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Transcript
Heart Physiology CH 11 Anatomy and Physiology Conduction of Impulses • Cardiac muscle can contract without nerve stimulation • Different cells contract at different rates – Atria 1/sec – Ventricles 1/5sec – Must be regulated • Autonomic nerves control rate • Intrinsic Conduction System (pacemaker) Intrinsic Conduction System • Also called the nodal system • Composed of a hybrid muscle/nervous tissue • Sets the rhythm at around 75 bpm • SA node is the pacemaker • AV node contracts atria • AV bundle carries signal to apex • Purkinje fibers contract ventricles Poor conduction • Damage to AV node is called heart block – Ventricles contract on their own • Damage to SA node – May need internal pacemaker • Major damage due to ischemia may disrupt normal conduction – Ventricles may go into fibrillation • Major cause of death in MI patients • Can require defibrillation (AED) Electrocardiogram (ECG) • Electrical signals in the heart can be traced externally • Electrodes are placed on the skin • P wave atria depolarize • QRS ventricles depolarize • T wave Vent Repolar Cardiac Cycle • • • • One complete heart beat Systole is contraction Diastole is relaxation In cardiac cycle they apply to the ventricles unless otherwise noted • Average 75 bpm or 1/0.8 sec • 1. heart relaxed and filling, AV valves open, end with atrial contraction • 2. AV valves close due to increased pressure, semilunar vales forced open, atria relax and fill • 3. ventricles relax, AV valves reopen and ventricles refill Heart Sounds • AV valve closing before ventricular contraction causes “lub” sound – Long and loud • Semilunar valves closing after contraction makes “dub” sound – Short and sharp • Any abnormal sound is called a murmur – May be normal or pathological Cardiac Output (CO) • Product of stroke volume (SV) and heart rate (HR) • CO = SV x HR • CO = 70 ml/beat x 75 beats/min = 5250 ml/min entire volume is 6000 ml • A change in either results in a change in CO Changing SV • Stroke volume related to stretch on ventricle wall • Increase venous return increases volume leading to more stretch – Exercise increases heart rate and muscular pump action Changing HR • ANS control – Sympathetic increases HR in times of demand – Parasympathetic slows the HR when crisis is over (Vagus Nerve) • Drugs such as digitalis • Hormones • Ions Homeostatic Imbalance of pumping • Congestive Heart Failure (CHF) – Progressive condition – Caused by coronary athlerosclerosis – Left side failure leads to lung congestion – Right side failure leads to peripheral swelling – Failure of one side puts stress on the other and will lead to total failure if untreated