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Transcript
REGULATION MEANS Intrinsic cardiac regulation of pumping in response to changes in volume of blood flowing into the heart. Autonomic nervous System by controlling heart rate and strength of heart pumping. INTRINSIC REGULATION (THE FRANK- STARLING MECHANISM) Amount of blood pumping by the heart is determined by rate of blood flow in the heart by veins (venous return.) The intrinsic ability of heart to adapt to increasing volume of inflowing blood is called Frank-Starling mechanism of heart. FSM means that the greater the heart muscle is stretched during filling ,the greater is force of contraction and greater the quantity of blood pumped in aorta. With in physiological limits the heart pumps all the blood that returns to it by the way of vaina cava . EXPLANATION OF FRANK-STARLING When extra amount of blood flows into ventricles ,cardiac muscle its gets stretched to greater length, this causes actin and myosin to overlap for force generation. Therefore, the ventricle ,because of its increased pumping ,automatically pumps the extra blood in to the arteries. The ability of stretching up to an optimal length is characteristic of all striated muscles. Stretch of the right atrial wall directly increases the heart rate by 10 to 20 percent. CONTROL BY SYMPATHETIC AND PARASYMPATHETIC NERVES For given level of input atrial pressures ,the amount of blood pumped each minute (cardiac output)often can be increased by more than 100 percent by sympathetic stimulation. The output can be decreased to as low as zero to almost zero by vagal (parasympathetic)stimulation. (Mechanism of excitation of heart by sympathetic nerves) Strong sympathetic stimulation can increase heart rate from normal 70beats per minute upto 180 to 200 or sometime 250times per minute. Heart contraction force increases to double than the normal. Cardiac output increased twice. PARASYMPATHETIC OR VAGAL STIMULATION. Strong stimulation of parasympathetic nerve fibres in the vagus nerve to heart can stop heart beat for few seconds but than the heart usually escapes and beats at a rate of 20 to 40 b/min as long as parasympathetic stimulation. Heart muscle contraction strength decreased by 20 to 30 percent. Vagal fibres are distributed mainly to the atrias so it decreases heart rate more than the strength of heart contraction. Great decrease in heart rate combined with slight decrease in contraction strength can decrease ventricular pumping 50 percent or more. EFFECT OF POTASSIUM AND CALCIUM IONS ON HEART FUNCTION Effect of potassium ions: Excess potassium in the ECF causes heart to become dilated and flaccid and slows the heart rate. Large quantities also can block conduction from the atria to ventricles. Elevation of potassium level from 8 to 12mEq/L can cause weakness of heart and abnormal rythm can cause death. Effect of calcium ions:Excess of calcium ions causes heart to go in spastic contraction. Defeciency of calcium ions causes cardiac flaccidity similar to effect in high potassium. EFFECT OF TEMPERATURE ON HEART FUNCTION Increased body temperature e.g fever causes a greatly increased heart rate sometime as fast as double. Decreased temperature causes greatly decreased heart rate falling to as low as few beats per minute. The Heart: Conduction System The heart pumps blood through the body This is accomplished by contraction and relaxation of the cardiac muscle tissue in the myocardium layer. Intercalated discs allow impulses to travel rapidly between adjacent cells so they function as one rather than individual cells Cardiac Muscle Tissue intercalated disc intercalated disc Conduction System Continued…. Cardiac conduction system: The electrical conduction system controls the heart rate This system creates the electrical impulses and sends them throughout the heart. These impulses make the heart contract and pump blood. : Components of the Conduction System Sinoatrial Node located in back wall of the right atrium near the entrance of vena cava initiates impulses 70-80 times per minute without any nerve stimulation from brain establishes basic rhythm of the heartbeat called the pacemaker of the heart impulses move through atria causing the two atria to contract. at the same time, impulses reach the second part of the conduction system Components of the Conduction System Continued …. Atrioventricular Node : located in the bottom of the right atrium near the septum cells in the AV node conduct impulses more slowly, so there is a delay as impulses travel through the node this allows time for atria to finish contraction before ventricles begin contracting Atrioventricular Bundle . “Bundle of His” From the AV node, impulses travel through to the right and left bundle branches These branches extend to the right and left sides of the septum and bottom of the heart. Atrioventricular Bundle Continued…. These branch a lot to form the Purkinje fibers that transmit the impulses to the myocardium (muscle tissue) The bundle of His, bundle branches and Purkinje fibers transmit quickly and cause both ventricles to contract at the same time Like a “phone tree” Atrioventricular Bundle Continued…. As the ventricles contract, blood is forced out through the semilunar valves into the pulmonary trunk and the aorta. After the ventricles complete their contraction phase, they relax and the SA node initiates another impulse to start another cardiac cycle. 1 - Sinoatrial node (SA node) 2 - Atrioventricular node (AV node) 3 – Bundle of His 4 - Right & Left Bundle Branches which lead to Purkinje Fibers