Cardiovascular System Definition & Functions PDF

DEFINITION AND FUNCTIONS OF THE CARDIOVASCULAR SYSTEM CARDIAC MUSCLES AND CARDIAC MYO- ELECTROPHYSIOLOGY BY Dr. Oguntola R.A 1 INTRODUCTION Cardiovascular system comprises of the heart (muscular pump) and the network of several b...

DEFINITION AND FUNCTIONS OF THE CARDIOVASCULAR SYSTEM CARDIAC MUSCLES AND CARDIAC MYO- ELECTROPHYSIOLOGY BY Dr. Oguntola R.A 1 INTRODUCTION Cardiovascular system comprises of the heart (muscular pump) and the network of several blood vessels through which blood from the heart is conveyed to all parts of the body and via which blood from the tissues is returned to the heart. It is a well organized transport system of the body by which the blood being circulated within a closed system under different pressure gradients, created by the pumping mechanism where heart acts as the central pump. 2 FUNCTION OF CARDIOVASCULAR SYSTEM Transport of oxygen from the lungs to the tissues. Transport of carbon dioxide from the tissues to the lungs where the CO2 is excreted. Transport of nutrients (digested food, electrolytes and vitamins) from the gastrointestinal tract to all parts to the body. Transport of waste products of cellular metabolism from the tissue to the kidneys and other excretory organs –lungs, gut (bilirubin, etc) and skin. Transport of hormones from the endocrine glands where they are formed to their target tissues/organs. Transport of heat between the body’s core and its surfaces thereby aiding temperature regulation. Transport of blood cells (mainly leucocytes) and chemical factors (immune substances) that defend the body against foreign proteins 3  The cardiovascular system includes (a) heart, (b) arteries, (c) capillaries and (d) veins. They all differ in structures as well as in functions. Blood is in circulation and is carried out to various tissue delivering oxygen and nutrients to them. Blood gets deoxygenated in the tissues and oxygenated in the lungs. Consequently, it has to pass alternately through lungs and tissues, doing opposite functions at these two places. Hence 4 5 6 HEART The heart is an hollow muscular organ situated between the 2 lungs in the mediastinum. It is made up of 4 chambers:2 Atria and 2 Ventricles’. The two left chambers are separated from the two right ones, by a continuous partition, the atrial portion of which is called the interatrial septum (fibrous). The ventricular part is known as the interventricular septum (upper one-fourth fibrous, lower three fourths muscular). The musculature is more and thick in the ventricle than in the atria. The force of contraction of the heart depends on the muscle. 7  RIGHT SIDE OF THE HEART Right side of the heart has two chambers, right atrium and right ventricle. Right atrium is a thin walled and low pressure chamber. It has got the pacemaker known as sinoatrial node that produces cardiac impulses and atrioventricular node that conducts the impulses to the ventricles. Right atrium receives venous (deoxygenated) blood via two large veins: 1. Superior vena cava that returns venous blood from the head, neck and upper limbs. 2. Inferior vena cava that returns venous blood from lower parts of the body 8  LEFT SIDE OF THE HEART Left side of the heart has two chambers, left atrium and left ventricle. Left atrium is a thin walled and low pressure chamber. It receives oxygenated blood from the lungs through pulmonary veins. This is the only exception in the body, where an artery carries venous blood and vein carries the arterial blood. Blood from left atrium enters the left ventricle through mitral valve (bicuspid valve). Wall of the left ventricle is very thick. Left ventricle pumps the arterial blood to different parts of the body through systemic aorta. 9 Valves of the Heart There should not be any admixture between arterial and venous blood. In other words, circulation must be strictly one way. This is done by the action of valves. There are four sets of valves in the heart: - The right atrioventricular opening is guarded by tricuspid valve [anterior (infundibular), posterior (marginal) and medial (septal) cusps]. - The left opening is guarded by the mitral (due to its resemblance to a Bishop's mitre) or bicuspid valve [anterior and posterior cusps]. - The openings of the aorta and pulmonary artery are guarded by semilunar valves (three cusps). 10 11 Action of the Valves 1. The atrioventricular (AV) valves open towards the ventricles and close towards the atria. 2. The semilunar (SL) valves open away from the ventricles and close towards the ventricles. So that when atria contract, atrioventricular valves open and blood passes into the ventricles. When ventricles contract, atrioventricular valves close, but semilunar valves open. This prevents regurgitation of blood into the atria but allows it to flow out of the ventricles. In this way circulation becomes one way. 12 13 14 LAYERS OF WALL OF THE HEART Heart is made up of three layers of tissues: 1. Outer pericardium 2. Middle myocardium 3. Inner endocardium. 15 PERICARDIUM Pericardium is the outer covering of the heart. It is made up of two layers: i. Outer parietal pericardium: Parietal pericardium forms a strong protective sac for the heart. Parietal pericardium is made up two layers: - Outer fibrous layer - Inner serous layer. ii. Inner visceral pericardium: Inner visceral pericardium lines the surface of myocardium. It is made up of flattened epithelial cells. This layer is also known as epicardium. The space between the two layers is called pericardial cavity or pericardial space and it contains a thin film of fluid. 16 MYOCARDIUM Myocardium is the middle layer of wall of the heart and it is formed by cardiac muscle fibers or cardiac myocytes. Myocardium forms the bulk of the heart and it is responsible for pumping action of the heart. Unlike skeletal muscle fibers, the cardiac muscle fibers are involuntary in nature. Myocardium has three types of muscle fibers: 1. Muscle fibers which form contractile unit of heart 2. Muscle fibers which form pacemaker 3. Muscle fibers which form conductive system 17 ENDOCARDIUM Endocardium is the inner most layer of heart wall. It is a thin, smooth and glistening membrane. It is formed by a single layer of endothelial cells, lining the inner surface of the heart. Endocardium continues as endothelium of the blood vessels. 18 Properties of the cardiac muscle The cardiac muscle contains two types of muscle fiber: Cardiac muscle proper Conductive system It has special properties that characterize it from other types of muscles, these properties include: 1- The histology of the cardiac muscle : Unlike skeletal muscle, there are anatomical connections between the myocardial fibers (intercalated discs and gap junctions) and unlike smooth muscle, the cardiac muscle is striated. 19 The muscle differences in histology 20 2- Functional syncytium Stimulation of one cardiac muscle cell results in stimulation of all the cells. This occurs due to presence of gap junctions & intercalated discs between the fibers (which are areas of low electrical resistance that allow movement of ions and therefore transmission of action potential easily between cells). Therefore, the heart contracts as one unit (as a syncytium which is multiple cells grouped together as one unit within a common cell membrane forming a single multi-nucleated cell). Heart is not an anatomical syncytium but it is a functional syncytium. 21 3- The main source of energy The cardiac muscle consumes fat as the main source of energy. Under basal conditions, 60% of the caloric needs are provided by free fatty acids, 35% by carbohydrates and 5% by ketones and amino acids. On the other hand, skeletal muscles utilize carbohydrates as the main source of energy, especially during exercise. 22  4- Blood flow (supply) The cardiac muscle receives its blood supply mainly during diastole (unlike skeletal muscles which receive their blood supply mainly during systole). The left ventricle receives its blood supply during the diastolic phase only whereas the right ventricle receives its blood supply during both systole & diastole. This is explained by many physiological & anatomical mechanisms: Physiological explanation: During systole, aortic pressure= 120 mmHg, left ventricular pressure= 120 mmHg and right ventricular pressure= 25 mmHg; therefore blood flows from the aorta through the right coronary artery but not the left one. 23  On the other hand, during diastole, aortic pressure= 80 mmHg, left ventricular pressure= 0 mmHg and right ventricular pressure= 0 mmHg; therefore blood flows from the aorta through both right and left coronary arteries. Anatomical explanation: during systole, leaflets of the aortic valve obstruct openings (sinuses) of coronary arteries. 24 5- Oxygen extraction - At basal conditions, the cardiac muscle extracts higher amount of oxygen than skeletal muscle. It consumes about 9 mL O2/ 100 g tissue/minute. This is approximately about 70-80% of the oxygen delivered by each unit of blood. 25 6- Metabolism The cardiac muscle depends on aerobic metabolism for generation of energy. Anaerobic metabolism provides less than 1% of the total energy. This figure may increase slightly during hypoxic states; however, there is no oxygen debt mechanism (obstruction of the coronary artery reduces oxygen delivery to the cardiac muscle and results in its necrosis and death). On the other hand, skeletal muscle depends on both aerobic & anaerobic metabolism for generation of energy, and there is oxygen debt mechanism (oxygen debt is the extra amount of oxygen consumed after exercise to repay oxygen taken from myoglobin & to get rid of lactate accumulating in muscle. 26 7- Electrical activity (action potential) Action potential of the cardiac muscle proper is characterized by the plateau phase (due to calcium influx), and that of the conductive system is characterized by the prepotential (see below). Theses phases are not found in action potentials of other types of muscles. 27 Actions potentials in the cardiac muscle 28 8- The refractory period It Is prolonged in cardiac muscle (200-400 ms) due to presence of the plateau phase. This protects the heart from tetanus (summation of contractions during successive stimulation) and therefore allows the cardiac muscle to relax; because relaxation is essential for ventricular filling. The refractory period in skeletal muscles is about 5-20 ms. 29 the refractory periods in muscles 30 9- ECF calcium: Contraction of the cardiac muscle depends on both ICF & ECF calcium, whereas contraction of skeletal muscle depends on ICF calcium only. The T tubules in the cardiac muscle allow influx of calcium during action potentials to stimulate contraction. Therefore, increasing ECF calcium increases contractility in cardiac muscle and may stop the heart in systole. 31 10- No Recruitment in cardiac muscle Increasing strength of stimulation in skeletal muscles results in recruitment (i.e. it increases number of stimulated fibers) whereas in the cardiac muscle there is no recruitment (no increase in the number of stimulated fibers). That’s because stimulation of one fiber in cardiac muscle results in stimulation of all the fibers. 32 11- Automaticity The various parts of the conductive system can discharge action potential spontaneously. This is explained by the prepotential phases that characterize action potentials of the different structures of the conductive system (the conductive system has no resting membrane potential but gradual elevation of prepotential towards the threshold, see below). This results in spontaneous generation of action potentials by the conductive system (especially the sino-atrial node). However, the cardiac muscle proper, which has no prepotential, can discharge action potentials spontaneously, but in abnormal conditions (e.g. ischemia). This is known as idio-ventricular rhythm. 33  12- Rhythmicity The cardiac muscle undergoes regular rhythmic contractions due to presence of the conductive system, which controls these rhythmic contractions whereas skeletal muscle cannot contract rhythmically. 13- Conductivity This is again a characteristic of the conductive system, which generates rhythmic action potentials and conducts it to the whole parts of the cardiac muscle 34 The cardiac muscle histology 35 ACTIONS OF THE HEART Actions of the heart are classified into four types: 1. Chronotropic action: Chronotropic action is the frequency of heartbeat or heart rate. It is of two types which include tachycardia or bradycardia 2. Inotropic action: Force of contraction of heart is called inotropic action. It is of two types which include Positive or Negative inotropic action. 3. Dromotropic action: Dromotropic action is the conduction of impulse through heart. It is of two types positive or negative dromotropic action 4. Bathmotropic action: Bathmotropic action is the excitability of cardiac muscle. It is also of two types positive or negative bothmotropic action. 36 BLOOD VESSELS Vessels of circulatory system are: Aorta Arteries Arterioles Capillaries Venules Veins and Venae cavae 37 ARTERIAL SYSTEM Arterial system comprises the aorta, arteries and arterioles. Walls of the aorta and arteries are formed by three layers: ₋ Outer tunica adventitia, which is made up of connective tissue layer. It is the continuation of fibrous layer of parietal pericardium. ₋ Middle tunica media, which is formed by smooth muscles ₋ Inner tunica intima, which is made up of endothelium. It is the continuation of endocardium. Aorta, arteries and arterioles have two laminae of elastic tissues: i. External elastic lamina between tunica adventitia and tunica media ii. Internal elastic lamina between tunica media and tunica intima. 38  Aorta and arteries have more elastic tissues and the arterioles have more smooth muscles. Arterial branches become narrower and their walls become thinner while reaching the periphery Resistance (peripheral resistance) is offered to blood flow in the arterioles and so these vessels are called resistant vessels. Arterioles are continued as capillaries, which are small, thin walled vessels having a diameter of about 5 to 8 μ. Capillaries are functionally very important because, the exchange of materials between the blood and the tissues occurs through these vessels. 39 VENOUS SYSTEM From the capillaries, venous system starts and it includes venules, veins and venae cavae. Capillaries end in venules, which are the smaller vessels with thin muscular wall than the arterioles. Diameter of the venules is about 20 μ. At a time, a large quantity of blood is held in venules and hence the venules are called capacitance vessels. Venules are continued as veins, which have the diameter of 5 mm. Veins form superior and inferior venae cavae, which have a diameter of about 30 mm. 40  Walls of the veins and venae cavae are made up of inner endothelium, elastic tissues, smooth muscles and outer connective tissue layer. In the veins and venae cavae, the elastic tissue is less but the smooth muscle fibers are more. 41 CAPILLARIES These make a connective link in between the arterioles and venules. They are linked by a single layer of flat endothelial cells which are the major component of the wall. The capillary endothelium does not directly touch the elements of other tissues and is always separated from a supporting bed of the connective tissue by an intervening layer-the basal lamina. Sinusoids: ⁻ Sinusoids and sinusoidal capillaries are not true capillaries and they have got relatively large caliber (30 μm) with irregular and tortuous walls. ⁻ The continuous endothelial lining is absent. ⁻ There is also some incomplete lining of phagocytic cells. ⁻ Due to absence of basal lamina, the blood gets direct contact with the tissue cells. 42  Blood flows through two divisions of circulatory system: 1. Systemic circulation 2. Pulmonary circulation. 43 DIVISIONS OF CIRCULATION Heart is a miracle of constants. The two ventricles contract simultaneously, as also the two atria. The same amount of blood passes out of the ventricles at the same time during systole. The same amount of blood enters the heart at the same time during diastole. Any discrepancy in the time or in the quantitative relations may ultimately cause heart failure. 44  Circulatory system has been divided into two functionally opposite parts: 1. Systemic circulation (greater circulation with high resistance circuit)- passing through the tissues. 2. Pulmonary circulation (lesser circulation with low resistance circuit)- passing through the lungs. The two systems again meet in the heart. The main functions of circulation are to make available to the tissues its different metabolic needs and on the other hand to carry away from the tissues the CO2 and other metabolic waste products for elimination from the body. These are done in two ways: i. (1) By maintaining patent circulation, so that blood is supplied adequately to every part of the body (in rest and activity). ii. (2) By maintaining an optimum blood pressure which is essential for capillary exchange. 45 SYSTEMIC CIRCULATION Systemic circulation is otherwise known as greater circulation with high resistance circuit. Blood pumped from left ventricle passes through the aorta to the systemic arteries and the systemic tissues (i.e. cerebral, coronary, renal, splanchnic, skeletal muscle, and skin). From the tissues to the systemic veins and vena cava. From the vena cava (mixed venous blood) to the right atrium From the right atrium to the right ventricle through the tricuspid valve. From the right ventricle to the pulmonary artery From the pulmonary artery to the lungs for oxygenation From the lungs to the left atrium via the pulmonary vein From the left atrium to the left ventricle through the mitral valve. From the left ventricle to the aorta through the aortic valve 46 PULMONARY CIRCULATION Pulmonary circulation is otherwise called lesser circulation. Blood is pumped from right ventricle to lungs through pulmonary artery. Exchange of gases occurs between blood and alveoli of the lungs at pulmonarycapillaries. Oxygenated blood returns to left atrium through the pulmonary veins. Thus, left side of the heart contains oxygenated or arterial blood and the right side of the heart contains deoxygenated or venous blood. 47 Functional divisions of the circulation. The pulmonary circulation (Adapted from circulatory physiology 48 49 Assigments Cardiac myo-electrophysiology 50