Cardiovascular 1st Lecture (PDF)

‫َللا ه ا ّل ِذ َ‬ ‫ين آ َم هنوا‬ ‫َ‬ ‫{ يَ ْر ِ ّ‬ ‫ع‬ ‫ف‬ ‫ين هأوتهوا‬‫َوا ّل ِذ َ‬ ‫ِمن هك ْم‬ ‫ا ْل ِع ْلمَ َد َر َجات }‬ ‫[ المجادلة ‪… ] 11 :‬‬  Introduction...

‫َللا ه ا ّل ِذ َ‬ ‫ين آ َم هنوا‬ ‫َ‬ ‫{ يَ ْر ِ ّ‬ ‫ع‬ ‫ف‬ ‫ين هأوتهوا‬‫َوا ّل ِذ َ‬ ‫ِمن هك ْم‬ ‫ا ْل ِع ْلمَ َد َر َجات }‬ ‫[ المجادلة ‪… ] 11 :‬‬  Introduction In the embryo, the heart begins to beat Beating even before its nerve supply has at 4 weeks of age. been established. The main function of the heart is to pump blood through the arteries, capillaries, and veins. Blood transports oxygen and nutrients and has other important functions as well. The heart is the pump that keeps blood circulating proper 1  10/6/2023 Imagine trying to squeeze a tennis ball 70 times a minute. Then imagine increasing your squeezing rate to 120 times a minute. A healthy heart can increase its rate and force of contraction to meet the body’s need for more oxygen, then return to its resting rate and keep on beating as if nothing extraordinary had happened. In fact, it isn’t extraordinary at all; this is the job the heart is meant to do. LOCATION It is located in the thoracic cavity between the lungs; mediastinum. The base of the cone-shaped heart is uppermost, behind the sternum, and the great vessels enter or leave here. The apex (tip) of the heart points downward and is just above the diaphragm to the left of the midline. 4  Figure 11.1 This virtual dissection, revealing the thoracic contents, shows reason why echocardiographic windows are limited when using the transthoracic approach 5  Blood Circulation Pericardial membranes The membranes which enclose the heart. There are three layers form the pericardium; 2 serous layers and 1 fibrous layer 1 The outermost is the fibrous pericardium, a loose fitting sac of strong fibrous connective tissue that extends inferiorly over the diaphragm and superiorly over the bases of the large vessels that enter and leave the heart. 2 The parietal pericardium: It is lining the fibrous pericardium. 3 The visceral pericardium: It is lining the surface of the heart muscle., often called the epicardium. Between the parietal and visceral pericardial membranes is serous fluid, which prevents friction as the heart beats. 6  CHAMBERS—VESSELS AND VALVES The walls of the 4 chambers of the heart are made of cardiac muscle; the myocardium. The chambers are lined with endocardium; it is a simple squamous epithelium that also covers the valves and continues into the vessels as their lining (endothelium). The important physical characteristics of the endocardium are its thinness and its smoothness; This very smooth tissue prevents abnormal blood clotting, because clotting would be initiated by contact of blood with a rough surface. The right and left atria (singular: atrium). The upper chambers of the heart are the right and left atria (singular: atrium). Atria are characterized by; Relatively thin walls. Separated by a common wall of myocardium called the interatrial septum. The atria receive blood, either from the body or the lungs, and the ventricles pump blood to either the lungs or the body. 7  The right and left ventricles (RV and LV). The lower chambers are the right and left ventricles (RV and LV) and characterized by: have thicker walls. Separated by the interventricular septum. The atria receive blood, either from the body or the lungs, and the ventricles pump blood to either the lungs or the body. RIGHT ATRIUM (RA) The 2 large caval veins return blood from the body to the RA. The superior vena cava (SVC) carries blood from the upper body. The inferior vena cava (IVC) carries blood from the lower body. From the RA, blood will flow through the tricuspid valve (TV), into the right ventricle (RV). The TV is made of three flaps (or cusps) of endocardium reinforced with connective tissue. The general purpose of all valves in the circulatory system is to prevent backflow of blood. The specific purpose of the TV is to prevent backflow of blood from the RV to the RA when the RV contracts. As the ventricle contracts, blood is forced behind the three valve flaps, forcing them upward and together to close the valve. 8  The left atrium (LA) The LA receives blood from the lungs, by way of four PVs. This blood will then flow into the left ventricle (LV) through the mitral valve (MV) valve. The MV prevents backflow of blood from the LV to the LA when the LV contracts. Another function of the atria is the production of a hormone involved in blood pressure maintenance. When the walls of the atria are stretched by increased blood volume or blood pressure, the cells produce atrial natriuretic peptide (ANP). The ventricles of the heart produce a similar hormone called B-type natriuretic peptide, or BNP. ANP decreases the reabsorption of sodium ions by the kidneys, so that more sodium ions are excreted in urine, which in turn increases the elimination of water. The loss of water lowers blood volume and blood pressure. You may have noticed that ANP is an antagonistto the hormone aldosterone, which raises blood pressure. RIGHT VENTRICLE (RV). When the RV contracts, the TV closes and the blood is pumped to the lungs through the pulmonary artery (PA). At the junction of this large artery and the RV is the PV. It is forced to open when the RV contracts and pumps blood into the PA. When the RV relaxes, blood tends to come back, but this fills the valve flaps and closes the PV to prevent backflow of blood into the RV. Projecting into the lower part of the right ventricle are columns of myocardiumcalled papillary muscles (PM). Chordae tendineae (from the papillary muscles to the flaps of the TV). When the RV contracts, the papillary muscles also contract and pull on the chordae tendineae to prevent inversion of the TV. 9  LEFT VENTRICLE (LV) The walls of the LV are thicker than those of the RV, which enables the LV to contract more forcefully. The LV pumps blood to the body through the aorta, the largest artery of the body. At the junction of the aorta and the LV is the aortic valve (AV). This AV is opened by the force of contraction of the LV, which also closes the MV. The AV closes when the LV relaxes, to prevent backflow of blood from the aorta to the LV. When the MV closes, it prevents backflow of blood to the LA; the flaps of the MV are also anchored by chordae tendineae and papillary muscles. The fibrous skeleton of the heart This is fibrous connective tissue that anchors the outer edges of the valve flaps and keeps the valve openings from stretching. It also separates the myocardium of the atria and ventricles and prevents the contraction of the atria from Heart valves in superior view. reaching the ventricles except by way The atria have been removed. The fibrous skeleton of the of the normal conduction pathway. heart is also shown. 1  So, we can conclude; The heart is really a double, or two-sided, pump. The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs to pick up oxygen and release carbon dioxide. The left side of the heart receives oxygenated blood from the lungs and pumps it to the body. Both pumps work simultaneously; that is, both atria contract together, followed by the contraction of both ventricles. Anterior view of the heart and major blood vessels. Frontal section in anterior view, showing internal structures. 1  (A) Coronary vessels in anterior view. The pulmonary artery has been cut to show the left coronary artery emerging from the ascending aorta. (B) Coronary vessels in posterior view. The coronary sinus empties blood into the right atrium CORONARY VESSELS Right Coronary artery (RCA) and Left Coronary Artery (LCA) The RCA and LCA are the 1st branches of the ascending aorta, just beyond the aortic valve. The two arteries branch into smaller arteries and arterioles, then to capillaries. The coronary capillaries merge to form coronary veins, which empty blood into a large coronary sinus that returns blood to the right atrium. The function of the coronary vessels is to supply blood to the myocardium itself, because oxygen is essential for normal myocardial contraction. If a coronary artery becomes obstructed, by a blood clot for example, part of the myocardium becomes ischemic, that is, deprived of its blood supply. Prolonged ischemia will create an infarct, an area of necrotic (dead) tissue. This is a myocardial infarction, commonly called a heart attack. 12  Summary CARDIAC CYCLE AND HEART SOUNDS The cardiac cycle is the sequence of events in one heartbeat. The cardiac cycle is the simultaneous contraction of the two atria, followed a fraction of a second later by the simultaneous contraction of the two ventricles. Systole is another term for contraction while the term for relaxation is diastole. The atrial systole is followed by ventricular systole. There is a significant difference between the movement of blood from the atria to the ventricles and the movement of blood from the ventricles to the arteries. 13 The cardiac cycle depicted in one heartbeat (pulse: 75). The outer circle represents the ventricles, the middle circle the atria, and the inner circle the movement of blood and its effect on the heart valves. The heart sounds The cardiac cycle also creates the heart sounds: Each heartbeat produces 2 sounds, often called (lub dup), that can be heard with a stethoscope. The 1st sound, the loudest and longest, is caused by ventricular systole closing the MV and TV. The 2nd sound is caused by the closure of the AV and PV. If any of the valves do not close properly, an extra sound called a heart murmur may be heard. 14  CARDIAC CONDUCTION PATHWAY The cardiac cycle is a sequence of mechanical events that is regulated by the electrical activity of the myocardium. Cardiac muscle cells have the ability to contract spontaneously; that is, nerve impulses are not required to cause contraction. The heart generates its own beat, and the electrical impulses follow a very specific route throughout the myocardium. Conduction pathway of the heart. Anterior view of the interior of the heart. The electrocardiogram tracing is of one normal heartbeat. 15  CARDIAC CONDUCTION PATHWAY The natural pacemaker of the heart is the sinoatrial (SA) node, a specialized group of cardiac muscle cells located in the wall of the RA just below the opening of the SVC. The SA node depolarizes more rapidly than any other part of the myocardium (60 to 80 times per minute). From the SA node, impulses for contraction travel to the AV node, located in the lower interatrial septum. The transmission of impulses from the SA node to the AV node and to the rest of the atrial myocardium brings about atrial systole. CARDIAC CONDUCTION PATHWAY The fibrous skeleton of the heart separates the atrial myocardium from the ventricular myocardium. It acts as electrical insulation between the two sets of chambers. The only pathway for impulses from the atria to the ventricles, therefore, is the atrioventricular bundle (AV bundle), also called the bundle of His. 16  CARDIAC CONDUCTION PATHWAY The AV bundle is within the upper interventricular septum; it receives impulses from the AV node and transmits them to the right and left bundle branches. From the bundle branches, impulses travel along Purkinje fibers to the rest of the ventricular myocardium and bring about ventricular systole. CARDIAC CONDUCTION PATHWAY The electrical activity of the atria and ventricles is depicted by an ECG. If the SA node does not function properly, the AV node will initiate the heart beat, but at a slower rate (50-60bpm). The AV bundle is also capable of generating the beat of the ventricles, but at a slower rate (15-40bpm). This may occur in certain kinds of diseases (transmission of impulses from the atria to the ventricles is blocked). 17  HEART RATE A healthy adult has a resting HR of 60 to 80bpm, which is the rate of depolarization of the SA node. A rate 100bpm is called tachycardia. A child’s HR may be as high as 100bpm, that of an infant as high as 120, and that of a near-term fetus as high as 140bpm. HEART RATE These higher rates are not related to only the age, but rather to size of the body. The smaller the body size, the higher the metabolic rate and the faster the heart rate (The HR of a mouse=200b/m, HR of an elephant~30bpm. Well-conditioned athletes have low resting pulse rates (Those of basketball players are often around 50 beats per minute, and the pulse of a marathon runner often ranges from 35 to 40 beats per minute). 18  Arrhythmias Arrhythmias are irregular heartbeats (from harmless to life- threatening ones). Nearly everyone experiences heart palpitations (becoming aware of an irregular beat) from time to time. These are usually not serious and may be the result of too much caffeine, nicotine, or alcohol. Ventricular fibrillation, a very rapid and uncoordinated ventricular beat that is totally ineffective for pumping blood. CARDIAC OUTPUT Cardiac output (COP) is the amount of blood pumped by a ventricle in 1 minute. A certain level of COP is needed at all times to transport oxygen to tissues and to remove waste products. During exercise, COP must increase to meet the body’s need for more oxygen. 17  CARDIAC OUTPUT To calculate COP, we must know the HR and stroke volume(SV). SV= How much blood is pumped per beat. An average resting SV is 60 to 80 mL per beat. COP= SV * HR. CARDIAC OUTPUT COP varies with the size of the person (average 5-6 L/m). The athlete’s resting SV is significantly higher than the average. The athlete’s heart pumps more blood with each beat and so can maintain a normal resting COP with fewer beats. 20  Starling’s law Starling’s law states that the more the cardiac muscle fibers are stretched, the more forcefully they contract. During exercise, more blood returns to the heart; this is called venous return. Increased venous return stretches the myocardium of the ventricles, which contract more forcefully and pump more blood, thereby increasing SV. How do the heart responds to exercise?? 1 Increase of HR. 2 Increase of stroke volume; the result of Starling’s law of the heart. 21  Cardiac reserve The COP of a healthy young person may increase up to 4 times the resting level during strenuous exercise. The cardiac reserve is the difference between exercise COP and rest COP (the extra volume the heart can pump when necessary). For example: If resting COP is 5L and exercise COP is 20L, the cardiac reserve is 15L. The marathon runner’s COP may increase six times or more. Because of Starling’s law, it is almost impossible to overwork a healthy heart. No matter how much the volume of venous return increases, the ventricles simply pump more forcefully and increase the SV and COP. The ejection fraction (EF%) This is the percent of the blood in a ventricle that is pumped during systole. A ventricle does not empty completely when it contracts, but should pump out 50-70% of the blood within it. A lower percentage would indicate that the ventricle is weakening. 22  Summary REGULATION OF HEART RATE The heart generates and maintains its own beat. The rate of contraction can be changed to adapt to different situations. The nervous system can and does bring about necessary changes in HR as well as in force of contraction. The medulla of the brain contains the two cardiac centers, the accelerator center and the inhibitory center. These centers send impulses along autonomic nerves. The autonomic nervous system has two divisions: sympathetic and parasympathetic. 21  Sympathetic Nervous system Sympathetic impulses from the accelerator center along sympathetic nerves increase HR and force of contraction during exercise and stressful situations. Parasympathetic impulses from the inhibitory center along the vagus nerves decrease the HR. At rest these impulses slow down the depolarization of the SA node to what we consider a normal resting rate, and they also slow the heart after exercise is over. What information is received by the medulla to initiate changes? The heart pumps blood; it is essential to maintain normal BP. Blood contains O2, which all tissues must receive continuously. Changes in BP or O2 level are stimuli for changes in HR. Pressoreceptors and chemoreceptors are located in the carotid arteries and aortic arch. 24  What information is received by the medulla to initiate changes? Chemoreceptors in the carotid bodies and aortic body detect changes in the oxygen content of the blood. The sensory nerves for the carotid receptors are the glossopharyngeal (9th cranial) nerve. The sensory nerves for the aortic arch receptors are the vagus (10th cranial) nerves. Example1 A person who stands up suddenly from a lying position may feel light-headed or dizzy for a few moments, because BP to the brain has decreased abruptly. The drop in BP is detected by pressoreceptors in the carotid sinuses—notice that they are “on the way” to the brain, a very strategic location. The drop in BP causes fewer impulses to be generated by the pressoreceptors. These impulses travel along the glossopharyngeal nerves to the medulla, and the decrease in the frequency of impulses stimulates the accelerator center. The accelerator center generates impulses that are carried by sympathetic nerves to the SA node, AV node, and ventricular myocardium. As HR and force increase, BP to the brain is raised to normal, and the sensation of light-headedness passes. When BP to the brain is restored to normal, the heart receives more parasympathetic impulses from the inhibitory center along the vagus nerves to the SA node and AV node. These parasympathetic impulses slow the HR to a normal resting pace. 25  Example2 The heart will also be the effector in a reflex stimulated by a decrease in the O2 content of the blood. The aortic receptors are strategically located so as to detect such an important change as soon as blood leaves the heart. The reflex arc in this situation would be: (1) aortic chemoreceptors, (2) vagus nerves (sensory), (3) accelerator center in the medulla, (4) sympathetic nerves, and (5) the heart muscle, which will increase its rate and force of contraction to circulate more oxygen to correct the hypoxia. The hormone epinephrine is secreted by the adrenal medulla in stressful situations. One of the many functions of epinephrine is to increase HR and force of contraction. AGING AND THE HEART The heart muscle becomes less efficient with age, and there is a decrease in both maximum cardiac output and HR, although resting levels may be more than adequate. The health of the myocardium depends on its blood supply, and with age there is greater likelihood that atherosclerosis will narrow the coronary arteries Atherosclerosis is the deposition of cholesterol on and in the walls of the arteries, which decreases blood flow and forms rough surfaces that may cause intravascular clot formation. 26  AGING AND THE HEART High blood pressure (hypertension) causes the left ventricle to work harder; it may enlarge and outgrow its blood supply, thus becoming weaker. A weak ventricle is not an efficient pump, and such weakness may progress to congestive heart failure. The heart valves may become thickened by fibrosis, leading to heart murmurs and less efficient pumping. Arrhythmias are also more common with age, as the cells of the conduction pathway become less efficient. Operation of Heart Valves The end of lecture 1 27