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Chapter 8 – Notes – Circulatory System 1.docx

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Department
Physiology
Course Code
Physiology 2130
Professor
Anita Woods

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Chapter 8 – Notes – Circulatory System 1: The Heart Objectives: - 4 functions of cardiovascular system - Types of myocardial cells and functions - Origin of self-excitability o Two characteristics of sinoatrial node  self-excitability - Action Potential in sinoatrial system (events) - Structure of heart and electrical conducting system - ECG (electrocardiogram) o Explain peaks o Tell you about health of heart - Cardiac cycle o Changes in aortic and ventrical pressure, volume o Relate these changes to activity in the valves - Calculate cardiac output and all factors controlling heart rate - Factors controlling stroke volume - Frank-Starling Law of the heart o How it returns cardiac output to normal Introduction Heart - Between lungs - Beat 2.5 billion times in life Functions of Cardio System: - Transport O2, nutrients - Transport CO2, waste - Regulate body temp, pH - Transport hormones, other substances Anatomy – The Heart - 2 side by side pumps o Right Atrium/ventricle – pump blood to lungs o Left Atrium/ventricle – pump blood to rest of body - Wall of LEFT ventricle thicker than wall of RIGHT ventricle Left Ventricle - Contracts more forcefully to pump to whole body Right Ventricle - Contracts with less force to deliver blood to lungs Heart Valves - Ensure 1 way flow of blood through heart Right Atrioventricular valve (AV) - Called tricuspid valve Left Atrioventricular valve (AV) - Bicuspid - Or mitral valve Circulation Through Heart - ENTER heart through right atrium - Pass through right atrioventricular valve - Into right ventricle - Right ventricle contracts - Blood pass through pulmonary valve - Into pulmonary artery - Into Lungs (pick up O2, drop CO2) - Through pulmonary vein - Into left Atrium - Through left atrioventricular valve - Into left ventricle (contracts) - Blood through aortic valve - Into Aorta - To rest of body Myocardial Cells - 2 myocardial cell types in the heart o Contractile cells (similar to skeletal) o Nodal/conducting cells (similar to nerve cells) Contractile Cells - Form walls of atria and ventricles - They contract similarly to muscle cells - Contain actin and myosin arranged in microfibrils - The microfibrils are surrounded by sarcoplasmic reticulum - DIFFERENCE from skeletal muscle: One nucleus but far more mitochondria - Efficient at extracting oxygen (80% of oxygen in passing blood) - Cells are shorter, branched and joined by special structures called intercalated discs - These discs contain “tight junctions” binding cells together - “Gap junctions” allow movement of ions and ion currents between myocardial cells - Can conduct action potentials from cell to cell WITHOUT need for nerves Myocardial Celle – Nodal/Conducting Cells - Contract weakly because the contain very few contractile elements - ABLE to spontaneously generate AP’s without help of nervous input (neurons) o Self-excitatory system for heart - Can rapidly conduct AP’s to atrial and ventricular muscle o Transmission system for rapid conduction of impulses throughout heart Origin of Self-Excitability - Sinoatrial node is general site of origin o Located in upper posterior wall of Right Atrium o First area that spontaneously depolarizes in AP o Also called “pacemaker of heart” - AP travels through atria to AV node and then to Bundle of His - Then through Purkinje Fibers then to ventricular muscle Myocardial Cells contd. - The same ions are responsible for AP’s in heart - But heart AP begins by itself - Important ions: Na, K, Ca o Ca important for heart beat SA Node Action Potential - Cause of spontaneous AP is controversial, but some factors: o Na move down concentration gradient o Na permeability here is higher than in other cells o Inside more POSITIVE (depolarized over time) o Ca also trying to move into cell (depolarizing) o Na and Ca moving in produce INITIAL depolarization, not yet AP - BUT, K movement is the main cause of the AP (leaving inside of cell) o If K moves out, inside gets more negative but for depolarization you want more positive inside o So K permeability decreases over time - Na/K pump is pumping K into cell, so both factors will cause these cells to depolarize Together: - Na and Ca moving in, K building up inside - Membrane potential repolarizes from -60 mV to -40 (threshold) - SA nodal cells don’t have resting membrane potential - This slow depolarization is spontaneous called pacemaker potential - Sets pace of heartbeat (changing this depolarization changes heart beat) Next step: - Membrane potential has depolarized to threshold (-40 mV) - Voltage gated Ca channels open, Ca rapidly flows in - Depolarization phase of SA node AP - Ca channels begin closing, K channel begin opening - Repolarization phase, allowing K out - Cell reaches original -60 mV and then cycle repeats (depolarizing) for another heart beat Myocardial Cells – Conducting System of the Heart - AP generated at the SA node - Travels through heart in coordinated manner - SA node  atrial muscle (contract)  atria  ventricles by a fibrous tissue - BUT the atria are isolated from the ventricles, so they must take another path to get there: o Through AV node  each branch of bundle of His  apex of heart  Purkinje fibers  distribute the AP to the ventricular muscle which then contracts - Well coordinated contraction necessary for heart to function - Conduction speed of the AP will vary as it moves through the heart o SA node has one of slowest conduction speeds o AP speeds up through atrial muscle (contracts simultaneously) o The contraction moves from top of heart to bottom o Blood forced down into ventricles o AV node slows the conduction speed in order to ensure that the atria has finished contracting before the ventricles contract o The AP must reach the base of the heart (conducted through the bundle of His) o Bundle of His conducts AP at FAST rate o Important for AP o reach apex of heart to contract first so blood can be forced up and through the valves at the top of the ventricles o After this the Purkinje fibers spread the AP through the ventricular muscle o Contracts from the apex upward Electrocardiogram (ECG) - Body fluids are good conductors of electricity - Heart sits in middle of conducting fluid - AP passes through heart and to different parts of the body (through fluids) and to the surface - Electrodes can be placed on skin around heart, electric potentials generated by heart are recorded - Recording during the cardiac cycle is called electrocardiogram P wave – electrical activity in heart associated with the depolarization of the atrial muscle leading to their contraction QRS complex – produced by depolarization of the ventricular muscle just prior to its contraction T wave – result of the repolarization of the ventricular muscle as it relaxes - No wave associated with the repolarization of the atrial muscle - This event which does occur is obscured by the much larger QRS complex The Cardiac Cycle - All mechanical, electrical and valvular events taking place in the heart during a single contraction - Two primary phases: o Systole o Diastole - Pay attention to: o ECG o Pressure changes o Volume in ventricle o Activity of the valves 5 Steps of Cardiac Cycle 1. Atrial Systole – First phase - Depolarization of the atria (P wave in the ECG) - Atria contract - Atrial pressure greater than ventricular pressure - AV (mitral) valve opens and blood flows into the ventricle - Ventricular volume increases slightly 2. Isovolum
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