Human Antomy & Physiology II Week 3
Human Antomy & Physiology II Week 3 BSC 2085
Popular in Human anatomy and Physiology II
Popular in Human anatomy and Physiology II
This 5 page Class Notes was uploaded by Alex on Wednesday August 31, 2016. The Class Notes belongs to BSC 2085 at 13810096322 taught by in Fall 2016. Since its upload, it has received 13 views. For similar materials see Human anatomy and Physiology II in Human anatomy and Physiology II at 13810096322.
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Date Created: 08/31/16
Chapter 18B: The Heart Heart Physiology: The Intrinsic Conduction System: Autorhythmic Cells: -Initiate spontaneous action potentials -have unstable resting potentials called pacemaker potentials -use calcium influx for rising phase of the action potential Intrinsic Cardiac Conduction: Sinoatrial Node (SA): -Located in wall of atrium -Center of autorhythmicity -Generates impulses about 57 times/minute called sinus rhythm Atrioventricular (AV): -has autorhythmicity (50 times/min) -wave of depolarization takes some time to reach it/ delay gives atria time to finish contracting Impulse passes from atria to ventricle via the atrioventricular bundle (bundle of HIS) AV Bundle: splits into 2 pathways in interventricular septum Purkinje fibers: carry impulse deep into the myocardium and ventricle contacts Electrocardiography Electrocardiogram (ECG): electrical activity is recoded by this test P- Wave: corresponds to depolarization of SA node QRS: corresponds to ventricular depolarization T- Wave: corresponds to ventricular repolarization The Cardiac Cycle The sequence of events that occur in the heart during one complete heartbeat-0.8 seconds. This includes: -Contraction (systole) and relaxation (diastole) of the chamber - Blood volume changes in the chambers; same in R & L sides -Pressure changes in the chambers; R is 20% less -Valves opening and closing -ECG -Heart sounds What causes blood to move through the heart? REMEMBER: movement of blood through the heart is determines entirely by the pressure (P) gradients. If Pa= Pv, no movement occurs Once Pa> Pv, the bicuspid/mitral valves open and blood moves Once Pv> P, the semilunar valves open and the blood moves Phases of the Cardiac Cycle Phase 1: Ventricular Filling -1) blood enters atria passively and flows into ventricles -2) 70% of blood flows immediately through the atria passively into the ventricle, since the valves are open -3) at the end, the SA node fires, followed by atrial contraction (systole) (This causes a sudden rise in atrial pressure which forces the residual blood (30%) in the atria to flow to the ventricles. The left and right atria contract simultaneously.) Phase 2: Ventricular Systole -1) action potential passes to AV node and begins to travel through the ventricles eventually causing them to contract (systole). -2) first split second of ventricular systole, both tricuspid and pulmonary semilunar valves are closed- Isovolumetric contraction phase of ventricular systole- pressure increases even more. -3) ventricular ejection opens semilunar valves (during ventricular systole, the atria relaxes (diastole) Phase 3: Isovolumetric relaxation (Diastole) -1) semilunar valves close to start ventricular diastole: second heart sound -2) closure of the aortic semilunar valves causes a brief increase in aortic pressure -3) for a split second all the valves are closed- isovolumetric relaxation. Ventricles relax -4) all during ventricular systole, the atria have been passively filling with blood. When the pressure in the atria exceeds that of the ventricle, the AV valves open. Heart Sounds Heart sounds (lub-dub) are associated with closing of heart - First sounds occurs as Av valves close and signify beginning of Systole - Second sound occurs when SL valves close at the beginning of ventricular diastole - Four sounds can actually be heard because of the mitral valve closes before the tricuspid and the aortic semilunar closes before the pulmonary semilunar Abnormalities in heart sounds reflect valve abnormalities (murmurs) Cardiac Output CO: the amount of blood pumped by each ventricle per minute (same on right and left sides) CO is the product of heart rate (HR) and stroke volume (SV) SV: is related to the force of ventricular contraction; the greater the force , the more blood ejected CO=SV x HR Cardiac output Examples: CO (ml/min)= HR (75 beats/min) x SV (70mL/miin) CO=5250 mL/min (5.25 L/min) The CO changes to meet body demands. Usually this is accomplished by increasing he HR. When HR and/or SV change, CO usually changes. The ventricle must out exactly what it receives. Even a tiny difference can accumulate over time and cause enlargement of the heart and eventually heart failure. Factors Affecting Stoke Volume and Cardiac Output Preload: is the degree of stretch of cardiac muscles cells when the ventricles are filled, just before they contract End Diastolic Volume Frank Starling Law of the Heart: the more the myocardium is stretched the harder it will contract. Venous return: important factor in stretching cardiac muscle. Anything that changes venous return to the heart will change the amount entering the ventricle. Contractility Is the increase in strength of contraction The greater the contractility the greater the SV and CO Afterload Pressure exerted by the blood in the aorta that must be overcome for the heart to eject its blood End systolic volume: the amount remaining in a ventricle AFTER it has contacted Afterload will lower SV and CO Afterload is really systemic blood pressure Regulation of Heart Rate CO=SV x HR Regulation of heart rate is the body’s principle means for short term control of CO and blood pressure Positive Chronotropic: factors increase heart rate Negative Chronotropic: factors decrease heart rate Regulation of Heart Rate: Autonomic Nervous System is the most important extrinsic control of the heart rate Cardioacceleratory Center (CAC): a group of sympathetic neurons in the medulla of the brain. -sends a message to activate the SA node, AV node and the myocardium. -epinephrine released to dilate the coronary artery -this increased both the heart rate and the force of contraction -stimuli include: fright, emotions, exercise, and messages from sensory receptors Cardioinhibitory center: group of parasympathetic neurons in the medulla of the brain. -when stimulated the CIC send messages that travel along the vagus nerve to the heart to slow HR and decrease SV - stimuli include: grief severe depression, and messages from sensory receptors Reflex regulation of the heart: Baroreceptors Sensory receptors that recognize pressure (mechanical changes in the vessel wall) When stimulated the receptors send messages to the cardiac centers to stimulate the CIC and depress the CAC -HR and SV decrease -the low stoke volume decreases blood pressure Reflex Regulation of the Heart: Atrial Bainbridge Reflex Vena cava and atria have baroreceptors With increased blood in the atria, the baroreceptors send messages to cardiac centers to increase HR and SV. Purpose: prevent blood from backing up into heart & get it moving into general circulation Congestive Heart Failure (CHF) CHF: inability of heart to pump enough oxygen to the tissues -Caused by: coronary artery disease -persistent high blood pressure -myocardial infarctions: death of cardiac muscle cells
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