Chapter 13 Cardiovascular System Cardiac Func9on The Heart Blood follows an essen1ally circular path as it travels through the body s vasculature Consists of two divisions the pulmonary circuit which consists of all the blood vessels connec1ng the lungs with the heart The systemic circuit which encompasses the rest of the blood vessels in the body Pacemaker Cells Contrac1ons of the heart are ini1ated by pacemaker cells which spontaneously generate ac1on poten1als Although located in nearly all parts of the heart the are concentrated in two speci c regions of the myocardium the sinoatrial node SA node located in the wall of the upper right atrium The atrioventricular node AV node located near he tricuspid valve Electrical Ac1vity in Cardiac Contrac1le Cells Electrocardiogram Electrocardiogram ECG or EKG is a noninvasive means for monitoring the electrical ac1vity of the heart Electrical ac1vity ini1ated by the heart spreads through the body whose uid func1ons as a conductor Electrodes are placed on the skin at corners of an imaginary equilateral triangle that measure the di erence in surface electrical poten1al between posi1ve and nega1ve electrodes Electrocardiogram The waveforms depend on whether the di erence between the two electrodes is posi1ve or nega1ve i e depolarizing wave traveling toward posi1ve electrode causes an upward de ec1on The EKG normally shows three characteris1c waveforms P wave Atrial depolariza1on QRS complex Ventricular depolariza1on T Wave Ventrical repolarizaton The p wave smallest in the gure is the atria that contracts rst Cardiac Arrhythmias Electrocardiogram Atriums input chambers Ventricles output chambers 1 Ventricular lling blood returning to the heart via the systemic and pulmonary veins enter the relaxed atria and passes into the ventricles Then atria contract driving more blood into the ventricles 2 Isovolumteric contrac1on ventricles contract closing the AV valces building up pressure 3 Ventricular ejec1on The pressure is great enough to force open the semilunar valves so blood can exit the ventricles 4 Isovolumetric relaxa1on Ventricular myocardium is relaxing and all valves are closed un1l the pressure is low enough that AV valves are open again Diastole relaxa1on Blood Pressure During diastole no blood enters the aorta because the aor1c valve is closed But blood is s1ll owing from the aorta through blood vessels causing a decrease in aor1c blood pressure DP In phase 3 aor1c valves open and aor1c pressure rises quickly and reaches a maximum called the systolic pressure SP Heart Beat The rst so er low pitched sound in the heartbeat is due to the start of systole phase 2 when the AV valves close The second louder sharper higher pitched sound is due to the start of the diastole phase 4 when the semilunar valves close Sounds are not due to valves slapping shut but rather the turbulent rush of blood through the valves as they are narrowing and about to close Cardiac Output Stroke Volume x Heart Rate Autonomic Control of the Heart Pacemaker cells of the SA node receive direct input from the autonomic nervous system which in uence the frequency of ac1on poten1als The AV node also gets input from both the sympathe1c and parasympathe1c nervous system which in uences the speed with which ac1on poten1als are conducted Autonomic Control of Heart Rate Sympathe1c neurons release norepinephrine which bind to 1 adrenergic receptors on the SA nodal cells ac1va1ng cAMP and ul1mately opening funny and T type calcium channels Parasympathe1c neurons release acetylcholine which binds to muscarinic receptors which suppress the opening of funny and T type calcium channels Autonomic Control of Stroke Volume The sympathe1c nervous system increases the strength and rate of contrac1on of ventricular contrac1le cells by 1 Increasing calcium into the cell through L type calcium channels 2 Enhance the release of calcium from the SR 3 Increases the rate of myosin ATPase ac1vity thereby increasing the rate of the crossbridge cycle 4 Enhance the rate of calcium uptake into the SR increasing the relaxa1on rate No need to know details of the channels Autonomic Control of the Heart Intrinsic Control of the Heart Starling s Law When the rate at which blood ows into the heart from the veins venous return changes the heart automa cally adjusts its output to match the in ow
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