Cardiovascular System I: Heart Function Lecture PDF

Cardiovascular System I: The Heart Cardiac Muscle  Cardiac muscle cells are only found in the heart wall Two specialized types of cardiac muscle cells 1) Contractile cells (99%) 2) Auto-rhythmic cells (1%) - both types connected by gap junctions  Function of Contracti...

Cardiovascular System I: The Heart Cardiac Muscle  Cardiac muscle cells are only found in the heart wall Two specialized types of cardiac muscle cells 1) Contractile cells (99%) 2) Auto-rhythmic cells (1%) - both types connected by gap junctions  Function of Contractile Cells 1) Rhythmic contraction and relaxation generates heart pumping action 2) Contraction pushes blood out of heart into vasculature 3) Relaxation allows heart to fill with blood Autorhythmic Cells 1) Initiate and propagate the action potentials responsible for contraction of the contractile cells 2) Do not contract- very few myofilaments 3) Two types of autorhythmic cells a) Pacemaker cells: initiate action potentials b) Conducting cells: propagate action potentials throughout heart  Pacemaker cells are located in two primary locations: 1. Sino-atrial (SA) node: collection of pacemaker cells in right atrium a) Initiate heartbeat (automatic) b) Primary pacemaker sets rate (heart rate) 2. Atrio-ventricular (AV) node: located between atria and ventricles  Pacemaker Cells 1) Produce pacemaker potentials = spontaneous depolarizations of membrane to threshold  results from spontaneous opening of ion channels 2) Does not require any neural or chemical signal = myogenic control 3) Depolarizations are transmitted to contractile cells via gap junctions causes them to contract Electrical Coupling of Cardiac Muscle Cells Figure 14.8  Heartbeat 1) Wave of contraction through cardiac muscle a) As contractile cells are depolarized to threshold, they contract 2) Atria contract as a unit 3) Ventricles contract as a unit  Spread of Excitation Between Cells 1) Atria contract first followed by ventricles a) Slight delay in propagation of action potentials through AV node into ventricles b) Why the delay? 2) Coordination due to presence of gap junctions and conduction pathways Conducting Cells  Rapidly conduct action potentials initiated by pacemaker cells throughout heart Three major pathways 1.Internodal pathways: between SA and AV nodes Ventricles 2.Atrioventricular Bundle (of His) 3.Purkinje fibers Anatomy of the Conduction System Figure 14.9  Spread of Excitation 1) Interatrial Pathway SA Node  right atrium  left atrium Rapid Simultaneous contraction right and left atria 2) Internodal Pathway SA Node  AV Node 3) AV Node Transmission Only pathway from atria to ventricles Slow conduction - AV Nodal Delay = 0.1 sec Atria contract before ventricles Spread of Excitation into Ventricles 1) Down Bundle of His 2) Up Purkinje Fibers a) Purkinje Fibers contact ventricle contractile cells b) Ventricle contracts from apex (tip) up  Electrocardiogram- ECG External measure of electrical activity of the heart 1) Non-invasive technique 2) Used to test for clinical abnormalities in conduction of electrical activity in the heart Standard ECG Trace 1) P wave = atrial depolariztion 2) QRS complex = ventricle depolarization 3) T wave = ventricle repolarization Figure 14.15b Ventricular Fibrillation Loss of coordination of electrical activity 1) Atrial fibrillation = weakness 2) Ventricular fibrillation = death within minutes Figure 14.16 Heart Anatomy – Four chambers 2 Atria 2 Ventricles – Septum Interatrial Interventricula r – Base – Apex Figure 14.1  Heart Valves  Heart chambers separated by one-way valve 1) Atrioventricular valves = AV valves a) Right AV valve (tricuspid valve) b) Left AV valve (bicuspid valve ) 2) Aortic Valve One-way flow into 3) Pulmonary Valvearteries leaving heart Valves and Unidirectional Blood Flow 1) Normal direction of blood flow a) Atria to ventricles b) Ventricles to arteries 2) Valves prevent backward flow of blood  Valves open passively due to pressure gradients a) AV valves open when: Pressure in atria > Pressure in ventricles b) Pulmonay and aortic valves open when P ventricles > P arteries  Cardiac Cycle Events associated with the flow of blood through the heart during a single complete heartbeat Two main periods of the cardiac cycle: 1. Systole  Ventricle contraction 2. Diastole  Ventricle relaxation Four Phases of Cardiac Cycle 1) Ventricular filling a) Pressure atria > Pressure ventricles b) AV valves open c) Passive phase - no atria or ventricular contraction (80% of filling) d) Active phase - atria contract (20% of filling) 2) Isovolumetric ventricular contraction a) Ventricles contracts - increases pressure b) AV and pulmonary and aortic valves closed c) No blood entering or exiting ventricle d) Ventricular pressure increases until it exceeds arterial pressure (aortic and pulmonary) Four Phases of Cardiac Cycle 3) Ventricular ejection a) Pressure ventricles > Pressure arteries b) Pulmonary and aortic valves open c) Blood ejected from ventricles 4) Isovolumetric Ventricular Relaxation a) Ventricle relaxes - decreases pressure b) AV and aortic and pulmonary valves closed c) No blood entering or exiting ventricle d) Pressure in ventricle continues dropping until it is less than atrial pressure Continuous Blood Flow During Cardiac Cycle 1) Aorta (and large arteries) – elastic a) Pressure reservoir 2) Store energy during systole as walls expand 3) Release energy during diastole as walls recoil inward 4) Maintains blood flow through entire cardiac cycle  Heart Sounds Sounds occur due to turbulent flow when valves close First sound = soft lubb Second sound = louder dubb AV valves close Aortic/pulmonary valves close Figure 14.21 The Cardiovascular System: Cardiac Function  Cardiac Output (CO) Volume of blood pumped by each ventricle per minute 1) Average CO = 5 liters/min at rest 2) Average blood volume = 5.5 liters 3) During heavy exercise: 25-30 liters/min  Cardiac Output is determine by two variables: 1. Heart rate (HR) = beats per minute 2. Stroke volume (SV) = volume of blood pumped by ventricles in one heart beat; typically about 70 ml at rest Therefore, CO = HR x SV - Can increase CO by increasing HR or SV or both  Both HR and SV are regulated by the cardiovascular system to control cardiac output  Regulation of Cardiac Output (CO) A. Heart Rate 1) HR is regulated by autonomic nervous system 2) Modifies rate set by pacemaker cells in nodes 3) Pacemaker cells receive input from both sympathetic and parasympathetic nervous systems Have both 1 adrenergic receptors and muscarinic, Ach receptors Autonomic Input to the Heart Figure 14.22  Heart Rate (HR) - Determined by SA Node Firing Rate 1. Sympathetic input: increases firing rate = increase in HR (via 1 adrenergic receptors) 2. Parasympathetic input: decreases firing rate = decrease in HR (via muscarinic, Ach receptors)  At rest, parasympathetic dominates: HR = 60-70 beats/min  Hormonal Control of Heart Rate Epinephrine - same effect as sympathetic nervous system -- large amounts of epinephrine released from adrenal medulla as part of fight or flight response increase in HR  AV Nodal Innervation Autonomic nervous system also innervates AV nodal cells 1)Sympathetic a) increases conduction velocity through node (speeds up conduction into ventricles) 2)Parasympathetic a) decreases conduction velocity through node