The Heart Chapter 19 & 20

Transcription

1 The Heart Chapter 19 & 20 Circulatory System heart, blood vessels and blood Cardiovascular System heart, arteries, veins and capillaries 2 major divisions Pulmonary circuit - right side of heart carries blood to lungs for gas exchange Systemic circuit - left side of heart supplies blood to all organs of the body Heart Anatomy Cardiac Cycle Myocardial Physiology Cardiac Conduction System and Innervation Coronary Circulation Blood Pressure

2 Cardiovascular System Circuit Cardiovascular System consists of: heart, arteries, veins and capillaries 2 major divisions Pulmonary Circuit - right side of heart carries blood to lungs for gas exchange Systemic Circuit - left side of heart supplies blood to all organs of the body

3 Heart Shape and Position Heart is located in the mediastinum, between lungs Base of the heart is the broad superior portion of heart Apex of the heart is the inferior end, tilts to the left, tapers to point

4 Heart Position

5 Pericardium Pericardium consists of the membranes around the heart that allows the heart to beat with minimal friction, provides room for the heart to expand but resists excessive expansion. Pericardial Sac (also called the parietal pericardium) is the membrane that loosely surrounds the heart. Pericardial sac has 2 layers: Fibrous layer: outer, tough, fibrous layer of CT Serous layer: inner, thin, smooth, moist membrane Pericardial Cavity is the space between heart and pericardial sac that is filled with pericardial fluid that acts as a lubricant. Visceral Pericardium (also called the epicardium) is the thin, smooth, moist serous layer that covers heart surface.

6 Pericardium and Heart Wall Figure 19.3

7 3 Layers of the Heart Wall Epicardium (visceral pericardium) serous membrane covers heart Myocardium thick muscular layer fibrous skeleton - network of collagen fibers and elastic fibers provides structural support attachment for cardiac muscle nonconductor important in coordinating contractile activity Endocardium smooth inner lining equivalent to the lining of blood vessels

8 Figure layers of the Heart Wall

9 Bacterial Endocarditis Gross pathology of subacute bacterial endocarditis involving mitral valve. Left ventricle of heart has been opened to show mitral valve fibrin vegetations due to infection with Haemophilus parainfluenzae. Autopsy. CDC/Dr. Edwin P. Ewing, Jr.

10 Cardiac Cycle One complete contraction (systole) and relaxation (diastole) of all 4 chambers of the heart followed by a brief period of inactivity: Cardiac Cycle Animation h/0028-swf_the_cardiac_cy.swf

11 Heart Sounds Heart sounds as heard through a stethoscope are due to turbulent flow of blood as heart valves close. Auscultation - listening to sounds made by body. First heart sound (S1) is louder and longer, sounds like LUBB, occurs when the AV valves close. Second heart sound (S2), is softer and sharper, sounds like DUPP, occurs when the semilunar valves close. S3 rarely audible in people older than 30 known as triple rhythm or gallop.

12 Heart Sounds S1 and S2 sounds of normal rhythm can be simulated by drumming two fingers on a table. If there is a third heart sound (S3) it can be simulated by drumming three fingers on a table, and it is described as a gallop. The cause of each heart sound is not known with certainty, but they are correlated with specific phases of the cardiac cycle. sounds/normal_first_and_second_heart_sounds.jhtml

13 Phases of Cardiac Cycle 1) Quiescent period 2) Ventricular filling 3) Isovolumetric contraction 4) Ventricular ejection 5) Isovolumetric relaxation

14 1) Quiescent period all chambers relaxed AV valves open blood flowing into ventricles 2) Ventricular Filling SA node fires, atria depolarize P wave appears on ECG atria contract, force additional blood into ventricles ventricles now contain end-diastolic volume (EDV) of about 130 ml of blood

15 3) Isovolumetric Contraction of Ventricles Atria repolarize and relax Ventricles depolarize QRS complex appears in ECG Ventricles contract Rising pressure closes AV valves - heart sound S1 occurs No ejection of blood (no change in volume)

16 4) Ventricular Ejection Rising pressure opens semilunar valves Rapid ejection of blood Stroke volume: amount ejected about 70 ml at rest End-systolic volume: amount left in heart SV/EDV= ejection fraction, at rest ~ 54%, during vigorous exercise as high as 90%, diseased heart < 50% Both ventricles must eject same amount of blood

17 5) Isovolumetric Relaxation of Ventricles T wave appears in ECG Ventricles repolarize and relax (begin to expand and fill) Semilunar valves close (dicrotic notch of aortic pressure curve) - heart sound S2 occurs AV valves remain closed (not shown) Ventricles expand but do not fill so there is no change in volume (not shown)

18 Cardiac Output (CO) Amount ejected by each ventricle in 1 minute CO = Heart Rate (beats/min) x Stroke Volume (ml/beat) Resting values, CO = 75 beats/min x70 ml/beat = 5,250 ml/min, usually about 4 to 6 liters/min Vigorous exercise CO to 21 L/min for fit person and up to 35 L/min for world class athlete Cardiac reserve: difference between a person s maximum CO and resting CO with fitness, with disease Right Ventricle and Left Ventricle eject the same amount of blood.

19 Preload Preload is the amount of tension in ventricular myocardium before it contracts. preload causes contraction strength exercise venous return, stretches myocardium ( preload), myocytes generate more tension during contraction, CO matches venous return Frank-Starling law of heart: StrokeVolume End Diastolic Volume ventricles eject as much blood as they receive; The more they are stretched ( preload) the harder they contract.

20 Unbalanced Ventricular Output

21 Unbalanced Ventricular Output

22 Heart Rate Heart Rate (HR) can be measured from pulse. Infants have HR of 120 beats per minute or more Young adult females average bpm Young adult males average 64 to 72 bpm HR rises again in the elderly Indurance trained athletes have resting heart rates around 40 bpm Miguel Indurain, 5 time Tour de France winner ( ), had a resting heart rate of 28 bpm and a cardiac output of 50 liters/minute

23 Heart Rate Tachycardia: persistent, resting adult HR > 100 can be caused by stress, anxiety, drugs, heart disease or body temperature Bradycardia: persistent, resting adult HR < 60 common during sleep and also in endurance trained athletes

24 Sympathetic Nervous System Cardioacceleratory Center stimulates sympathetic nerves to SA node, AV node and myocardium these nerves secrete norepinephrine, which binds to -adrenergic receptors in the heart CO normally peaks at HR of 160 to 180 bpm Sympathetic n.s. can HR up to 230 bpm, (limited by refractory period of SA node), but SV and CO (less filling time)

25 Parasympathetic Nervous System Cardioinhibitory Center stimulates vagus nerves right vagus nerve - SA node left vagus nerve - AV node secrete ACh (acetylcholine), binds to muscarinic receptors opens K + channels in nodal cells, hyperpolarized, fire less frequently, HR slows down vagal tone: background firing rate holds HR to sinus rhythm of 70 to 80 bpm severed vagus nerves results in the SA node firing at its intrinsic rate of about 100bpm maximum vagal stimulation HR as low as 20 bpm

26 Structure of Cardiac Muscle Short, thick, branched cells with central nuclei. Sarcoplasmic reticulum is less developed than in skeletal muscle. cardiac cells must get more Ca 2+ from extracellular fluid during excitation T-tubules are invaginations of sarcolemma (muscle cell plasma membrane) that carry depolarization into the cells. Intercalated Discs, join cells end to end. interdigitating folds - surface area for strength and communication mechanical junctions tightly join cells adherent junctions and desmosomes electrical junctions gap junctions form channels that allow ions to flow directly from cell to cell to synchronize contractions

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28 Intercalated Disk

29 Cardiac Muscle Histology

30 Metabolism of Cardiac Muscle Fatigue resistant aerobic respiration Cells have abundant myoglobin most tissues extract about 25% of the oxygen from the blood as it circulates, but cardiac muscle extracts about 65% of the oxygen Abundant, large mitochondria Organic fuels: fatty acids, glucose, glycogen, ketones

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32 Cardiac Conduction System Cardiac Conduction System is Myogenic - heartbeat originates within heart Cardiac Conduction System is Autorhythmic heartbeats are regular and spontaneous

33 Cardiac Conduction System Cardiac Conduction System Components: Cardiac Conduction System is composed of Nodes and Bundles of specialized cardiac cells. Sinoatrial (SA) Node is the pacemaker that initiates heartbeat and sets heart rate. Atrioventricular (AV) node is an electrical relay to the ventricles AV bundle (bundle of His) is a pathway for signals from the AV node to muscles of ventricles. Right and left bundle branches are divisions of the AV bundle that enter the interventricular septum and descend to the apex. Purkinje Fibers project upward from apex and spread throughout ventricular myocardium. Moderator Band is a muscular bundle of heart tissue crossing the ventricular cavity from the interventricular septum to the base of the paillary muscles of the right ventricle. fibrous skeleton of the heart is connective tissue that insulates atria from ventricles.

34 Cardiac Conduction System

35 Impulse Conduction SA node signal travels at 1 m/sec across the atria AV node slows signal to 0.05 m/sec composed of thinner myocytes with fewer gap junctions delays signal 100 msec, allows ventricles to fill AV bundle and purkinje fibers speeds up signal to ventricles at 4 m/sec Papillary muscles get the signal first, and their contraction stabilizes the AV valves Ventricular systole begins at apex, progresses up spiral arrangement of myocytes twists ventricles slightly during contraction which increases the efficiency of blood ejection from the chambers

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37 Cardiac Rhythm Sinus Rhythm is set by the SA node adult sinus rhythm at rest is 70 to 80 bpm Nodal Rhythm is set by the AV node adult nodal rhythm is 40 to 50 bpm but it is normally overridden by the sinus rhythm Ectopic Foci are regions of spontaneous firing outside of the SA node ectopic foci can be caused by hypoxia, caffeine, nicotine, electrolyte imbalance, drugs Arrhythmia any abnormal cardiac rhythm heart block: failure of conduction system due to disease bundle branch block (caused by damage to bundle branches) total heart block (caused by damage to AV node)

38 Physiology of the SA Node SA node - no stable resting membrane potential Pacemaker potential gradual depolarization of SA cells from a slow inflow of Na + into the SA cells membrane potential rises from -60 mv to -40 mv Action potential occurs at threshold of -40 mv depolarizing phase from -40 mv to 0 mv Caused by opening of fast Ca +2 channels (lets Ca +2 in) repolarizing phase Caused by opening of K + channels (let K + out) at -60 mv K + channels close, pacemaker potential starts over Each depolarization creates one heartbeat SA node at rest fires every 0.8 sec or about 75 bpm at rest

39 Membrane Potentials of SA Node Cells

40 Contraction of Myocardium Myocytes have stable resting potential of -90 mv Depolarization (very brief) stimulus opens voltage regulated Na + channels and allows Na + to rush in which rapidly depolarizes the membrane action potential peaks at +30 mv Na + channels close again quickly Plateau of 200 to 250 msec, sustains contraction slow Ca +2 channels open, Ca +2 binds to Ca +2 channels on sarcoplasmic reticulum which releases Ca +2 into cytoplasm and results in muscle contraction Repolarization - Ca +2 channels close, K + channels open, rapid K + outflow returns cell to resting potential

41 1) Voltage gated Na + channels open 2) Rapid depolarization due to Na + inflow 3) Na + channels close 4) K + channels and slow Ca +2 channels open. K + flows out restoring membrane polarity, but Ca +2 flowing in prolongs membrane depolarization 5) Ca +2 channels close and Ca +2 is pumped out K + channels still open and re-polarizes as K + flows out Myocardial Contraction and Action Potential

42 Electrocardiogram (ECG) Composite of action potentials of nodal and myocardial cells Detected, amplified and recorded by electrodes on arms, legs and chest

43 ECG P wave SA node fires, atrial depolarization atrial systole QRS complex AV node fires, ventricular depolarization ventricular systole (atrial repolarization and diastole - signal obscured) T wave ventricular repolarization

44 ECG and the Cardiac Cycle 1) atrial depolarization begins 2) atrial depolarization complete (atria contracted) 3) ventricles begin to depolarize at apex; atria repolarize (atria relaxed) 4) ventricular depolarization complete (ventricles contracted) 5) ventricles begin to repolarize at apex 6) ventricular repolarization complete (ventricles relaxed)

45 Diagnostic Value of ECG Valuable for diagnosing abnormalities in conduction pathways, myocardial infarction (MI), heart enlargement, electrolyte imbalances and hormone imbalances.

46 Myocardial Infarction Sudden death of heart tissue caused by interruption of blood flow from coronary artery narrowing or occlusion. Dead myocardium is replaced with thin, noncontractile connective tissue. Anastomoses defend against interruption by providing alternate blood pathways. circumflex artery and right coronary artery join and form the posterior interventricular artery. anterior and posterior interventricular arteries join at apex of heart.

47 Coronary Blood Vessels

48 Coronary Vessel Atherosclerosis Plaque Artery Wall

49 Major Events of Cardiac Cycle

50 Chapter 20 Principles of Blood Flow Blood Flow: amount of blood flowing through a tissue in a given time (ml/min) Perfusion: rate of blood flow per given mass of tissue (ml/min/g) Important for delivery of nutrients and oxygen, and removal of metabolic wastes

51 Variations in Blood Flow in the Circulatory System From aorta to capillaries, blood pressure and flow DECREASES because: friction between blood and vessels smaller radii of arterioles and capillaries farther from the heart, more vessels, greater total cross sectional area From capillaries to vena cava, rate of flow INCREASES because: large amount of blood is funneled back into fewer vessels.

52 Blood Pressure Changes With Distance More pulsatile closer to heart

53 Blood Pressure Usually measured at brachial artery of arm Systolic pressure: BP during ventricular systole Diastolic pressure: BP during ventricular diastole Normal value, young adult: 120/75 mm Hg

54 Neural Blood Pressure Control: Baroreflex Changes in BP detected by stretch receptors (baroreceptors) in large arteries above heart in the: aortic arch aortic sinuses (behind aortic valve cusps) carotid sinus (base of each internal carotid artery) Autonomic negative feedback response baroreceptors send signals to brainstem BP increases rate of signals to brainstem which inhibits the sympathetic nerves to blood vessels causing vasodilation (causes BP ) BP causes rate of signals to drop, excites vasomotor center, sympathetic nerves, causes vasoconstriction and BP

55 Baroreceptors

56 Baroreflex Negative Feedback Response

57 Neural Control: Chemoreflex Chemoreceptors in aortic bodies and carotid bodies are located in aortic arch, subclavian arteries, external carotid arteries Chemoreceptors monitor ph drop in ph of blood and cerebrospinal fluid due to increase in CO 2 concentration in tissues triggers an increase in blood pressure and respiration rate Carbon dioxide dissolves in water to form carbonic acid: CO 2 + H 2 O H 2 CO 3 carbonic acid dissociates in water into H + (acidic hydrogen ions) and HCO 3 - (bicarbonate ions)

58 Chemoreceptors Carotid body Aortic body Aortic body

59 Hormonal Control of BP and Flow Angiotensin II ( BP) potent vasoconstictor produced by liver, kidneys and lungs Aldosterone ( BP) Na + and water retention by the kidneys increases blood volume Vasopressin (ADH) ( BP) causes water retention by the kidneys which increases blood volume also causes generalized vasoconstriction Atrial Natriuretic Factor ( BP) produced by atria of heart urinary sodium excretion which lowers blood volume also causes generalized vasodilation Epinephrine and Norepinephrine ( and BP) binds to -adrenergic receptors on smooth muscle of blood vessels causing vasoconstriction which increases blood pressure binds to -adrenergic receptors on blood vessels in skeletal and cardiac muscle causing vasodilation which decreases blood pressure

60 Abnormalities of Blood Pressure Hypertension chronic resting BP above 140/90 can damage capillaries and weaken small arteries causing aneurysms Hypotension is chronic low resting BP Blood pressure lower than 120/80 mm Hg is considered normal. There is no specific number at which day-to-day blood pressure is considered too low, as long as no symptoms of trouble are present. Hypotension can be caused by blood loss or dehydration Hypotension lowers renal function, causes dizziness and impairs brain function