CHAPTER 19( HEART) DR PARIA PARTO 3  4. During fetal life the ductus arteriosus shunts blood from the pulmonary trunk into the aorta. At birth the ductus arteriosus closes and becomes the ligamentum arteriosum G. Myocardial Thickness and Function 1. The thickness of the myocardium of the four chambers varies according to the function of each chamber. 2. The atria walls are thin because they deliver blood to the ventricles 3. The ventricle walls are thicker because they pump blood greater distances. a. The right ventricle walls are thinner than the left because they pump blood into the lungs, which are nearby and offer very little resistance to blood flow. b. The left ventricle walls are thicker because they pump blood through the body where the resistance to blood flow is greater IV. HEART VALVES ENSURE ONE-WAY FLOW OF BLOOD. A. Valves open and close in response to pressure changes as the heart contracts and relaxes. Blood flows from areas of higher blood pressure to areas of lower blood pressure. B. Operation of the Atrioventricular Valves 1. Atrioventricular (AV) valves prevent blood flow from the ventricles back into the atria. When the AV valve is open, the rounded ends of the cusps project into the ventricle. The papillary muscles and chordae tendineae are slack and blood moves from a higher pressure in the atria to the lower pressure in the ventricles. 2. When the ventricles contract, the pressure of the ventricular blood drives the cusps upward until their edges meet and close the valve opening. Back flow of blood is prevented by the contraction of papillary muscles tightening the chordae tendineae that prevent the valve cusps from pushing up into the atria. CHAPTER 19( HEART) DR PARIA PARTO 4  C. Operation of the Semilunar Valves 1. The semilunar (SL) valves are made up of three crescent moon-shaped cusps and allow ejection of blood from the heart into arteries but prevent back flow of blood into the ventricles. 2. Semilunar valves open when the ventricles contract and the blood pressure in the ventricles exceeds the blood pressure in the arteries.Blood is ejected into the pulmonary trunk and aorta. 3. As the ventricles relax, blood starts to flow back toward the heart. This blood fills the cusps and causes them to close D. Clinical Connection: Heart Valve Disorders A narrowing of a heart valve opening that restricts blood flow is known as stenosis. A failure of a valve to close completely is termed insufficiency or incompetence. One example of this is mitral valve prolapse (MVP). In MVP one or both cusps of the mitral valve protrude into the left atrium during contraction. It is the most common valve disorder and is more prevalent in women. In aortic stenosis the aortic valve is narrowed while in aortic insufficiency there is backflow of blood into the left ventricle. Some infectious diseases can damage or destroy heart valves. One example is rheumatic fever which usually occurs after a streptococcal infection of the throat. Antibodies are produced which attack the joints, heart valves, and other organs. Valves are replaced if they limit the ability to complete daily activities. The aortic valve is the most commonly replaced heart valve. V. THE HEART PUMPS BLOOD TO THE LUNGS FOR OXYGENATION, THEN PUMPS OXYGEN-RICH BLOOD THROUGHOUT THE BODY. A. Systemic and Pulmonary Circulations 1. The right side of the heart is the pump for the pulmonary circulation. The right atrium receives deoxygenated blood from the body and sends it to the right CHAPTER 19( HEART) DR PARIA PARTO 5  ventricle and on to the lungs (via pulmonary trunk and pulmonary arteries and pulmonary capillaries) for oxygenation 2. The left side of the heart is the pump for the systemic circulation. The left atrium receives oxygenated blood from the pulmonary veins and pumps it to the left ventricle which contract to move oxygenated blood out into the aorta and systemic arteries to systemic tissues. Exchange of nutrients and gasses occurs in systemic capillaries. These capillaries are drained by veins which ultimately return blood to the SVC, IVC, and coronary sinus B. Coronary Circulation 1. The flow of blood through the many vessels that flow through the myocardium of the heart is called the coronary (cardiac) circulation; it delivers oxygenated blood and nutrients to and removes carbon dioxide and wastes from the myocardium 2. The principal arteries, branching from the ascending aorta and carrying oxygenated blood, are the right and left coronary arteries. 3. The left coronary artery divides into the anterior interventricular branch or left anterior descending (LAD) artery and the circumflex branch. 4. The right coronary artery divides into the posterior interventricular branch and the marginal branch 5. Connections between arteries are termed anastomoses and provide alternate routes for blood to reach a particular organ or tissue. The myocardium contains many anastomoses to provide detours for arterial blood is a main route becomes obstructed. 6. Deoxygenated blood returns to the right atrium primarily via the principal vein, the coronary sinus. The coronary sinus drains blood into the right atrium and receives blood from the great, middle, small, and anterior cardiac veins. 7. Clinical Connection: Myocardial Ischemia and Infarction CHAPTER 19( HEART) DR PARIA PARTO 6  Partial obstruction of blood flow in the coronary arteries may cause myocardial ischemia which causes hypoxia. Angina pectoris is severe chest pain that accompanies myocardial ischemia. Silent myocardial ischemia is ischemic episodes without pain and are dangerous because the person has no forewarning of an impending heart attack. A complete obstruction to blood flow in a coronary artery may result in a myocardial infarction (MI). Infarction means the death of tissue due to interrupted blood supply. This tissue is replaced with scar tissue and the heart muscle loses some of its strength. The infarct may disrupt the conduction system of the heart and cause sudden death by triggering ventricular fibrillation. Treatment for a MI may involve a thrombolytic agent, an anticoagulant, and possible surgery. VI. THE CARDIAC CONDUCTION SYSTEM COORDINATES HEART CONTRACTIONS FOR EFFECTIVE PUMPING. A. Cardiac Muscle Tissue 1. Cardiac muscle fibers are shorter in length than skeletal muscle fibers and exhibit branching 2. The ends of cardiac muscle fibers connect to adjacent fibers via intercalated discs. The discs contain desmosomes and gap junctions. B. Cardiac muscle has the same arrangement of myofilaments but the mitochondria are larger and more numerous than in skeletal muscle fibers. The sarcoplasmic reticulum of cardiac muscle fibers is smaller than the SR of skeletal muscle fibers and cardiac muscle has a smaller intracellular reserve of Ca2+. C. Autorhythmic Cells: The Conduction System 1. Cardiac muscle cells are autorhythmic cells because they are self-excitable. They repeatedly generate spontaneous action potentials that then trigger heart contractions. CHAPTER 19( HEART) DR PARIA PARTO 7  a. These cells act as a pacemaker to set the rhythm for the entire heart. b. They form the conduction system, the route for propagating action potential through the heart muscle. 2. The SA nodal cells do not have a stable resting potential. They repeatedly depolarize to threshold spontaneously. This is called a pacemaker potential. When the pacemaker potential reaches threshold, it triggers an action potential. 3. The cardiac action potential then propagates through the conduction system in the following sequence: the sinoatrial (SA) node (pacemaker), atrioventricular (AV) node, atrioventricular bundle (bundle of His), right and left bundle branches, and the Purkinje fibers D. Contraction of Contractile Fibers 1. The action potential initiated by the SA node travels along the cardiac conduction system and spreads out to excite the contractile fibers of the atria and ventricles. 2. The action potential occurs through depolarization (entrance of Na+), plateau (inflow of Ca2+), and repolarization (outflow of K+). The action potential leads to a mechanical response as in skeletal muscle. 3. There is also a refractory period that takes place. The refractory period is longer than the contraction and prevents tetanus. E. ATP Production in Cardiac Muscle 1. Most ATP in cardiac muscle is produced by aerobic cellular respiration. Cardiac muscle utilizes fatty acids, glucose, lactic acid, amino acids, ketone bodies, and creatine phosphate. Cardiac muscle cells contain creatine kinase (CK) for ATP production from creatine phosphate. CHAPTER 19( HEART) DR PARIA PARTO 8 VII. THE ELECTROCARDIOGRAM IS A RECORD OF ELECTRICAL ACTIVITY ASSOCIATED WITH EACH HEARTBEAT. A. Electrocardiogram 1. 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). a. The ECG helps to determine if the conduction pathway is abnormal, if the heart is enlarged, and if certain regions are damaged. b. Figure 19.9 shows a typical ECG. 2. In a typical record, three clearly visible waves accompany each heartbeat. a. P wave (atrial depolarization - spread of impulse from SA node over atria) b. QRS complex (ventricular depolarization - spread of impulse through ventricles) c. T wave (ventricular repolarization) 1. Correlation of ECG Waves With Heart Activity A cardiac action potential arises in the SA node and propagates to the AV node. As atrial fibers depolarize, the P wave appears. 2. After the P wave begins, the atria contract (atrial systole). 3. Action potential slows at the AV node giving the atria time to contract. 4. The action potential moves rapidly through the bundle branches, Purkinje fibers, and the ventricular myocardium producing the QRS complex to depolarize the ventricles (during this time the atria repolarize) 5. Ventricular contraction (ventricle systole) begins after the QRS is complete. 6. Repolarization of the ventricles produces the T wave and the ventricles relax (ventricular diastole). CHAPTER 19( HEART) DR PARIA PARTO 9  7. Briefly contractile fibers in both the atria and ventricles are relaxed and then the P wave appears again and the cycle repeats. VIII. THE CARDIAC CYCLE REPRESENTS ALL OF THE EVENTS ASSOCIATED WITH ONE HEARTBEAT. 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. Heart Sounds During Cardiac Cycle 1. 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 2. The first heart sound (S1) which can be described as a lubb sound. Itis created by blood turbulence associated with the closing of the atrioventricular valves soon after ventricular systole begins. 3. The second heart sound (S2) can be described as a dupp sound. It represents the closing of the semilunar valves close to the end of the ventricular systole. 4. Additional heart sounds that are not normally loud enough to be heard are S3 (due to turbulence as the ventricles fill) and S4 (due to turbulence during atrial systole). 5. Clinical Connection: Heart Murmurs A heart murmur is an abnormal sound consisting of a clicking, rushing, or gurgling noise that is heard before, between, during or after the normal heart sounds. They are common in children and are termed innocent or functional heart murmurs; and often disappear with growth. A valve disorder such as a stenotic or incompetent valve will produce a murmur. CHAPTER 19( HEART) DR PARIA PARTO 10 C. Pressure and Volume Changes During the Cardiac Cycle 1. During a cardiac cycle, atria and ventricles alternately contract and relax forcing blood from areas of high pressure to areas of lower pressure. 2. Figure 19.12 shows the relation between the ECG and changes in atrial pressure, ventricular pressure, aortic pressure, and ventricular volume during the cardiac cycle. 3. The phases of the cardiac cycle are: atrial systole, ventricular systole, and atrial and ventricular diastole. a. Atrial systole occurs at the same time that the ventricles are relaxed (ventricular diastole). The atria contract, increasing pressure forces the AV valves to open. The amount of blood in the ventricle at the end of diastole is the End Diastolic Volume (EDV) and is about 130 mL. b. Ventricular systole occurs at the same time that the two atria are relaxed (atrial diastole). Ventricles contract and increasing pressure forces the AV valves to close which produces the first audible heart sound Once AV and SL valves are all closed (isovolumetric contraction), ventricularpressure continues to rise, opening the SL valves leading to ventricular ejection. The amount of blood in the left ventricle at the end of systole is End Systolic Volume (ESV) and is about 60 mL. c. Ventricular repolarization causes ventricular diastole d. Pressure in the ventricles fall and blood in the aorta and pulmonary trunk flows backward, catches the cusps and closes the SL valves.. The brief time all four valves are closed is termed isovolumetric relaxation. Pressure in the ventricles continues to fall; the AV valves open and ventricular filling begins CHAPTER 19( HEART) DR PARIA PARTO 11 IX. CARDIAC OUTPUT IS THE BLOOD VOLUME EJECTED BY A VENTRICLE EACH MINUTE. 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 volume of blood ejected from the ventricle into the aorta (or pulmonary trunk) each minute. 1. Cardiac output equals the stroke volume (SV), the volume of blood ejected by the ventricle with each contraction, multiplied by the heart rate (HR), the number of beats per minute; CO = SV X HR. 2. The stroke volume is the difference between the EDV and the ESV. C. Regulation of Stroke Volume 1. Three factors regulate stroke volume: preload, the degree of stretch in the heart before it contracts; contractility, the forcefulness of contraction of individual ventricular muscle fibers; and afterload, the pressure that must be exceeded if ejection of blood from the ventricles is to occur. a. Preload: Effect of Stretching 1) 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. 2) The Frank-Starling law of the heart equalizes the output of the right and left ventricles and keeps the same volume of blood flowing to both the systemic and pulmonary circulations. b. Myocardial contractility, the strength of contraction at any given preload, is affected by stimulation or inhibition of the sympathetic nervous system. CHAPTER 19( HEART) DR PARIA PARTO 12  1) Stimulation of the sympathetic nervous system causes release of hormones like epinephrine and norepinephrine that increase calcium levels in the interstitial fluid and cause the heart to contract more forcefully. 2) Inhibition of the sympathetic nervous system, some anesthetics, and increased potassium levels in the interstitial fluid, decrease contraction force. Calcium channel blockers are drugs that reduce Ca2+ inflow, thereby decreasing the strength of the heartbeat. 3) Thus, for a constant preload, the stroke volume increases when contractility increase and decreases when contractility decreases. c. The pressure that must be overcome before a semilunar valve can open is the afterload. When afterload increases, the valves open later than normal and stroke volume decreases. Conditions that increase afterload include hypertension and atherosclerosis. D. Regulation of Heart Rate 1. 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. 2. Autonomic Regulation of the Heart a. Nervous control of the cardiovascular system stems from the cardiovascular center in the medulla oblongata b. Proprioceptors (for the onset of physical activity), baroreceptors (monitor blood pressure), and chemoreceptors monitor factors that influence the heart rate. c. Sympathetic impulses increase heart rate and force of contraction via cardiac accelerator nerves. These nerves release NE which speeds up CHAPTER 19( HEART) DR PARIA PARTO 13  spontaneous depolarization at the SA and AV nodes. NE also enhances Ca2+ entrance through Ca2+ channels thereby increasing contractility. d. Parasympathetic impulses reach the heart via the vagus nerves. They release acetylcholine which slows the rate of spontaneous depolarization to decrease heart rate. e. At rest, parasympathetic stimulation predominates and the resting HR is about 75 bpm. 3. Chemical Regulation of Heart Rate a. Heart rate is affected by hormones (epinephrine, norepinephrine, thyroid hormones). These hormones also increase contractility. b. Cations (Na+, K+, Ca+2) also affect heart rate. Increased blood levels of Na+ block Ca2+ inflow into cardiac muscle fibers, decreasing the force of contraction. Excess K+ in the blood decreases HR by blocking the generation of action potentials. A moderate increase in interstitial Ca2+ level increases HR and contractility. 4. Other factors such as age, gender (females have higher HRs than males), physical fitness, and temperature (hypothermia decreases HR) also affect heart rate. CHAPTER 19( HEART) DR PARIA PARTO 14