Chapter 19b- The Circulatory System: Physiology of the Heart
Chapter 19b- The Circulatory System: Physiology of the Heart BIOL 20214
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This 7 page Study Guide was uploaded by Marlee Porter on Wednesday February 3, 2016. The Study Guide belongs to BIOL 20214 at Texas Christian University taught by Mrs. Crenshaw in Spring 2016. Since its upload, it has received 35 views. For similar materials see Anatomy and Physiology in Biology at Texas Christian University.
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Date Created: 02/03/16
Chapter 19b The Circulatory system: Cardiac Physiology Cardiac Muscle Cardiocytes cardiac muscle cells Intercalated discs thick connections between cardiocytes Gap Junctions channels allowing ion flow between cells Desmosomes anchor cells together Interdigitating folds increase surface area between cells Cardiocytes rely almost exclusively on aerobic cellular respiration The heart is fatigue resistant, but requires constant oxygen supply Adults derive 6090% of ATP from fatty acids The heart can adapt to metabolize whatever nutrient source is available (even lactic acid) Damaged cardiac muscle is replaced almost entirely with fibrosis (scar tissue) Two types of cardiocytes: Contractile Cardiocyte Cells Most numerous Gap junctions form a functional syncytium to allow all connected cardiocytes to contract simultaneously Pacemaker Cells Noncontractile cells Capable of selfdepolarization; do not require extrinsic stimulation to facilitate cardiac contraction Bundled together to form nodes, which collectively form the Intrinsic Conduction System Connected to one another and to cardiocytes via gap junctions Electrical Events of the Heart Pacemaker Cells and the Intrinsic Conduction System Determines the innate rhythm of the heart Anatomy consists of a series of pacemaker cells which form the: Sinoatrial Node Atrioventricular Node Bundle of His and the bundle branches Purkinje Fibers Sa node regulates heart rate; failure for the SA node to function will results in one of the slower nodes to take over The rhythm of these cells can be influenced by numerous chemical and mechanical elements Normal rhythm is generally 7080bpm The Sinoatrial Node (SA)= “the pacemaker” Generates the sinus rhythm of the heart and stimulate the two atria to contract simultaneously As long as the SA node is active, all other cells of the cardiac conduction system will depolarize at the sinus rhythm At rest, it receives input from the parasympathetic Vagus Nerve This Vagal Tone keeps resting heart rate around 75bpm Innate rhythm is closer to 100bpm without Vagal tone Atrioventricular Node (AV) Receives depolarization from SA node due to gap junctions Delays impulse travel by 100ms, which allows atria to finish contraction before the ventricles begin contraction Without the SA node, the AV node would generate 4050 impulses per minute (nodal or junctional rhythm) Sufficient to maintain life Bundle of His (Atrioventricular Bundle) -Carries signals from the AV node through the ventricular septum and forks into the Right and Left Bundle branches -Without the AV node, the bundle would generate 30 impulses per minute -Too slow to maintain life -Purkinje Fibers -Arise from the bundle branches -Controls the bulk of ventricular depolarization -More elaborate network in the left ventricle -Entire ventricular myocardium depolarizes about 200ms after the SA node fires Potentials of Pacemaker Cells 1. Leakage channels allow the slow, constant influx of Na+ (pacemaker potential) 2. Slow gain of Na+ enables membrane potential to reach threshold (-40mV) 3. At threshold potential, voltage-gated Ca2+ channels open, allowing a fast Ca2+ influx, resulting in depolarization 4. As Ca3+ channels close, K+ channels open, resulting in the efflux of K+ 5. K+ efflux results in repolarization, returning the SA node cells to resting membrane potential (-60mV) Potentials of Cardiocytes -Impulse generated by the SA node causes both atria to fire simultaneously -Cardiocyte: -Only contract when stimulated- they do not exhibit pacemaker potentials seen in the SA node -Resting membrane potential is -90mV -Stimulus opens Na+ voltage-gated channels in the cardiocytes, which results in depolarization of the cardiocytes -At threshold, more Na+ channels open, resulting in positive feedback of signal propagation -Plateau Phase -Slow Ca2+ channels are opened by depolarization -Prolong depolarization for 200-250ms, which prolongs time of contraction -Prevents a brief “muscle twitch” to ensure that ventricles contract fully to expel blood -Long Absolute Refractory Period -About 250ms -Longer than in skeletal muscle -Prevents wave summation and tetanus in the heart, which would stop the pumping action of the heart Electrical Events of the Heart -Electrocardiogram (ECG or EKG) -All action potentials generated by both the SA/AV nodes, and the contractile cells at a given time -Recorded by attaching leads to various places on the body -Tracings- series of waves that indicate abnormalities with the heart’s electrical activities -The letters represent the electrical activity that proceeds the mechanical activity -Electrical Events -Described as depolarization or repolarization -Electrical leads to mechanical -Mechanical Events -Systole- contraction -Diastole- relaxation P wave- depolarization of the atria QRS wave- depolarization of the ventricles; repolarization of the atria T Wave- repolarization of the ventricles RST (RT)= systole TPQ (TQ)= diastole RR= whole time it takes to get through one cardiac event PR= onset of atrial depolarization to the onset of ventricular depolarization QT= onset of ventricular depolarization to end of ventricular repolarization (refractory period of the ventricles) T-P segment- end of one cardiac event, beginning of a second Cardiac Arrhythmias -Arrhythmia- any deviation from the sinus rhythm -Ventricular Fibrillation- hallmark of a heart attack -Random ventricular depolarization that repeatedly re-stimulates ventricular regions without patterned diastole -Atrial Fibrillation- high frequency depolarization that fails to stimulate the ventricle -Common in elderly -Less fatal -Defibrillation- electric shock used to “reset” the SA node -Heart Block- failure for any part of the conduction system to transmit signals, usually due to disease or degeneration -Named for the region of the conduction system that fails -Total heart block follows damage to AV node -Ectopic Focus- any region of spontaneous firing other than the SA node -Extrasystole- Premature ventricular contraction (PVC) -Type of ectopic focus -When other parts of the conduction system fire before the SA node -Can be stimulated by caffeine, nicotine, stress, or electrolyte imbalance Mechanical Events of the Heart The Cardiac Cycle -All events associated with blood flow through the heart during one complete heart beat; follow electrical activity 1. Ventricular Filling- mid to late (atrial) diastole -AV Valves open, SL valves close -80% blood enter ventricles passively, remaining 20% enters as atria contract; all from superior and inferior vena cava, and coronary sinus -End Diastolic Volume (EDV) -Maximum amount of blood the ventricles will contain following atrial contraction -At the end, the atria relax and the ventricles begin to depolarize 2. Isovolumetric Contraction Phase -Ventricles begin to contract and increased blood pressure forces AV valves open -Short period in which both AV and SL valves are close and blood is “trapped” in each ventricle 3. Ventricular Ejection Phase -Increased pressure inside the ventricles overcomes the pressure on the opposing side of the SL valves, and the two SL valves open -Pulmonary trunk- goes from diastolic to systolic -Aorta- goes from diastolic to systolic -Same volume of blood ejected on both sides of the heart -Atria are in diastole and are filling with blood 4. Isovolumetric Relaxation -Early ventricular diastole -SL valves close due to change in pressure system and the AV valves have not opened yet -Dicrotic Notch- indicates closure of the aortic SL valves -End Systolic Volume (ESV) -Small amount of blood remaining in ventricle -Ventricular filling will begin again, once atrial pressure exceeds ventricular pressure, and the AV valves open Heart Sounds -Lub- AV valves close, mitral valves close; louder, longer sound -Dup- SL valves close, aortic valve closes; short, sharp sound -Heart murmur- abnormal heart sounds as blood flows through the heart -Incompetent Valve- “leaky”; same blood re-pumped multiple times -Stenosis- valves are stiff and do not open completely; heart contracts more forcefully
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