Transcript Chapter 20
Chapter 20: The Cardiovascular System THE HEART Heart Anatomy Location Orientation diaphragm, mediastinum, 2/3 left of midline Apex- points anterior, inferior, left Base- directed posterior, superior, right Vessels Superior and Inferior Vena Cava Pulmonary trunk pulmonary arteries(lungs) Pulmonary veins Aorta Pericardium- figure 20.2 Membrane that surrounds & protects Confines to position in mediastinum 2 main parts: Fibrous pericardium- superficial, anchor Tough, inelastic, dense irregular CT Baglike, open end attached to vessels Prevents overstretching of heart Serous pericardium- thinner, delicate Forms double layer (pericardial fluid in pericardial cavity reduces friction, allows movement): Parietal layer- fused to fibrous Visceral layer- inner = EPICARDIUM- adheres tightly to heart surface Layers of the heart wall Epicardium- thin, transparent, outer Myocardium- middle Visceral layer of serous pericardium Smooth slippery outside of heart Cardiac muscle- striated but involuntary Bulk of heart Pumping action Endocardium- inner Thin endothelium over CT Smooth lining of chambers and valves Continuous with b.v. Heart Anatomy fig 20.3-6 Heart chambers = 4 2 Atria Right- receives blood from vena cavae Left- receives blood from pulmonary veins 2 Ventricles Right- pumps deoxygenated blood to lungs Left- pumps oxygenated blood to systemic circ Myocardium much thicker than right ventricle Heart valves = 4 Atrioventricular valves = tricuspid & bicuspid Semilunar valves = aortic and pulmonary Valve function When AV valve open: Ventricle contracts, pressure cusps up, close Cusps project into ventricle Ventricle relaxed papillary muscle relaxed chordae tendineae slack Blood: pressure atria pressure ventricle Papillary muscles contract chordae tendineae tighten SL valves open when pressure in ventricles exceeds pressure in arteries As ventricles relax blood moves back toward heart SL valves close Terms Auricles – on anterior surface of atria Increases capacity of each atrium so each can hold a greater volume of blood Coronary sulcus – separation between atria and ventricles Systole – contraction Diastole – relaxation Tachycardia – high heart rate, > 100bpm Bradycardia – low heart rate, 50 bpm Pulmonary and systemic circuits Coronary circulation (1) Coronary circulation (2) Coronary – “crown,” encircles heart contracts, little blood flows coronary artery but as relaxes, aorta pushes blood thru coronary arteries Anastomoses – area where 2 or more arteries supply the same region Provide alternate routes for blood to reach a particular organ or tissue Myocardium contains Provides detours if main route is obstructed Problems… Myocardial ischemia – partial obstruction of blood flow in coronary arteries blood flow to myocardium hypoxia may weaken cells w/out killing them Silent = episodes without pain, dangerous in that no forewarning to attack Angina pectoris – “strangled chest” Severe pain usually accompanies myocardial ischemia Tightness or squeezing sensation Can occur during exertion when requires more O2 Pain referred to neck, chin, left arm Myocardial infarction (MI) Heart attack Complete obstruction of blood flow to coronary artery Infarction = death of tissue area due to interrupted blood supply Tissue distal to obstruction dies, replaced by noncontractile scar tissue loses strength May also disrupt conduction system and cause sudden death – ventricular fibrillation – rapid uncoordinated twitching that disrupts regular rhythm treatment: injection of clot dissolver, plus heparin, coronary angioplasty or coronary artery bypass Properties of cardiac muscle cells Shorter than skeletal Branching Central nucleus, sometimes binucleate Intercalated discs- thickenings of sarcolemma, contain: Desmosomes- hold fibers together Gap junctions- for AP conduction Mitochondria large & numerous Like skeletal- arrangement of proteins SR smaller less intracellular Ca2+ T-tubules wider but less abundant Functional syncytium stimulation of individual muscle cell results in contraction of all muscle cells due to gap junctions in intercalated discs an application of the all-or-none principle If stimulus in cardiac muscle is great enough to initiate contraction of a single cell, the entire muscular syncytium will undergo contraction Contraction physiology 1% of cardiac fibers become autorhythmic during embryonic development Pacemaker function- set rhythm of electrical excitation Conduction system- network of specialized fibers provide path for excitation to progress thru heart Ensuring coordinated contraction of chambers Both atria contract at same time Both ventricles contract at same time Cardiac AP goes thru following sequence… Contraction physiology (2) Pathway of stimulation 1. Sinoatrial (SA) node- cells do not have a stable resting membrane potential depolarized spontaneously = pacemaker potential 2. Atrioventricular (AV) node 3. Bundle of His 4. Bundle branches 5. Purkinje fibers 6. Ventricular cells- contraction pushes blood up to SL valves Cardiac Action Potentials, 20.11 Depolarization: Na+ gates open= fast channels Rapid depolarization because they open fast Plateau: opening of slow Ca2+ channels in the sarcolemma More Ca2+ outside cell cytosol also causing Ca2+ to pour out of SR Ca2+ contraction K+ channels opening but Ca2+ balances it remains depolarized for about 0.25 sec (in skeletal muscle 0.001 sec, no plateau phase) Repolarization: K+ outflow restores resting m.p. Ca2+ channels also are closing Cardiac Action Potentials (2) Positive inotropic agents contractility (substances promote inflow of Ca2+ channels strength contractions NE and Epinephrine modify Timing strength of contraction Do NOT establish a rhythm Digitalis interstitial Ca2+ Negative inotropic agents contractility Ach released by Parasymp NS slows SA node pacing from 100 to about 75 AP/minute Also: anoxia, acidosis, some anesthetics, K+, Ca2+ channel blockers Long refractory pd- cardiac muscle Refractory pd- time interval during which second contraction cannot be triggered In cardiac- longer than contraction pd Another contraction cannot happen until relaxation is happening Tetanus cannot occur If tetanus occurred blood flow would cease Arrhythmias Irregular rhythm due to conduction defect Causes: Caffeine, nicotine, alcohol, other drugs, anxiety, hyperthyroidism, K+ deficiency, & some heart disease Examples: Heart block – AP slowed or blocked (3 types) 1st °= AP slow thru AV, 2nd °= some AP not thru AV node, 3rd ° = no AP thru AV node Atrial flutter – rapid atrial contractions Atrial fibrillation – asynchronous cont- atrial fibers Ventricular fibrillation– async cont ventricular fibers* Premature ventricular contraction – ectopic area of high excitation abnormal AP (before SA node intends) Cardiac excitation and the ECG Electrocardiogram (ECG) P wave – atrial depolarization atrial contraction ventricular filling QRS complex – ventricular depolarization ventricular contraction SL valves open blood ejection Rt ventriclepulmonary trunk pul arteries lungs Left ventricle aorta systemic circulation T wave – ventricular repolarization Heart sounds A. Normal First sound – lubb – closure of AV valves Second sound – dupp – closure of SL valves B. Abnormal sounds (murmurs) 1. stenosis – failure of valve to open 2. insufficiency – failure of valve to close The Cardiac Cycle Ventricular filling AV open, SL closed Isovolumetric contraction AV closed, SL closed Ventricular ejection AV closed, SL open Isovolumetric relaxation AV closed, SL closed Ventricular filling Regulation of Cardiac Output Cardiac output = stroke volume x heart rate CO = SV x HR Stroke volume = ml/ beat EDV - ESV Heart rate = beats/ min Cardiac output = L/ min rest = 5.25 L/min (70 mL/beat x 75 bpm) exercise = 19.5 L/min (130mL/beat x 150bpm) Regulation of stroke volume 1. Effect of preload = Frank-Starling Law of the Heart > preload > force of contraction 2. Effect of afterload rubberband Pressure rqrd for ejection of blood 3. Effect of contractility-each individual fiber Positive inotropic agents- eg. norepinephrine Negative inotropic agents - eg. propranolol Regulation of Heart Rate 1. Normal rate = vagal tone 2. Regulation 1. Autonomic Nervous system 2. Chemical a. Hormones b. Ions