HUMAN CIRCULATORY SYSTEM

Human circulatory system consist of a muscular chambered heart a network of blood vessel and blood the fluid which is circulated. HEART"the mesodermally derived organ is situated tilted to the left.It has the size of a clenched fist.It is protected by a double walled membranous bag called pericardium enclosing pericardial fluid.Our heart has four chambers called ATRIA and VENTRICLES.The atria are separated by interatrial septum and the ventricles are separated by inter ventricular septum.The atria and ventricles are separated by atrio-ventricular septum.The opening between right atria and right ventricle is guarded by tricuspid valve and the left atria and left ventricle is separated by bicupsid valve.The opening between right and left ventricles into the pulmonary artery and the aorta respectively provided by semilunar valve.This valve helps to prevent backward flow. The entire heart is made up of cardiac muscles.The walls of ventricles are much thicker than atria. A specialized cardiac musculature called the nodal tissue is also distributed in the heart. A patch of the tissue is present in the right upper corner of the right atrium called SINO-ATRIAL NODE . Another mass of the tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum called the atrio-ventricular node. A bundle of nodal fibers,atrio ventricular continues from the AVN which passes through the atrio-ventricular septa to emerge on the top of the interventricular septum and immediately divides into a right and left bundle. These branches give rise to minute fibers throughout the ventricular musculature of the respective sides called purkinje fibres. The nodal musculature has the ability to generate action potentialwithout any external stimuli.ie It is auto excitable. Therefore it is called the cardiac pacemaker. CARDIAC CYCLE To begin with ,all the four chambers of heart in a relaxed state.they are called joint diastole, As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and venecava flows into the left and right atria. The semilunar valves are closed at this stage. The SAN now generates an action potential which stimulates both the atria to undergo a simulataneous contraction-the atrialsystole. The increases the flow of blood into the ventricles by about 30 percent. The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle from where the bundle of His transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract, the atria undergoes relaxation, coinciding with the ventricular systole. Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves due to attempt backflow of blood into the atria. As the ventricular pressure increases further,the semilunar valves guarding the pulmonary artery and the aorta are forced open,allowing the blood in the ventricles to flow through these vessels into the circulatory pathways. The ventricular now relax and the backflow of blood into the ventricles. As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by veins. The blood now once again moves freely to the ventricles.The ventricles and atria are now again in a relaxed state. Duration of cardiac cycle about 0.8 seconds. REGULATION OF CARDIAC ACTIVITY Normal activities of the heart are regulated intrinsically,ie,auto regulated by specialized muscles,hence the heart is called myogenic. A special neural centre in the medulla oblongata can moderate the cardiac function through autonomic nervous system. Neural signals through the sympathetic nerves can increase the rate of heart beat,the strength of ventricular contraction and thereby the cardiac output.On the other hand,parasympathetic neural signals decrease the rate of heart beat ,speed of conduction of action potential and thereby the cardiac output. Adrenal medullary hormones can also increase cardiac output. DISORDERS OF CIRCULATORY SYSTEM HYPERTENSION: The term for blood pressure that is normal 120/80. If the blood pressure of an individual is 140/90 or higher,it shows hypertension. CORONARY ARTERY DISEASE (CAD): It is reffered to as atherosclerosis,affects the vessels that supply blood to the heart muscles. It is caused by deposits of calcium,fat,cholestrol and fibrous tissues,which makes the lumen of arteris narrower. ANGINA: It is also called 'angina pectoris'. A symptom of acute chest pain appears when no enough oxygen is reaching the heart muscle. Angina can occur in men and women of any age but it is more common among the middle aged. HEART FAILURE: It means that state of heart when it is pumping blood effectively enough to meet the needs of the body. It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease. Heart failure is not same as cardiac arrest or a heart attack. ______________________________________