The Cardiovascular System The Heart-2 PDF

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Lisa Nelson...

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The Heart The Pulmonary and Systemic Circuits ● Heart is a transport system consisting of two side by side pumps ● Right side receives oxygen-poor blood from tissues ○ Pulmonary circuit ● Left side receives oxygenated blood from lungs ○ Systemic circuit ● Receiving chambers of heart ● Right atrium ○ Receives blood from returning from systemic circuit ● Left atrium ○ Receives blood returning from pulmonary circuit ○ Pumping chambers of heart ● Right ventricle ○ Pumps blood through pulmonary circuit ● Left ventricle ○ Pumps blood through systemic circuit Size, Location, and Orientation of Heart ● Approximately the size of a fist ● Weighs less than 1 pound ● Locations ○ In mediastinum ○ On superior surface of diaphragm ● Base (posterior surface) leans toward right shoulder ● Apex points toward left hip ● Apical impulse Coverings of the Heart ● Pericardium 1. Superficial fibrous pericardium 2. Deep two-layered serous pericardium a. Parietal layer b. Visceral layer (epicardium) c. Two layers separated by fluid-filled pericardial cavity (decreases friction) Homeostatic Imbalance ● Pericarditis ○ Inflammation of pericardium ○ Roughens membrane surfaces, causing pericardial friction rub (creaking sound) heard with stethoscope ● Cardiac tamponade ○ Excess fluid that leaks into pericardial space ○ Can compress heart’s pumping ability ○ Treatment: fluid is drawn out of cavity (usually with syringe) Layers of the Heart Wall ● Three layers of heart wall

1. Epicardium 2. Myocardium: circular or spiral bundles of contractile cardiac muscle cells a. Cardiac skeleton: crisscrossing, interlacing layer of connective tissue 3. Endocardium: innermost layer, is continuous with endothelial lining of blood vessels a. Lines heart chambers and covers cardiac skeleton of valves Chambers and Associated Great Vessels ● Internal features ● Four chambers ○ Two superior atria ○ Two inferior ventricles ● Interatrial septum: separates atria ○ Fossa ovalis: remnant of foramen ovale of fetal heart ● Interventricular septum: separates ventricles ● Surface features ● Coronary sulcus (atrioventricular groove) ○ Encircles junction of atria and ventricles ● Anterior interventricular sulcus ○ Anterior position of interventricular septum ● Posterior interventricular sulcus ○ Landmark on posteroinferior surface Atria: the receiving chambers ● Small, thin-walled chambers ● Auricles: appendages that increase atrial volume ● Right atrium: receives deoxygenated blood from body ○ Posterior portion ○ Anterior portion ○ Posterior and anterior regions are separated by crista terminalis ● Three veins empty into right atrium: ○ Superior vena cava: returns blood from body regions above the diaphragm ○ Inferior vena cava: returns blood from body regions below the diaphragm ○ Coronary sinus: returns blood from coronary veins ● Left atrium: receives oxygenated blood from lungs ○ Four pulmonary veins return blood from lungs Ventricles: the discharging chambers ● Make up most of the volume of heart ● Right ventricle: most of anterior surface ● Left ventricle: posteroinferior surface ● Trabeculae carneae: irregular ridges of muscle on ventricular walls ● Papillary muscles: project into ventricular cavity ● Anchor chordae tendineae that are attached to heart valves ● Thicker walls that atria ● Actual pumps of heart ○ Right ventricle ○ Left ventricle

Heart Valves ● Ensure unidirectional blood flow through heart ● Open and close in response to pressure changes ● Two major types of valves ○ Atrioventricular ○ Semilunar valves ● No valves are found between major veins and atria; not a problem because ○ Inertia of incoming blood prevents backflow ○ Heart contractions compress venous openings Atrioventricular (AV) Valves ● Two atrioventricular (AV) valves prevent backflow into atria when ventricles contract ○ Tricuspid valve (right AV valve) ○ Mitral valve (left AV valve), bicuspid valve ○ Chordae tendineae: anchor cusps of AV valves to papillary muscles that function to hold valve flaps in closed position, prevent flaps from everting back into atria Semilunar (SL) valves ● Two semilunar (SL) valves prevent backflow from major arteries back into ventricles ● Open and close in response to pressure changes ● Each valve consists of three cusps that roughly resemble a half moon ● Pulmonary semilunar valve ● Aortic semilunar valve Pathway of Blood Through Heart ● Right side of heart ○ Superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus ○ Right atrium ○ Tricuspid valve ○ Right ventricle ○ Pulmonary semilunar valve ○ Pulmonary trunk ○ Pulmonary arteries ○ Lungs ● Left side of heart ○ Four pulmonary veins ○ Left atrium ○ Mitral valve ○ Left ventricle ○ Aortic semilunar valve ○ Aorta ○ Systemic circulation ● Equal volumes of blood are pumped to pulmonary and systemic circuits ● Pulmonary circuit is short, low-pressure circulation ● Systemic circuit is long, high friction circulation ● Anatomy of ventricles reflects differences ● Left ventricle walls are 3x thicker than right

● Pumps with greater pressure Coronary Circulation ● Functional blood supply to heart muscle itself ● Shortest circulation in body ● Delivered when heart is relaxed ● Left ventricle receives most of coronary blood supply Coronary arteries ● Both left and right coronary arteries arise from base of aorta and supply arterial blood to heart ● Both encircle heart in coronary sulcus ● Arteries contain many anastomoses (junctions) ● Provide additional routes for blood delivery ● Cannot compensate for coronary artery occlusion ● Heart receives 1/20 of body’s blood supply Cardiac Muscle Fibers ● Microscopic Anatomy ● Cardiac muscle cells: striated, short, branched, fat, interconnected ○ One central nucleus, at most 2 nuclei ● Intercalated discs are connecting junctions between cardiac cells that contain… ○ Desmosomes: hold cells together, prevent cells from separating during contraction ○ Gap junctions: allow ions to pass from cell to cell; electrically couple adjacent cells ● Allows heart to be a functional syncytium, a single coordinated unit ● Contain numerous large mitochondria (25-35% of cell volume) that afford resistance to fatigue ● Rest of volume composed of sarcomeres ○ Z discs, A bands, and I bands all present ● T tubules are wider, but less numerous ○ Enter cell only once at Z disc ● SR simpler than in skeletal muscle; no triads Differences in Physiology of Skeletal and Cardiac Muscle ● Similarities with skeletal ○ Muscle contraction is preceded by depolarizing action potential ○ Depolarization wave travels down T tubules; causes sarcoplasmic reticulum (SR) to release Ca2+ ○ Excitation-contraction coupling occurs ■ Ca2+ binds troponin causing filaments to slide ● Differences ○ Some cardiac muscle cells are self-excitable ○ Two kinds of myocytes ■ Contractile cells: responsible for contraction ■ Pacemaker cells: noncontractile cells that spontaneously depolarize ○ Initiate depolarization of entire heart

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Do not need nervous system stimulation, in contrast to skeletal muscle fibers Heart contracts as a unit ■ All cardiomyocytes contract as unit (functional syncytium), or none contract ○ Influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR ■ Depolarization opens slow Ca2+ channels in sarcolemma, allowing Ca2+ to enter cell ■ Extracellular Ca2+ then causes SR to release its intracellular Ca2+ ■ Skeletal muscles do not use extracellular Ca2+ ○ Tetanic contractions cannot occur in cardiac muscles ■ Cardiac muscle fibers have longer absolute refractory period than skeletal muscle fibers ● Absolute refractory period is almost as long as contraction itself ● Prevents tetanic contractions ● Allows heart to relax and fill as needed to be an efficient pump ○ The heart relies almost exclusively on aerobic respiration ■ Cardiac muscle has more mitochondria than skeletal muscle so has greater dependance on oxygen ● Cannot function without oxygen ■ Skeletal muscle can go through fermentation when oxygen not present ■ Both types of tissues can use other fuel sources ● Cardiac is more adaptable to other fuels, including lactic acid, but must have oxygen Electrical Events of the Heart ● Heart depolarizes and contracts without nervous system stimulation ● Setting the Basic Rhythm: The Intrinsic Conduction System ● Coordinated heartbeat in a function of: 1. Presence of gap junctions 2. Intrinsic cardiac conduction system a. Network of noncontractile (autorhythmic) cells b. Initiate and distribute impulses to coordinate depolarization and contraction of heart ● Action potential initiation by pacemaker cells ○ Cardiac pacemaker cells have unstable resting membrane potentials cells pacemaker potentials or prepotentials ○ Three parts of action potential 1. Pacemaker potential: K+ channels are closed, but slow Na+ channels are open, causing interior to become more positive 2. Depolarization: Ca+ channels open (around -40mV), allowing huge influx of Ca2+, leading to rising phase of action potential 3. Repolarization: K+ channels open, allowing efflux of K+, and cell becomes more negative ● Sequence of Excitation

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Cardiac pacemaker cells pass impulses, in following order, across heart in ~0.22 seconds 1. Sinoatrial node a. Pacemaker of heart in right atrial wall b. Depolarizes faster than rest of myocardium c. Generates impulses about 75x/minute (sinus rhythm) d. Inherent rate of 100x/minute tempered by extrinsic factors e. Impulse spreads across atria, and to AV node 2. Atrioventricular node a. In inferior interatrial septum b. Delays impulses approximately 0.1 second c. Because fibers are smaller in diameter, have fewer gap junctions d. Allows atrial contraction prior to ventricular contraction e. Inherent rate of 50x/minute in absence of SA node input 3. Atrioventricular bundle a. In superior interventricular septum b. Only electrical connection between atria and ventricles c. Atria and ventricles not connected via gap junctions 4. Right and left bundle branches a. Two pathways in interventricular septum b. Carry impulses toward apex of heart 5. Subendocardial conducting network (Purkinje fibers) a. Also referred to as Purkinje fibers b. Complete pathway through interventricular septum into apex and ventricular walls c. More elaborate on left side of heart d. AV bundle and subendocardial conducting network depolarize 30x/min in absence of AV node input e. Ventricular contraction immediately follows from apex toward atria f. Process from initiation at SA node to complete contraction takes ~0.22 seconds Modifying the Basic Rhthym: Extrinsic Innervation of the Heart ● Heartbeat modified by ANS via cardiac centers in medulla oblongata ● Cardioacceleratory center: sends signals through sympathetic trunk to increase both rate and force ● Stimulates SA and AV nodes, heart muscle, and coronary arteries ● Cardioinhibitory center: parasympathetic signals via vagus nerve to decrease rate ● Inhibits SA and AV nodes via vagus nerves Action Potentials of Contractile Cardiac Muscle Cells ● Contractile muscle fibers make up bulk of heart and are responsible for pumping action ● Different from skeletal muscle contraction; cardiac muscle action potentials have plateau ● Steps involved in AP 1. Depolarization opens fast voltage-gated Na+ channels: Na+ enters cell a. Positive feedback influx of Na+ causes rising phase of AP (from -90mV to +30mV)

2. Depolarization by Na+ also opens slow Ca2+ channels a. At 士30mV, Na+ close, but slow Ca2+ channels remain open, prolonging depolarization, seen as plateau 3. After about 200 ms, slow Ca2+ channels are closed, and voltage-gated K+ channels are open a. Rapid efflux of K+ repolarizes cell to RMP b. Ca2+ is pumped both back into SR and out of cell into extracellular space Difference between contractile muscle fiber and skeletal muscle fiber contractions ● AP in skeletal muscle lasts 1-2ms; in cardiac muscle it lasts 200 ms ● Contraction in skeletal muscle lasts 15-100ms; in cardiac contraction lasts over 200ms ● Benefit of longer AP and contraction ○ Sustained contraction ensures efficient ejection of blood ○ Longer refractory period prevents tetanic contractions Electrocardiography ● Electrocardiogram can detect electrical currents generated by heart ● Electrocardiogram (ECG or EKG) is a graphic recording of electrical activity ○ Composite of all action potentials at given time; not a tracing of a single AP ○ Electrodes are placed at various points on body to measure voltage differences ● 12 lead ECG is most typical ● Main features: P wave, QRS complex, T wave, P-R interval, S-T segment, Q-T interval Mechanical Events of Heart ● Systole ● Diastole ● Cardiac cycle ○ Atrial systole and diastole are followed by ventricular systole and diastole ○ Cycle represents series of pressure and blood volume changes ○ Mechanical events follow electrical events seen on ECG ● Three phases of the cardiac cycle 1. Ventricular filling: mid to late diastole a. Pressure is low b. Atrial depolarization triggers atrial systole (P wave) c. End diastolic volume (EDV) d. Depolarization spreads to ventricles (QRS wave) e. Atria finish contracting and return to diastole while ventricles begin systole 2. Ventricular Systole a. Atria relax; ventricles begin to contract b. Rising ventricular pressure causes closing of AV valves c. Two phase i. Isovolumetric contraction phase ii. Ejection phase, pressure in aorta around 120mm Hg 3. Isovolumetric relaxation: early diastole a. Following ventricular repolarization (T wave), ventricles are relaxed; atria are relaxed and filling

b. End systolic volume (ESV): volume of blood remaining in each ventricle after systole c. Backflow of blood in aorta and pulmonary trunk closes SL valves i. Causes dicrotic notch (brief rise in aortic pressure as blood rebounds off closed valve) ii. Ventricles are totally closed chambers (isovolumetric) d. When atrial pressure exceeds ventricular pressure, AV valves open; cycle begins again Heart Sounds ● Two sounds (lub-dup) associated with closing of heart valves ● First is closing of AV valves at beginning of ventricular systole ● Second is closing of of SL valves at beginning of ventricular diastole ● Pause between them indicates heart relaxation ● Mitral valve closes slightly before tricuspid, and aortic closes slightly before pulmonary valve ○ Differences allow auscultation of each valve when stethoscope is placed in four different regions Cardiac Output ● Volume of blood pumped by each ventricle in 1 minute ● CO = heart rate (HR) x stroke volume (SV) ○ HR: number of beats per minute ○ SV: volume of blood pumped out by one ventricle with each beat ● Normal: 5.25L/min ● Regulation of Pumping ○ Correlates with force of contraction ○ At rest: CO (ml.min)= HR (75beats/min) x SV(70ml/beat) = 5.25L/mon ● Cardiac Reserve ○ Maximal CO is 4-5 times resting CO in nonathletic people (20-25L/min) ○ Maximal CO may reach 35L/min in trained athletes ○ Cardiac reserve: difference between resting and maximal CO ○ CO changes (increases/decreases) if either or both SV or HR is changed ○ CO is affected by factors leading to ■ Regulation of stroke volume ■ Regulation of heart rates ● Regulation of Stroke Volume ○ Mathematically: SV = EDV - ESV ○ EDV is affected by length of ventricular diastole and venous pressure (~120mL/beat) ○ ESV is affected by arterial BP and force of ventricular contraction (~50mL/beat) ○ Normal SV- 120mL/beat-50mL/beat= 70mL/beat ○ Three main factors that affect SV: ■ Preload ■ Contractility ■ Afterload

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Autonomic nervous system regulation of heart rate ○ Sympathetic nervous system can be activated by emotional or physical stressors ○ Norepinephrine is released and binds to beta1-adrenergic receptors on heart, causing… ■ Pacemaker to fire more rapidly, increasing HR, EDV decreased because of decreased fill time ■ Increased contractility: ESV decreased because of increased volume of ejected blood, because both EDV and ESV decrease, SV can remain unchanged ● Parasympathetic nervous system opposes sympathetic effects ○ Acetylcholine hyperpolarizes pacemaker cells by opening K+ channels, which slows HR ○ Has little to no effect on contractility ● Heart at rest exhibits vagal tone ○ Parasympathetic is dominant influence on heart rate ○ Cutting vagal nerve leads to increase rate about 25 beats/min ○ Which reflects inherent rate of pacemaking SA node (HR of ~100b/m) ○ When sympathetic is activated, parasympathetic is inhibited, and vice versa ○ Atrial (Bainbridge) reflex: sympathetic reflex initiated by increased venous return, hence increase atrial filling ○ Atrial walls are stretched with increased volume ○ Stimulates SA node, which increases HR ○ Also stimulates atrial stretch receptors that activate sympathetic reflexes Chemical Regulation of Heart Rate ● Hormones ● Epinephrine from adrenal medulla increases heart rate and contractility ● Thyroxine increases heart rate; enhances effects of norepinephrine and epinephrine ● Ions ● Intra and extracellular ion concentrations must be maintained for normal heart function ● Imbalances are very dangerous to heart Other factors that influence heart Rate ● Age ○ Fetus has faster HR, declines with age ● Gender ○ Females have faster HR than males ● Exercise ○ Increases HR, trained athletes can have slow HR ● Body temperature ○ HR increases with increased body temperature...