Control of Heart Rate IB Topic 6.2

Your Heart is a Muscle  Cardiac muscle Spontaneously contracts and relaxes without nervous system control  Two atria Relatively thin walls; receives blood  Two ventricles Relatively thick walls; pumps blood  Needs to be controlled in order to keep the timing of the contractions unified

Cardiac Cycle  The heart contracts and relaxes in a rhythmic cycle  When the heart contracts, it pumps blood  When the heart relaxes, its chambers fill with blood  One complete sequence of pumping and filling is called the cardiac cycle

Systole and Diastole  Systole: Contraction phase  Diastole: Relaxation phase  AV valves Between atrium and ventricle  Semi-lunar valves Between two exits (L ventricle and aorta & R ventricle and pulmonary artery)

Maintaining Your Beat  The right atrium has a mass of tissue within its walls near the superior vena cava Called the sinoatrial node (SA node) It acts as the pacemaker for the heart  It sends out an electrical signal to initiate the contraction of both atria  For a person with a resting heart rate of 72 beats a minute, signals from SA node are sent out every 0.8 seconds

AV node  Also within the right atrium is another mass of tissue near the base Called the atrioventricular node (AV node) Receives signal from SA node, waits ~ 0.1 seconds, and sends out another electrical signal This signal goes to the muscular ventricles, causing them to contract

Therefore …  Both atria contract first, then both ventricles contract together

Physiological Cues  The SA node sets the tempo for the entire heart  Tempo may be influenced by: Exercise (increased demand for oxygen and your cells are producing more carbon dioxide)  High levels of CO2 trigger the medulla  signal to cranial nerves  increase heart rate Chemicals  Adrenaline  High stress  adrenal glands secrete adrenaline into your blood stream  Adrenaline causes the SA node to “fire” more frequently

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