CVS Heart Reading Notes PDF

THE CARDIOVASCULAR SYSTEM: HEART STRUCTURE AND LOCATION OF THE HEART The heart is located between the two lungs. Approximately two- thirds of its mass is to the left of midline. The apex is the inferior pointed end of the heart. It is formed by the tip of the left ventricle and rests on the diaphra...

THE CARDIOVASCULAR SYSTEM: HEART STRUCTURE AND LOCATION OF THE HEART The heart is located between the two lungs. Approximately two- thirds of its mass is to the left of midline. The apex is the inferior pointed end of the heart. It is formed by the tip of the left ventricle and rests on the diaphragm. The base of the heart is the superior portion. The major blood vessels enter and exit at the base. PERICARDIUM The heart is enclosed and held in position by the pericardium, which consists of an outer fibrous pericardium and an inner serous pericardium. The fibrous pericardium prevents overstretching of the heart, provides a tough protective membrane, and anchors the heart in place. The serous pericardium is more delicate and forms a double layer around the heart. This double layer is comprised of the outer parietal pericardium, which is fused to the fibrous pericardium, and the inner visceral pericardium, which adheres to the heart's surface. The visceral pericardium is also known as the epicardium. Between the parietal and visceral pericardium is the pericardial cavity, a space filled with pericardial fluid that reduces friction between the two membranes. Inflammation of this membrane is known as pericarditis. HEART WALL The wall of the heart has three layers: epicardium, myocardium, and endocardium. The epicardium is the visceral layer of the serous pericardium and is comprised of mesothelium and connective tissue. The myocardium consists of cardiac muscle, which constitutes the bulk of the heart and is responsible for the heart's pumping action. Cardiac muscle cells are involuntary, striated, and branched, and the tissue is arranged in interlacing bundles of fibers. The cardiac muscle cells (fibers) are connected to each other by intercalated discs (thickenings of the sarcolemma). These connections allow the cells to contract as functional units (networks) rather than individually. There are two separate networks of connected muscle cells, one network in the upper heart chambers (atria) and one network in the lower heart chambers (ventricles). These networks allow the atria to contract separately from the ventricles. The endocardium is a layer of 1|Page simple squamous epithelium that lines the myocardium and covers the valves of the heart and tendons that hold them. It is continuous with the inner lining of the blood vessels. CHAMBERS OF THE HEART The chambers of the heart include two upper atria and two lower ventricles. The right atrium and left atrium are separated by a thin-walled interatrial septum; a prominent feature of this septum is an oval depression called the fossa ovalis. Wrinkled, pouch-like auricles are found on the outer, anterior surface of each atrium. The right ventricle and left ventricle are separated by the interventricular septum. The thickness of the myocardium of the heart chambers varies depending upon their functions. Atria have less myocardium than the ventricles because they only pump blood into the ventricles. Thicker walled ventricles pump blood into the lungs (right ventricle) and to the whole body (left ventricle). GREAT VESSELS OF THE HEART AND THE VALVES OF THE HEART Great vessels and blood flow Deoxygenated blood flows into the right atrium through the superior vena cava (blood from upper body), the inferior vena cava (blood from lower body), and the coronary sinus (blood from the heart's vessels). The deoxygenated blood from the right atrium is delivered to the right ventricle, which then pumps the blood into the pulmonary trunk. The pulmonary trunk divides into right and left pulmonary arteries that deliver the blood to the corresponding lungs. At the lungs, the blood is oxygenated and returned to the heart through four pulmonary veins that enter the left atrium. The oxygenated blood passes into the left ventricle and then into the ascending aorta and is delivered to the body. HEART VALVES The heart contains four valves – two atrioventricular (AV) valves and two semilunar valves. The valves are composed of dense connective tissue and covered by endothelium. The function of the valves is to prevent the backflow of blood in the heart. The two atrioventricular valves are located between the atria and ventricles. The tricuspid valve is located between right atrium and right ventricle. The bicuspid (mitral) valve is located between the left atrium and left ventricle. Associated with the AV valves are tendon-like structures called the chordae tendineae. They are 2|Page attached to papillary muscles (cardiac muscle projections) located in the ventricular chambers. These structures keep the cusps (flaps) of the valves from pushing up into the atria, preventing the backflow of blood into the atria when the ventricles contract. The two semilunar valves prevent the backflow of blood into the heart as it leaves the ventricles. The pulmonary valve is located between the right ventricle and the pulmonary trunk. The aortic valve is located between the left ventricle and the aorta. BLOOD FLOW THROUGH THE HEART The movement of blood through the heart is directly related to changes in pressure, which causes the opening and closing of valves. Blood will flow through the heart from areas of higher pressure to areas of lower pressure. When the walls of the atria are stimulated to contract, the subsequent increase in pressure forces the AV valves to open, and atrial blood flows into the ventricles. When the ventricle walls contract, the blood pressure increases which closes the AV valves and pushes the semilunar valves open, causing the blood to enter the pulmonary trunk and aorta. As the ventricles relax, the blood from the pulmonary trunk and aorta starts to flow back toward the ventricles, which closes the semilunar valves. BLOOD SUPPLY TO THE HEART The flow of blood through the numerous vessels in the myocardium is called coronary (cardiac) circulation. The coronary arteries deliver oxygenated blood to the myocardium. The two primary coronary arteries are the right and left coronary arteries. There are many anastomoses (connections) between the coronary artery branches, providing detours for oxygenated blood in case a main route is blocked. The deoxygenated blood of the myocardium is eventually collected by the coronary sinus, which empties into the right atrium. Most heart attacks, or myocardial infarctions, result from obstruction of blood flow in the coronary arteries supplying the myocardium. The heart tissue dies and is replaced by scar tissue. Reduced oxygen supplies to the heart muscle cells will weaken them in a condition known as ischemia. A painful clinical condition, known as angina pectoris, is a result from myocardial ischemia. 3|Page CONDUCTION SYSTEM The heart is capable of beating even if removed from the body and having its nerve supply cut. This is because the heart has some specialized muscle fibers that can generate and distribute action potentials to the myocardium without outside influence. THE COMPONENTS OF THIS CONDUCTION SYSTEM INCLUDE THE FOLLOWING: Sinoatrial (SA) node, also known as the pacemaker (located in the right atrium near the opening of the superior vena cava). Atrioventricular (AV) node (in the right atrium near the opening of the coronary sinus). Atrioventricular bundle or bundle of His (in the interventricular septum). Right and left bundle branches (in the interventricular septum). Purkinje fibers (in the ventricular myocardium). The conduction system spontaneously and rhythmically generates action potentials that result in the contraction of heart muscle. This spontaneous generation of impulses is controlled by the sinoatrial node. When an action potential is initiated by the SA node, it spreads over both atria, causing them to contract simultaneously. The action potential then reaches the AV node where the action potential slows down, allowing the atria to empty their blood into the ventricles. From the AV node, the action potential enters the atrioventricular bundle, which splits into right and left bundle branches. The bundle branches distribute the action potential to the Purkinje fibers which transmit the action potential into the ventricular myocardium, causing the simultaneous contraction of the ventricles. To meet changes in the body’s blood supply, the heart’s conduction system can be influenced and adjusted by factors such as hormones and neurotransmitters. 4|Page ELECTROCARDIOGRAM The electrical currents created by the action potentials of the conduction system can be detected by electrodes placed on the skin’s surface, producing a recording called an electrocardiogram (ECG or EKG). A normal ECG consists of a P wave, QRS complex, and a T wave. The P wave represents atrial depolarization as the action potential spreads from the SA node through both atria, resulting in atrial contraction. The QRS complex represents the depolarization and spread of impulses through the ventricles, causing ventricular contraction. The T wave represents the repolarization of the ventricles, as the ventricles begin to relax. Repolarization of the atria occurs during the QRS complex. The large QRS complex masks the phase of atrial repolarization. The ECG is a valuable tool for the diagnosis of abnormal cardiac rhythms and conduction patterns and following recovery from a heart attack. CARDIAC CYCLE In the normal heart, the two atria contract while the two ventricles relax. Then the two ventricles contract while the atria relax. The term systole refers to the phase of contraction, and diastole refers to the phase of relaxation. One cardiac cycle equals one complete heartbeat and consists of atrial systole and ventricular diastole occurring simultaneously, followed by ventricular systole and atrial diastole occurring simultaneously. The cardiac cycle is divided into three major phases: 1) relaxation period; 2) atrial systole (contraction); and 3) ventricular systole (contraction). The relaxation period begins at the end of the cardiac cycle, and all four chambers are in diastole. The repolarization of the ventricular muscle fibers (the T wave in the ECG) initiates the relaxation period. As the ventricles relax, pressure within them drops. Pressure continues to drop within the ventricles to a point where it falls below atrial pressure. This drop in pressure in the ventricles causes the AV valves to open and ventricular filling begins as blood flows from the atria 5|Page into the ventricles. An action potential from the SA node causes atrial depolarization (P wave of the ECG), followed by atrial systole (contraction). This signifies the end of the relaxation period. Throughout the period of ventricular filling, the AV valves are open and the semilunar valves are closed. Ventricular depolarization (QRS complex of the ECG) leads to ventricular systole (contraction). When ventricular contraction occurs, the blood pushes against the AV valves, forcing them shut. As ventricular contraction continues, the pressure in the ventricles rises above pressure in the aorta and pulmonary trunk, causing the semilunar valves to open and blood to be ejected from the heart. When the ventricles start to relax, the ventricular pressure drops and the semilunar valves close. This starts the return to the relaxation period. The average resting heart rate is 75 beats per minute and a complete cardiac cycle lasts approximately 0.8 seconds. HEART SOUNDS The sound of the heartbeat is due to turbulence in blood flow caused by the closure of the heart valves. The first heart sound, “lubb”, represents the closure of the atrioventricular valves after ventricular systole begins. The second heart sound, “dupp”, represents closure of the semilunar valves close to the end of ventricular systole. CARDIAC OUTPUT The amount of blood ejected per minute from the left ventricle into the aorta is called cardiac output (CO). Although the same amount of blood is ejected from both the right and left ventricles, the CO typically refers to the amount of oxygenated blood leaving the left ventricle each minute. CARDIAC OUTPUT IS DETERMINED BY TWO FACTORS: The volume of blood pumped by the left ventricle (called the stroke volume or SV) during each beat The number of beats per minute (heart rate or HR). Multiplying the stroke volume by the heart rate will determine the cardiac output; CO = SV x HR In a resting adult, the stroke volume averages about 70 mL, and the average heart rate is 75 beats per minute. Therefore, average cardiac output is 5.25 liters per minute. Average CO = 70 mL/beat x 75 beats/min = 5250 mL/min or 5.25 liters per minute The cardiac output will change if either the stroke volume or heart rate changes. 6|Page REGULATING STROKE VOLUME When the heart chambers fill with blood, muscle fibers stretch. With greater filling, additional stretching occurs, causing the heart to contract with greater force to eject the additional blood. This relationship is known as the Frank-Starling law of the heart. The forcefulness of contraction (increased or decreased) can also be influenced by the autonomic nervous system, hormones, chemicals, and drugs. The amount of pressure required to open the semilunar valves determine when the valves open and can affect how much blood is ejected. REGULATING HEART RATE Heart rate is influenced by autonomic control, chemicals, temperature, emotions, gender, physical fitness, and age. Autonomic regulation of heart rate originates in the cardiovascular (CV) center of the medulla. Sympathetic impulses from the CV center reach the heart’s conduction system, atria, and ventricles through the cardiac accelerator nerves. These nerves release norepinephrine to increase the heart rate. Parasympathetic impulses from the CV center reach the heart’s conduction system and atria through the vagus nerves (cranial nerve X). The vagus nerves release acetylcholine, which decreases the heart rate by slowing the pacemaking activity of the SA node. CARDIAC OUTPUT IMPACTS BLOOD PRESSURE Increasing cardiac output causes blood pressure to rise; decreasing cardiac output causes it to drop. Special pressure sensitive receptors (baroreceptors) can provide input to the CV center, changing the heart rate, cardiac output, and blood pressure. Certain chemicals have an effect on heart rate. The adrenal medullae hormones epinephrine and norepinephrine cause an increase in the heart rate and strength of contraction. Exercise, stress, and excitement increase the release of these hormones from the adrenal medullae. Thyroid hormones increase heart rate. Elevated potassium or sodium ions decrease the heart rate and strength of contraction. An excess of calcium will increase the heart rate and contraction force. Body temperature affects the heart rate. Increased body temperature due to fever or strenuous exercise will cause the SA node to discharge impulses faster and thereby increase heart rate. Decreased body temperature decreases heart rate and strength of contraction. Gender is another influencing factor. Adult females have slightly faster 7|Page heartbeats than adult males. Age affects heart rate, with newborns having a rapid resting heart rate that gradually declines to the adult level. EXERCISE AND THE HEART Regular aerobic exercise can increase cardiac output and improve cardiovascular fitness. Exercise increases the oxygen demand of skeletal muscles. Long-term exercise training ensures that the muscle’s oxygen demands are met by increasing cardiac output and by development of additional capillaries in the skeletal muscles. At rest, a well-trained athlete has an increased stroke volume and a decreased heart rate but maintains approximately the same cardiac output as a healthy, untrained individual. Common Disorders In coronary artery disease (CAD), the heart muscle does not receive an adequate amount of blood because of an interruption in the blood supply through the coronary arteries. This can be caused by atherosclerosis in the coronary arteries. Atherosclerosis is a process in which fatty substances are deposited in the walls of arteries resulting in an impeded blood flow. The atherosclerotic plaques have a cap that can break open, causing a clot to form. If the clot is large enough, it will interfere with or stop blood flow in the coronary arteries resulting in a heart attack. Congenital defects include patent ductus arteriosus (PDA), atrial septal defects (ASD), ventricular septal defect (VSD), valvular stenosis, and tetralogy of Fallot. Arrhythmias and dysrhythmias are a result of problems in the heart’s conduction system. They may be caused by a variety of factors, including stress, drugs, valve defects, heart disease, hormones, and chemicals. 8|Page