Ch 18 the cardiovascular system PDF

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Chapter 18 The Cardiovascular System: The Heart

Heart - Gross Anatomy • Roughly cone-shaped organ, “big as your fist” − Apex – point of cone; points toward left hip − Base – Flattened posterior side (not inferior) facing right shoulder • Weighs 250-300 g (slightly less than one pound) • Location − Inferior mediastinum − Rests on superior surface of diaphragm − Anterior to the vertebral column, posterior to the sternum

Location and Basic Structure of the Heart

Coverings of the Heart – Pericardium Heart is enclosed in double-walled sac called the pericardium • Superficial fibrous pericardium − Made of dense connective tissue

− Protects, anchors, and prevents overfilling • Deep two-layered serous pericardium − Parietal layer tightly adherent to internal surface of fibrous pericardium − Visceral layer (epicardium) on external surface of the heart − Parietal and visceral layers separated by fluid-filled pericardial cavity (decreases friction) Opened Fibrous Pericardium Parietal layer of serous pericardium exposed on internal surface of fibrous pericardium

Layers of the Heart Wall 1. Epicardium - visceral layer of the serous pericardium 2. Myocardium - composed of: • Spiral bundles of cardiac muscle cells • Fibrous cardiac skeleton of the heart: crisscrossing, interlacing network of connective tissue -

Anchors cardiac muscle fibers

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Supports great vessels and valves

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Restricts flow of electrical impulses between the atria and ventricles

3. Endocardium is continuous with endothelial lining of blood vessels

Fibrous Skeleton of the Heart (atria have been removed)

Spiral orientation of cardiac muscle fibers allows wringing action to help empty heart chambers

Chambers of the heart Four chambers • Two atria − Separated internally by the interatrial septum − Coronary sulcus (atrioventricular groove) on external surface of heart − encircles heart at junction of atria and ventricles

− External auricles modestly increase atrial volume • Two ventricles − Separated by the interventricular septum − Anterior and posterior interventricular sulci mark the position of the septum externally Atria: The Receiving Chambers • Internal walls are ridged by pectinate muscles • 3 veins return blood from systemic circulation to right atrium − Superior vena cava − Inferior vena cava − Coronary sinus (vein from heart) • 4 veins return blood from pulmonary circulation to left atrium 2 right and 2 left pulmonary veins Ventricles: The Discharging Chambers • Internal walls have irregular ridges of muscle called trabeculae carneae (“crossbars of flesh”) • Papillary muscles project from walls into the ventricular cavities − Help keep atrioventricular valves closed during contraction of ventricles • Only blood vessel connected to right ventricle is pulmonary trunk • Only blood vessel connected to left ventricle is aorta Heart Valves • Ensure unidirectional blood flow through the heart • Two atrioventricular (AV) valves − Tricuspid valve (right) − Mitral valve (left) – also called bicuspid valve − Prevent backflow of blood into the atria when ventricles contract • Chordae tendineae anchor AV valve cusps to papillary muscles Heart Valves (continued) • Two semilunar valves − Aortic semilunar valve between aorta and left ventricle

− Pulmonary semilunar valve between pulmonary trunk and right ventricle − Prevent backflow of arterial blood into ventricles when ventricles relax

Pathway of Blood Through the Heart The heart is two side-by-side pumps • Right ventricle is the pump for the pulmonary circuit -

Pulmonary arteries carry blood to air sacs of lungs

• Left ventricle is the pump for the systemic circuit -

Aorta carries blood to all body tissues except air sacs of lungs

Pathway of Blood Flow Through the Heart • Right atrium  tricuspid valve  right ventricle • Right ventricle  pulmonary semilunar valve  pulmonary trunk  pulmonary arteries  lungs • Lungs  pulmonary veins  left atrium • Left atrium  bicuspid valve  left ventricle • Left ventricle  aortic semilunar valve  aorta • Aorta  systemic circulation

Pathway of Blood Through the Heart • Equal volumes of blood are pumped to the pulmonary and systemic circuits • Pulmonary circuit is short, low-pressure • Systemic circuit is long, high pressure -

Higher pressure is due to fact that blood encounters much more resistance (friction) in the long pathways of systemic circuit

• Anatomy of the ventricles reflects these differences Coronary Circulation • The functional blood supply to the heart muscle itself • Arterial supply varies considerably and contains many anastomoses (junctions) among branches • Collateral routes provide additional routes for blood delivery

Coronary Circulation • Arteries • Two major arteries: Right and left coronary arteries lie in coronary sulcus − First branches off of aorta  heart muscle gets immediate delivery of highly oxygenated blood • Veins • All cardiac veins drain into one big vein called coronary sinus − Located on posterior heart − Empties directly into right atrium Coronary Circulation • Arteries (most important in heart disease) − Right and left coronary arteries (in coronary sulcus) 

Right has two main branches: marginal and posterior interventricular arteries

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Left has two main branches: circumflex and anterior interventricular arteries

• Veins − Located alongside arteries − All major veins drain into coronary sinus − Great cardiac vein runs in anterior interventricular sulcus; middle cardiac vein runs in posterior interventricular sulcus Homeostatic Imbalances • Angina pectoris -

Thoracic pain caused by brief deficiency in blood delivery to myocardium

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Cells are weakened

• Myocardial infarction (heart attack) -

Prolonged coronary artery blockage

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Areas of cell death are repaired with noncontractile scar tissue

Microscopic Anatomy of Cardiac Muscle

• Cardiac muscle cells are striated, short, fat, branched, and interconnected

• Connective tissue matrix (endomysium) connects muscle cells to cardiac skeleton • T tubules are wide but less numerous (one per sarcomere) • SR is simpler than in skeletal muscle  no terminal cisternae means there are no triads • Numerous large mitochondria make up 25–35% of cell volume (only 2% of cell volume in skeletal muscle cells) • In contrast to skeletal muscle, myofibrils of cardiac muscle cells vary greatly in diameter and branch extensively  makes banding pattern less distinct than that of skeletal muscle Microscopic Anatomy of Cardiac Muscle • Intercalated discs: specialized junctions between cardiac muscle cells − Desmosomes anchor adjacent cells to each other and prevent them from separating during contraction − Gap junctions allow ions to pass directly from one cell to another; electrically couple adjacent cells • These structures allow heart muscle to behave as a functional syncytium • In contrast, skeletal muscle cells are independent of one another both structurally and functionally Heart Physiology: Electrical and Mechanical Activity • In PowerPoint A, we went over the macroscopic and microscopic structure of the heart • In this PowerPoint, we will learn about − The electrical activity of cardiac muscle cells − The electrical activity of the heart as a whole − The sequence of mechanical events associated with the pumping action of the heart Electrical Activity of Cardiac Muscle Cells There are two major types of cardiac muscle cells in the heart: 1. Contractile cells – 99% of cardiac muscle cells − responsible for physical pumping action of heart 2. Pacemaker cells – only 1% of cells − specialized non-contractile cells − responsible for generating heart’s electrical signal and controlling its movement through the heart as a whole

Cardiac Muscle Contraction is Different from Skeletal Muscle Contraction • Depolarization of the heart is rhythmic and spontaneous − Pacemaker cells have automaticity, or autorhythmicity (self-excitable). − Skeletal muscle cells are not autorhythmic • Gap junctions ensure the heart contracts as a unit − Skeletal muscle cells do not have gap junctions • Long absolute refractory period (250 ms) prevents tetanic contractions, which would stop the heart’s pumping action − Absolute refractory period in skeletal muscle cells ≈ 2 ms Contractile Cells – Excitation and Contraction • Depolarization opens voltage-gated fast Na+ channels in the sarcolemma • Reversal of membrane potential from –90 mV to +30 mV • Depolarization wave in T tubules causes the SR to release Ca2+ • Depolarization wave also opens voltage-gated slow Ca2+ channels in sarcolemma − 10-20% of calcium pulse needed for contraction comes from extracellular fluid Ca2+ surge prolongs depolarization phase (plateau)

Contractile Cells – Excitation and Contraction • Influx of Ca2+ from extracellular fluid triggers release of additional Ca 2+ from the SR • E-C coupling occurs as Ca2+ binds to troponin and sliding of the filaments begins • Duration of the AP and the contractile phase is much longer in cardiac muscle than in skeletal muscle • Repolarization results from inactivation of Ca 2+ channels and opening of voltage-gated K+ channels Pacemaker Cells – Excitation • Unlike contractile cells, pacemaker cells have unstable resting potentials (pacemaker potentials or prepotentials) due to open slow Na+ channels • At threshold, voltage-gated fast Ca2+ channels open • Explosive Ca2+ influx produces the rising phase of the action potential

• Repolarization phase of AP results from inactivation of Ca2+ channels and opening of voltagegated K+ channels Electrical Activity of the Whole Heart Intrinsic cardiac conduction system • Network of pacemaker cells that initiates and distributes action potentials to coordinate depolarization and contraction of heart

Intrinsic Cardiac Conduction System: Sequence of Excitation 1. Sinoatrial (SA) node (pacemaker) • Generates impulses about 75 times/minute (sinus rhythm) • Depolarizes faster than any other part of the myocardium 2. Atrioventricular (AV) node • Smaller diameter fibers having fewer gap junctions with internodal fibers carrying electrical signal from SA node • Delays impulses approximately 0.1 second to allow atria time to finish contraction • Depolarizes 50 times per minute in absence of SA node input

Intrinsic Cardiac Conduction System: Sequence of Excitation (continued) 3. Atrioventricular (AV) bundle (bundle of His) • Only electrical connection between the atria and ventricles 4. Right and left bundle branches • Two pathways in the interventricular septum that carry the impulses toward the apex of the heart Intrinsic Cardiac Conduction System: Sequence of Excitation (continued) 5. Subendocardial conducting network (Purkinje fibers) • Complete the pathway into the apex and ventricular walls • AV bundle and Purkinje fibers depolarize only 30 times per minute in absence of AV node input

Electrocardiography

• Electrocardiogram (ECG or EKG): a composite of all the action potentials generated by pacemaker and contractile cells at a given time • Three waves 1. P wave: depolarization of SA node and atria 2. QRS complex: ventricular depolarization T wave: ventricular repolarization Three Waves of Electrocardiogram

Homeostatic Imbalances Defects in the intrinsic conduction system may result in 1. Arrhythmias: irregular heart rhythms 2. Uncoordinated atrial and ventricular contractions 3. Fibrillation: rapid, irregular contractions; useless for pumping blood -atrial fibrillation, which is not always symptomatic, is the most common arrhythmia

Homeostatic Imbalances • Defective SA node may result in • Ectopic focus: abnormal pacemaker takes over • If AV node takes over, there will be a junctional rhythm (40–60 bpm) • Defective AV node may result in • Partial or total heart block

• Few or no impulses from SA node reach the ventricles

Extrinsic Innervation of the Heart • Heartbeat is modified by the autonomic nervous system • Cardiac centers are located in the medulla oblongata − Cardioacceleratory center innervates SA and AV nodes, heart muscle, and coronary arteries through sympathetic neurons − Cardioinhibitory center inhibits SA and AV nodes through parasympathetic fibers in the vagus nerves

Autonomic Nervous System Regulation • Sympathetic nervous system is activated by emotional or physical stressors − Norepinephrine causes the pacemaker to fire more rapidly (and at the same time increases contractility) • Parasympathetic nervous system opposes sympathetic effects − Acetylcholine hyperpolarizes pacemaker cells by opening K + channels − The heart at rest exhibits vagal tone resting heart rate reflects tonic inhibition by parasympathetic input If all nerves to the heart are cut, resting HR increases to ~100 BPM Heart Sounds • Two sounds (lub-dup) associated with closing of heart valves • First sound occurs as AV valves close and signifies beginning of ventricular systole • Second sound occurs when SL valves close at the beginning of ventricular diastole • Heart murmurs: abnormal heart sounds most often indicative of valve problems

Mechanical Events: The Cardiac Cycle • Cardiac cycle: all mechanical events associated with blood flow through the heart during one complete heartbeat • The period between the start of one heartbeat and the beginning of the next (duration = 0.8 sec )

• Systole—contraction • Diastole—relaxation • These terms are used for both the atria and ventricles: -atrial systole and diastole -ventricular systole and diastole • Cycle represents series of pressure and blood volume changes organized into 3 phases • Mechanical events follow electrical events seen on ECG Cardiac Cycle – Phase 1 1. Ventricular filling: mid-to-late diastole • Pressure is low; 80% of blood flows passively from atria through open AV valves into ventricles (SL valves closed) • Atrial depolarization triggers atrial systole (P wave), atria contract, pushing remaining 20% of blood into ventricle − End diastolic volume (EDV): volume of blood in each ventricle at end of ventricular diastole (≈120 ml) • Depolarization spreads to ventricles (QRS wave) • Atria finish contracting and return to diastole Cardiac Cycle – Phase 2 2. Ventricular systole • Atria relax; ventricles begin to contract • Rising ventricular pressure causes closing of AV valves • Two subphases 2a: Isovolumetric contraction phase: all valves are closed 2b: Ejection phase: ventricular pressure exceeds pressure in large arteries, forcing SL valves open • End systolic volume (ESV): volume of blood remaining in each ventricle after systole (≈50 ml) Phase 3: Isovolumetric relaxation 3. Isovolumetric relaxation: early diastole • Following ventricular repolarization (T wave), ventricles are relaxed; atria are relaxed and filling

• Backflow of blood in aorta and pulmonary trunk closes SL valves − Ventricles are briefly totally closed chambers (isovolumetric) • When atrial pressure exceeds ventricular pressure, AV valves open; cycle begins again − Atria and ventricles are both filling passively

Cardiac Cycle - Timing of major events Assuming the average heart beats 75 times/min, the cardiac cycle lasts 0.8 seconds •

Atrial systole = 0.1 seconds

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Ventricular systole = 0.3 seconds

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Remaining 0.4 seconds is a period of total heart relaxation called the quiescent period

Cardiac Output (CO) • Volume of blood pumped by each ventricle in one minute • CO = heart rate (HR) x stroke volume (SV) • HR = number of beats per minute • SV = volume of blood pumped out by a ventricle with each beat SV = EDV – ESV

Cardiac Output (CO) At rest • CO (ml/min) = HR (75 beats/min)  SV (70 ml/beat) = 5.25 L/min • Maximal CO is 4–5 times resting CO in nonathletic people • Maximal CO may reach 35 L/min in trained athletes • Cardiac reserve: difference between resting and maximal CO

Chemical Regulation of Heart Rate 1. Hormones

• Epinephrine from adrenal medulla enhances heart rate and contractility • Thyroxine increases heart rate and enhances the effects of norepinephrine and epinephrine 2. Intra- and extracellular ion concentrations (e.g., Ca 2+ and K+) must be maintained for normal heart function

Homeostatic Imbalances • Tachycardia: abnormally fast heart rate (>100 bpm) • If persistent, may lead to fibrillation • Bradycardia: heart rate slower than 60 bpm • May result in grossly inadequate blood circulation May be desirable result of endurance training

Congestive Heart Failure (CHF) • Progressive condition where cardiac output is so low that blood circulation is inadequate to meet tissue needs • Caused by • Coronary atherosclerosis • Persistent high blood pressure • Multiple myocardial infarcts Dilated cardiomyopathy (DCM)

Developmental Aspects of the Heart • Since fetus does not breath, there is no need for large volume of blood to go to lungs • Structure of fetal heart allows bypass of pulmonary circulation • Foramen ovale in interatrial septum connects the two atria – allows bypass of right ventricle • Ductus arteriosus connects the pulmonary trunk and the aorta – further minimizes amount of blood going to lungs Developmental Aspects of the Heart

• Congenital heart defects • Most common birth defect (1:200 live births) • Lead to mixing of systemic and pulmonary blood (“blue baby”) • Involve defects in heart, valves, and/or vessels that increase the workload on the heart...