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Cardiac Cycle and Conduction System of the Heart The Cardiac Cycle The mechanical events that take place in one heartbeat. The Cardiac Cycle Contraction of a chamber - systole Relaxation of a chamber - diastole 1 full contraction and relaxation - cardiac cycle Systole and Diastole Peak pressure of a chamber or vessel Stroke volume amount of blood ejected by the heart (LV) in one contraction • aprox. 70 cc (60-100 cc) Relaxation of a chamber Pressure at its lowest Cardiac Output Cardiac Output amount of blood ejected every minute • aprox. 5-6L • SV X HR = CO • The strength of contraction is directly related to venous return Starling’s Law The more a muscle is stretched the harder it will contract (up to a certain point) Similar to a spring Muscle fibers will stretch and contract harder when there is a increased blood return Blood Pressure The pressure exerted by blood against the walls of a given artery. Factors Affecting Blood Pressure 1. 2. 3. 4. Blood Volume Strength of heart contractions Heart rate Blood viscosity – thickness of blood Electrophysiology Before the heart can contract it must be stimulated. This is accomplished through a specialized network of cells called the conduction system. There are numerous “pacemaker” cells to stimulate the heart to contract. Sinoatrial Node (SA) Located in high RA Main pacemaker of heart Sends out regular impulses at 60- 100 times per minute Regulated by the Autonomic Nervous System (vagus nerve) Atrioventricular Node (AV) Located on floor of TA next to septum and behind CS os Has 2 main jobs Delay the impulse from the SA node long enough so the ventricles can fill with blood Protect the ventricles from rapid atrial rates Inherent rate of 40-70 bpm Bundle of His (HB) A fibrous ring around the 4 valves Only muscular connection between the atria and ventricles AV node and HB collectively are called the AV junction The AV junction can pace the heart at rate of 40-60 bpm Bundle Branches HB branches into rt.... And lt.. Branches Lt.. Bundle branch (LBB) then divides into anterior and posterior sections Purkinje Fibers Terminal portion of the conduction system Located in ventricular muscle Can pace the heart at 20-40 bpm Electrical Properties of Cardiac Muscle Electrophysiology cont..... Automaticity Property by which a cell can reach threshold potential (depolarize) and send an impulse without help from an external source Excitability Ability of cell to reach threshold potential in response to an outside stimulus Conductivity The propagation of an impulse form cell to cell through intercalated disks Contractility Ability of a muscle to shorten (contract) when stimulated Electrophysiology Cont..... The end result of contraction of the heart is electrochemical in nature. The impulse is sent along the conduction system because of changes in cellular polarity. Cardiac Emergencies A thrombosis or embolism may plug the arteries causing a myocardial infarction or “heart attack” Angina pectoris is used to describe the severe chest pain infarct