20-1 chapter 20 cardiovascular system the heart. 20-2 the heart the heart is two pumps. –pulmonary...
TRANSCRIPT
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20-1
Chapter 20
Cardiovascular System
The HeartThe Heart
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20-2
The Heart• The Heart is two pumps.
– Pulmonary circulation• Carries blood to lungs
• Returns to the left side of heart
– Systemic circulation• Delivers oxygen and
nutrients to the body
• Returns to the right side of the heart
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Functions of the Heart
• Generating blood pressure• Routing blood
– Heart separates pulmonary and systemic circulations
– Ensures oxygenation of blood flowing to tissues.
• Ensuring one-way blood flow– Heart valves ensure one-way flow
• Regulating blood supply– Changes in contraction rate and force match
blood delivery to changing metabolic needs
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Size, Shape, Location of the Heart
• Size of a closed fist• Shape
– Apex: rounded tip of heart
– Base: Flat part at opposite of end of cone
• Located in thoracic cavity within the mediastinum
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Heart Cross Section
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Pericardium
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Heart Wall
• Three layers of tissue– Epicardium: This serous membrane of smooth
outer surface of heart– Myocardium: Middle layer composed of
cardiac muscle cell and responsibility for heart contracting
– Endocardium: Smooth inner surface of heart chambers
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Heart Wall
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External Anatomy• Four chambers
– 2 atria– 2 ventricles
• Auricles• Major veins
– Superior vena cava– Inferior vena cava– Pulmonary veins
• Major arteries– Aorta– Pulmonary trunk
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External Anatomy
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Coronary Circulation
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Heart Valves
• Atrioventricular– Tricuspid
– Bicuspid or mitral
• Semilunar– Aortic
– Pulmonary
• Prevent blood from flowing back
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Heart Valves
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Function of the Heart Valves
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Blood Flow Through Heart
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Systemic and PulmonaryCirculation
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Heart Skeleton
• Consists of plate of fibrous connective tissue between atria and ventricles
• Fibrous rings around valves to support
• Serves as electrical insulation between atria and ventricles
• Provides site for muscle attachment
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Cardiac Muscle
• Elongated, branching cells containing 1-2 centrally located nuclei
• Contains actin and myosin myofilaments
• Intercalated disks: Specialized cell-cell contacts
• Desmosomes hold cells together and gap junctions allow action potentials
• Electrically, cardiac muscle behaves as single unit
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Conducting System of Heart
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Electrical Properties• Resting membrane potential (RMP) present
– Depends on low permiability to Na+ and Ca++ and high permiability to K+.
• Action potential– Depolarization (Fast voltage gated Na+ channels)
– Early partial repolarization (Voltage gated Na+ channels close and a small number of K+ channels open.)
– Plateau phase - Prolonged period of slow repolarization (Slow Ca++ channels are open)
– Rapid final repolarization phase (Voltage gated Ca++ channels close and more K+ channels open)
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Action Potentials inSkeletal and Cardiac Muscle
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SA Node Action PotentialPermiability changes in the pacemaker cells (Autorhythmicity):1. Prepotential
• Small number of Na+ channels open
• Voltage-gated K+ channels are closing
• Voltage-gated Ca++ channels begin to open
2. Depolarization Phase• Voltage-gated Ca++ channels are
open• Voltage-gated K+ channels are
closed.3. Repolarization phase
• Voltage-gated Ca++ channels close.
• Voltage-gated K+ channels open.
Note: Ca++ channel blockers like verapamil are used to treat tachycardia and arrhythmias.
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Refractory Period
• Absolute: Cardiac muscle cell completely insensitive to further stimulation
• Relative: Cell exhibits reduced sensitivity to additional stimulation
• Long refractory period prevents tetanic contractions
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Electrocardiogram
• Action potentials through myocardium during cardiac cycle produces electric currents than can be measured
• Pattern– P wave
• Atria depolarization
– QRS complex• Ventricle depolarization
• Atria repolarization
– T wave: • Ventricle repolarization
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Cardiac Arrhythmias
• Tachycardia: Heart rate in excess of 100bpm• Bradycardia: Heart rate less than 60 bpm• Sinus arrhythmia: Heart rate varies 5%
during respiratory cycle and up to 30% during deep respiration
• Premature atrial contractions: Occasional shortened intervals between one contraction and succeeding, frequently occurs in healthy people
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Alterations in Electrocardiogram
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Cardiac Cycle
• Heart is two pumps that work together, right and left half
• Repetitive contraction (systole) and relaxation (diastole) of heart chambers
• Blood moves through circulatory system from areas of higher to lower pressure.– Contraction of heart produces the pressure
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Cardiac Cycle
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Heart Sounds• First heart sound or “lubb”
– Atrioventricular valves and surrounding fluid vibrations as valves close at beginning of ventricular systole
• Second heart sound or “dupp”– Results from closure of aortic and pulmonary semilunar
valves at beginning of ventricular diastole, lasts longer
• Third heart sound (occasional)– Caused by turbulent blood flow into ventricles and detected
near end of first one-third of diastole
• Clinical considerations:– Murmurs: abnormal heart sounds– Incompetent valve - leaks excessively (gurguling,swishing)– Stenosis: abnormally small valve openings (rushing sound before valve closes.– Causes: genetic or due to rhematic fever scaring or myocardial infarction
affecting the papillary muscles.
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Location of Heart Valves
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Mean Arterial Blood Pressure• Stroke volume
– Volume of blood pumped during each cardiac cycle• at rest ~70 ml• can increase to ~200 ml during exercise
• Heart Rate– ~72 BPM and can increase to over 120 BPM during exercise.
• Cardiac Output =stroke volume X heart rate– Resting: 72 beats/min X 70 ml/beat = 5040 ml / min– Exercise: 120 beats/min X 200 ml/beat = 24,000 ml/min.
• Cardiac reserve – max cardiac output - resting cardiac output– = 24,000-5040ml /min = 18960 ml– C.O. is the major factor in determining blood pressure.
• Blood pressure– Blood pressure reflects pressure changes in the aorta not the ventricle– Normal BP 120 systolic / 80 diastolic
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Factors Affecting MAP
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Regulation of the Heart• Intrinsic regulation: Results from normal
functional characteristics, not on neural or hormonal regulation– Starling’s law of the heart– Stretching of the SA node.
• Extrinsic regulation: Involves neural and hormonal control– Parasympathetic stimulation
• Supplied by vagus nerve, decreases heart rate, acetylcholine secreted
– Sympathetic stimulation• Supplied by cardiac nerves, increases heart rate and force of
contraction, epinephrine and norepinephrine released
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Heart Homeostasis• Effect of blood pressure
– Baroreceptors monitor blood pressure
• Effect of pH, carbon dioxide, oxygen– Chemoreceptors monitor
• Effect of extracellular ion concentration– Increase or decrease in extracellular K+ decreases heart
rate
• Effect of body temperature– Heart rate increases when body temperature increases,
heart rate decreases when body temperature decreases
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Baroreceptor and ChemoreceptorReflexes
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Baroreceptor Reflex
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Chemoreceptor Reflex-pH
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Effects of Aging on the Heart
• Gradual changes in heart function, minor under resting condition, more significant during exercise
• Hypertrophy of left ventricle
• Maximum heart rate decreases
• Increased tendency for valves to function abnormally and arrhythmias to occur
• Increased oxygen consumption required to pump same amount of blood