Cardio Vascular System

Circulatory System in Human - 168 - THE CARDIO VASCULAR SYSTEM Circulatory system of human being is of closed type. It consists of (i) Heart (pumping organ) (ii) Arteries and arterioles, and venules (bring blood back to heart blood vessels. (iii) Capillaries (connecting arterioles and venules) Blood Vessels Blood vessels are an intricate network of tubes that transport blood throughout the body. Blood vessels are made up of three layers. (i) Tunica externa (outermost) : Formed of connective tissue containing collagen & elastic fibres, lymph vessels & nerve fibres. (ii) Tunical media (middle) : Formed of elastic connective tissue and smooth muscle fibre. (iii) Tunica interna (innermost) : Made up of two parts elastic membrane & endothelium. Capillaries They are made up of tunica interna (single layer of endothelium) They are most effective. blood vessels because exchange of substances takes place through them. It contains mixed, oxygenated and deoxygenated blood. It transports blood from the arteries to the veins. Blood flow is at its slowest through the capillaries because they are the narrowest vessels , they have the greatest friction which restricts blood flow. This slow velocity aids the exchange of materials. Circulatory System in Human - 169 STRUCTURE OF HUMAN HEART It is a blunt conical organ of about 12 cm long and 9 cm broad, Its narrow apex is pointed downward and to the left. Heart is enclosed in a double walled sac called Pericardium outer membrane is parietal pericardial membrane (fibrous) and inner one is visceral pericardial membrane (serous). In between the two layers is a fluid-filled space, the fluid is called pericardial fluid & it prevents any friction between the heart walls and the surrounding tissues, protection from shock and mechanical injury. Heart-wall consists of three layers–epicardium (outermost), Myocardium (middle one) and endocardium (inner most). Among these three layers, myocardium consists of cardiac muscles, resembling the striated muscles structurally and smooth muscles functionally because of involuntary muscle. The human heart consists of four chambers two auricles (atria) and two ventricles. The auricles are located at the broader end and the ventricles are towards the posterior narrow end. There is an external transverse groove between the auricles and ventricles, it is called coronary sulcus. - 170 Internally, the two (right and left) auricles are separated by an inter atrial septum. It has an oval depression called fossa ovalis, which is the remnant of foramen ovale of foetal heart. The two ventricles are separated by inter-ventricular septum. The atria and ventricles are separated by auriculo-ventricular septum (Av septum). It has an opening between the right auricle and right ventricle, guarded by tricuspid valve. The opening in the A-v septum between the left auricle and left ventricle is guarded by biscuspid or mitral valves. Circulatory System in Human The pointed ends of the flaps of A.V. valve are attached to the ventricular wall by chordae tendinae. The right auricle receives inferior and superior vena cava and the left auricle receives two pairs of pulmonary veins. The two major arteries leaving the heart are pulmonary artery (from right ventricle) and aorta from left ventricles) their openings are guarded by semilunar valves. The wall of the heart is made of cardiac muscle fibres which branch & connect with one another forming a network, this facilitates quick conduction of the cardiac impulse. Coronary arteries & vein supply blood to the cardiac muscles. CONDUCTING SYSTEM OF HEART In mammals, the heart is myogenic i.e. the cardiac impulse is initiated by the cardiac muscle fibres. The cardiac impulse is initiated by the sino-atrial node (SA- node), situated in the upper lateral wall of the right atrium. The fibres of SA-node show the ability of self-excitation and by determining the rate of discharge of cardiac impulse, it determines the heart rate (hence called pacemaker). The SA node is in close association with the muscles of auricles & conducts the impulse to auricles. The impulse is conducted to the ventricles through the auriculo-ventricular node (Av-node) located in the posterior part of interatrial septum. From this node, the AV bundle (Bundle of His) arises and immediately branches in a right and a left bundle, that enter the wall of right & left ventricles respectively. These two bundle branches produce many branches called Purkinje fibres throughout the ventricular wall and conduct the impulse to the muscles of ventricles. Circulatory System in Human CARDIAC CYCLE The contraction (systole) and relaxation (diastole) of auricles & ventricles alternately resulting in one heart beat constitute a cardiac cycle. When the auricles contract (auricular systole) the ventricles relax (ventricular diastole). When the ventricles contract (ventricular systole), the auricle relax (auricular diastole). For a very brief period, both auricle and ventricles are in their diastole & relax simultaneously, which is called as joint diastole. During systole of a cardiac chamber, blood is pumped out from there and during diastole of a cardiac chamber, blood enters that chamber. BLOOD FLOW IN THE HEART DURING A CARDIAC CYCLE AND THE HEART SOUNDS Cardiac cycle refers to all the events which occur during one heart beat i.e. its contraction and relaxation. The successive stages are as follows : - 172 - Circulatory System in Human (i) Atrial Systole : As SA node is stimulated a wave of contraction spreads through atria and the blood is forced into ventricles as bicuspid and tricuspid valves are open. (ii) Ventricular systole : A wave of contraction spreads through both ventricles stimulated by AV node contraction of ventricles result in closure of biscuspid and tricuspid valves producing first heart sound, lubb. (It lasts for 0.16-0.90 sec) Due to contraction of ventricles blood flows into pulmonary artery from right ventricle & dorsal aorta from left-ventricle. (iii) Ventricular diastole : As ventricles relax both semilunar valves close resulting in second heart sound, dupp (it lasts for only 0.10 second). Pressure within ventricles keeps decreasing when this pressure decreases below the atrial pressure both bicuspid & tricuspid valve open and blood flows again into ventricles. (iv) Joint Diastole : Before the atrial systole again starts, a time comes when both atria and ventricles are in a relaxed state. At this state blood flows into atria from superior and inferior vena cava and from atria to ventricles also. But semilunar valve remain closed. So blood does not flow into the aortic arch and pulmonary aorta. Heart Beat The rhythmic contraction and relaxation of the cardiac chamber is known as heart beat. The beat of a heart may originate in one of the two ways. Neurogenically or myogenically. REGULATION OF HEART BEAT - 173 Heart beat is at a fixed rate due to inherent property of the fibres of SA node but it can also be controlled by nervous & hormonal mechanisms. Centre of heart beat regulation is medulla oblongata. (i) Nervous regulation Sympathetic nervous system having accelerator nerves increases the heart beat by secreting adrenaline hormone. Parasympathetic nervous system supplying with vagus nerves decreases the heart beat by secreting acetylcholine (Ach). (ii) Hormonal Regulation Thyroxine : Secreted by thyroid gland increases heart beat indirectly by increasing basal metabolic rate which requires more oxygen. Epinephrine and nor-epinephrine secreted by adrenal medulla, also have a role in regulation. Epinephrine increases heart beat in emergency. Nor-epinephrine increases heart beat under normal condition. COURSE OF CIRCULATION OF BLOOD Physiology of blood circulation was first described by Sir William Harvey. Human heart is completely divided into four chambers due to which there is no inter-mixing of oxygenated and deoxygenated blood. Blood that returns from various parts of the body is collected in the right auricle. It passes into right ventricle from where it pumped into pulmonary aorta. Which takes the blood to lungs for oxygenation. Oxygenated blood returns to left auricle through pulmonary veins and is carried to left ventricle which distributes it to different body parts through dorsal aorta. So to complete one circulation blood goes twice through the heart and this type of circulation is called double circulation. Circulation of blood from heart to lungs and back into heart is called pulmonary circulation. while circulation to and from body parts is called systemic circulation. It can be schematically represented as follows : Circulatory System in Human CORONARY CIRCULATION Coronary circulation is accomplished by the coronary arteries. Coronary arteries arise from a branch of the aorta. The coronary veins formed on the myocardium ultimately pour into the superior vena cava, that reaches the blood to the right auricle.- 174 PORTAL CIRCULATION It is constituted by a portal vein and the system of capillaries to which the portal vein supplies blood. Hepatic portal vein returns blood from the intestine and breaks into a second system of capillaries in the liver. Hypophyseal portal vein arising from hypothalamus breaks into a second system of capillaries in the anterior pituitary. Circulatory System in Human HEART SOUNDS Heart sound are of two types which occurs due to the closure of cuspid valves and semilunar valves & are respectively called “lubb” & ‘dupp’. Heart sounds give valuable information about working of valves. So any damage of these valves (either bicuspid/tricuspid or semilunar valves) effects the quality of sound. This is called heart murmur. Heart murmurs are caused by turbulent blood flow through a narrowed valve or by changes in the direction of the blood flow. Heart beat/heart sound are measured or listened by means of instrument called stethoscope. - 175 BLOOD PRESSURE It is the arterial pressure of blood exerted on the wall of arteries with each heart beat. It is measured from the brachial artery in the elbow pit. The blood pressure can be measured by sphygmomanometer. It has first measured by S. Hales. Blood pressure is expressed as. Normal B.P. has high systolic value & low diastolic value i.e. 120 mm Hg/80 mm Hg in arteries. B.P. In arterioles = 85/60 mm Hg. In capillaries = 30/10 mm Hg in vein = 10/0 mm Hg Hypertension : means high blood pressure. It has systolic value more than 140 mm Hg and diastolic more than 90 mm Hg. Hypotension : means low blood pressure with systolic below 110 mm Hg and diastolic below 70 mm Hg. CARDIAC OUTPUT The amount of blood flowing from the heart (i.e. from the left ventricle into aorta) over a given period of time is known as cardiac output. Cardiac output = stroke volume (the volume of blood pumped by heart/heart beat) × Heart rate (ventricular systole/min) 70 ml × 72 /min = 5040 ml/min. = about 5 litre/min. PACEMAKER SA node controls the heart beat & hence, it is the natural pace maker of the heart. An artificial pacemaker is an electronic device, which regularly sends a small amount of electrical charge for maintaining the rhythmicity of the heart. It is implanted subcutaneously in the upper thoracic region which has connection with the heart. In patients having the symptoms of ventricular escape (stokes Adams syndrome), in which the atrial impulse suddenly fails to be transmitted to the ventricle, which may last for few seconds to few hours even, the artificial pacemaker is connected to the right ventricle for controlling its rhythm. The pacemaker consists of pulse generator containing cell, to produce electricals impulse, that lead in the form of wire and an electrode connected to the portion of the heart when impulse is to be transmitted. Circulatory System in Human PULSE It is wave of distension felt in the arteries with each heart beat. It is counted from radial artery of wrist. Normal pulse rate ranges 70-90 per minute. 72/min in man & 80/min in women. Pulse rate in children is more rapid than adult. It is more rapid in female then in the male. When any strong emotion is experienced the pulse rate is increased for e.g. anger, excitement etc. Exercise increases the rate of pulse. HEART RATE It is pulse per minute Heart rate increased by Increase in blood pressure in the vena cava. Increase in blood CO2 concentration. Decrease in blood pH Increase in body temperature (core temperature) Increase in hormone adrenaline. Decrease in hormone thyroxin (increase in BMR) Increase in the nervous input from pain receptors. Heart rate decreased by Increase in blood pressure in aorta and large arteries. Decrease in blood CO2 concentration. Increase in blood pH. Decrease in body temperature (core temperature). Decrease in adrenaline. Increase in thyroxine. DISORDERS OF THE CIRCULATORY SYSTEM (i) Hypertension : Hypertension means high blood pressure. It has systolic value more than 140 mm Hg and diastolic more than 90 mm Hg. Reasons : Stiffening of arterial walls due to ageing, thickening of arterial walls due to cholesterol deposition, chronic vasoconstriction of arterial walls toxins, hormones etc. Hypertension caused by hormones is called primary or essential hypertension. It affects the heart, brain and kidneys. In the brain, it causes haemorrhage or infractions, leading to various disabilities. Circulatory System in Human (ii) Hypotension Hypotension means low blood pressure with systolic below 110 mm Hg and diastolic below 70 mm Hg. Reason : Low metabolic rate, starvation, anaemia (most common cause), chronic vasodilation of arterioles, nervous disorders. (iii) Atherosclerosis It refers to the deposition of lipids (specially cholesterol) on the wall lining the lumen of large and medium sized arteries. Such a deposition is called atheromatous or atherosclerotic plaque. The formation of atheromatous plaque starts with the deposition of minute cholesterol particles in the tunica intima and smooth muscles. Gradually these plaques grow due to proliferation of the fibres & small muscles around it. This results in the reduction of the lumen size of the artery & so the flow of blood is reduced. In extreme circumstances, these plaques may completely block the artery. The proliferation of smooth muscles occurs because these plaques provide a rough surface to the platelets causing the release of platelet derived growth factor (PDGF). Such plaques, if formed in the coronary artery, reduce the blood supply to the heart, or may stop the supply due to complete blockage. This may result in heart attack or stroke. (iv) Arteriosclerosis It refers to the hardening of the arteries due to deposition of cholesterol and thickining of them by calcium salt. This calcification makes the arteries stiff and rigid and the artery loses the property of distension and hence its wall may rupture. The blood coagulates and blocks the pathway of blood (thrombosis). If the thrombosis or clot appears in the coronary artery, it leads to heart attack and death. (v) Coronary Heart Disease Coronary arteries supply oxygen and nutrients to the heart muscle & remove CO2 and other metabolic wastes from it. Unhealthy coronary arteries cause heart diseases. These includes angina pectoris and heart attack. (a) Angina Pectoris : Angina pectoris literally means “pain in the chest”. It results from arteriosclerosis of the arteries that supply the blood to the heart muscle itself i.e. the coronary arteries. Due to the lack of required expansion, these arteries are unable to carry extra blood to the heart muscle at the time of stress when the heart is beating more vigorously. Deprived of oxygen, the heart muscle experiences constricting pain. (b) Heart Break : Formation of a clot (thrombus) in a narrowed coronary artery stops blood supply to the part of the heart muscle beyond the clot. The muscle cells of this part die due to lack of oxygen and glucose. This condition is called “heart attack” (coronary thrombosis or myocardial infraction or MI). The heart attack is characterised by severe pain in the heart, breathlessness, restlessness, nausia & vomiting. (vi) Rheumatic Heart Disease (RHD) These result from repeated attacks of rheumatic fever in childhood. This fever is caused by infection of the throat with the bacterium streptococcus. Bacterial infection may reach the heart and the bacterial toxins affect the auriculoventricular valves. The valves develop two defects. Failure to close completely and narrowing of the openings they surrounds. This makes the function of the valves irregular & lowers the blood pressure. (vii) Tachycardia Increased rate of heart beat. (viii) Bradycardia Decreases rate of heart beat. Circulatory System in Human (ix) Heart block When heart beat is not passed to the ventricles properly. This is the deflect of conducting system of the heart. (x) Arrythmia It is irregular heart beat. 178 (xi) Coronary thrombosis It happens due to formation of clot in coronary artery. (xii) Myocardial infraction It is the death of a part of heart muscle following cessation of blood supply to it. It is acute heart attack. (xiii) Palpitation Awareness of heart beat or a feeling of having a rapid and unusually forceful heart beat, especially of they last for several hours or over several days and/or cause chest pain, breathlessness or dizziness. (xiv) Fibrillation It is a condition in which the heart muscle is contracting very rapidly but in an uncoordinated fashion. There are atrial and ventricular fibrillation. Ventricular fibrillation is immediately life threatening unless it can be stopped by defibrillation. A machine called a defibrillator which gives a DC voltage shock, is used to do this. (xv) Anaemia It is a condition where the number of blood corpuscles is reduced below the normal limit. The anaemia leads to fatigue, paleness and less resistance to cold. (xvi) Aplastic anaemia This abnormality is due to the defective formation of red blood cells in bone narrow. It may be due to deposition of fatty tissue, fibrous tissue or tumer cells in bone marrow. (xvii) Familial haemolytic anaemia In this case RBC formation is abnormal making it fragile and can be easily broken down. (xviii) Mediterranean anaemia It is also called thalassemia or cooley’s anaemia. This is due to very small fragile RBCs, containing haemoglobin F. (xix) Jaundice The increased RBC destruction, due to haemolysis causes a rise in plasma bile pigment (bilirubin) and there might be Jaundice along with the anaemia. (xx) Pernicious anaemia This is the most common type of megaloblastic or macrocytic or nutritional or Addison’s anaemia, in which lack of intrinsic factor in the Stomach makes it impossible for vitamin B12 to be absorbed. In other words, it is due to lack of maturing factors in the diet. In this abnormality the maturation of erythrocytes is delayed and immature red cells come in the circulation & may rupture there. (xxi) Polycythemia Vera (erythremia) It is a pathological condition having RBC count well above normal. Circulatory System in Human (xxii) Perpura In this disease there is dimination of platelets in the blood. Haemorrhage occurs beneath the skin and mucous membrane. The coagulation time remains normal, but bleeding time is prolonged. (xxiii) Haemoglobinuria -Red cells are agglutinated and then haemolysed. Large amount of haemoglobin is released and partly converted into bilirubin. Part of the released haemoglobin is excreted in urine as such. (xxiv) Hypochromic anaemia This is a microcytic of nutritional anaemia. It is caused due to deficiency of iron. The red cells are smaller, irregular shape and contain less amount of haemoglobin. This condition is seen in middle aged women during or after the menopause. (xxv) Arteriosclerosis Rise in blood cholesterol may lead to a deposition of cholesterol on the walls of blood vessels. This causes the arteries to lose their elasticity and get stiffened. This is called hardening of arteries to lose their elasticity and get stiffened. This is called hardening of arteries or arteriosclerosis. This also results in hypertension. (xxvi) Thrombosis It involves the formation of a blood clot called thrombus inside the blood vessels. It blocks the blood flow and proves fatal, it formed inside the coronary vessels or carotid vessel. When a thrombus is transported by the blood flow then it is called embolus which when deposited in a specific part of the vessel and acts off the circulation, then it is called embolism. - 180 - Circulatory System in Human - 182 -