chapter 20: the cardiovascular system the heart
TRANSCRIPT
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Chapter 20: The Cardiovascular
System
THE HEART
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Heart Anatomy Location
diaphragm, mediastinum, 2/3 left of midline Orientation
Apex- points anterior, inferior, left Base- directed posterior, superior, right
Vessels Superior and Inferior Vena Cava Pulmonary trunk pulmonary arteries(lungs) Pulmonary veins Aorta
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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
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Layers of the heart wall Epicardium- thin, transparent, outer
Visceral layer of serous pericardium Smooth slippery outside of heart
Myocardium- middle 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.
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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
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Valve function When AV valve open:
Cusps project into ventricle Ventricle relaxed papillary muscle relaxed chordae
tendineae slack Blood: pressure atria pressure ventricle
Ventricle contracts, pressure cusps up, close 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
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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
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Pulmonary and systemic circuits
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Coronary circulation (1)
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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
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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
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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 non-
contractile 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
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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
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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
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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…
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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
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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
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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
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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
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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)
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Cardiac excitation and the ECG
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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
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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
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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
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Regulation of Cardiac Output Cardiac output = stroke volume x heart rateCO = 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)
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Regulation of stroke volume 1. Effect of preload = Frank-Starling Law
of the Heart > preload > force of contraction
rubberband
2. Effect of afterload Pressure rqrd for ejection of blood
3. Effect of contractility-each individual fiber Positive inotropic agents- eg. norepinephrine Negative inotropic agents - eg. propranolol
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Regulation of Heart Rate 1. Normal rate = vagal tone 2. Regulation
1. Autonomic Nervous system 2. Chemical
a. Hormones b. Ions
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