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Cardiac Muscle:
• autorhythmic tissue with branched fibers interconnected by special lowresistance junctions (= intercalated discs = gap junctions) (fig. 18 - 11)
• synchronized electrical activity spreads instantaneously as if no intervening
cell membranes existed (functional syncytium); wave of electrical activity occurs
independently of nervous system and causes the contractile activity of the heart
• Action Potential of Cardiac Muscle: spontaneous depolarization with state of
prolonged refractoriness; eliminates tetanic contractions (figs. 18 - 12 & 18 - 13)
Pacemaker and Conducting System (fig. 18 - 14):
• tissue with fastest rhythm sets the pace of the entire myocardium; heart's
electrical system has specialized fibers serving as pacemaker and conducting system:
a. Sinoatrial (SA) Node: located in back wall of right atrium; initiates atrial
depolarization/repolarization sequence
b. Atrioventricular (AV) Node: located in right atrium near interatrial septum
and atrioventricular junction; serves as only path for electrical conduction to ventricular
myocardium; relays signal after delay of approx. one-tenth second (ensures atrial
contraction precedes ventricular contraction)
c. Conducting System: AV node conveys signal directly to AV bundle (bundle of
His), onward to bundle branches and ultimately to network of Purkinje fibers
(distribute signal throughout ventricular myocardium)
Electrocardiography (ECG) (figs. 18 - 16 & 18 - 17):
• non-invasive monitoring of heart’s electrical activity; possible due to
electrolytic properties of body fluids
• typical electrocardiogram features standard deflections:
P Wave: signifying atrial depolarization (discharge of pacemaker)
QRS Wave or Complex: caused by depolarization of ventricles (atrial
repolarization masked by this)
T Wave: caused by repolarization of ventricles
PR Interval: time elapsed between start of P wave and R wave (period of delay
at AV node) (called PQ interval in text)
• ECG Leads (12 combinations of electrode placements routinely used)
a. 6 limb leads (3 bipolar: I, II, III; 3 unipolar: aVL, aVR, aVF)
b. 6 chest leads (all unipolar: V1 - V6)
• ECG normal pattern = sinus rhythm
• digressions from normal pattern (arrhythmias) signify various disorders:
tachycardia (elevated heart rate), bradycardia (depressed heart rate), flutter (rapid rate
in a particular chamber), & fibrillation (loss of coordination)
• Depressed ST Segment: usually coronary ischemia with angina pectoris
Bio 108/508
lec. 17 - p. 2
• Elevated ST Segment (sometimes accompanied by inverted T wave): usually
signifies heart attack (myocardial infarction)
• Prolonged PR Interval: various types of heart block
• QT Interval: duration of ventricular electrical signal
Bio 108/508
lec. 17 - p. 3
Cardiac Blood Flow :
• blood flow is one-way due to operation of valves (AV valves and SL valves)
(figs. 18 - 8 to 18 - 10):
right atrium --(tricuspid valve)--> right ventricle --(pulmonary valve)-->
pulmonary trunk --> lungs --> pulmonary veins --> left atrium--(mitral valve)--> left
ventricle --(aortic valve)--> aorta --> body tissues --> venae cavae --> right atrium
• valve operation is passive (no muscle tissue); respond to external pressures:
a. AV valves open when atrial pressure > ventricular pressure
b. AV valves close when atrial pressure < ventricular pressure
c. SL valves open when arterial pressure < ventricular pressure
d. SL valves close when arterial pressure > ventricular pressure
• Pressure Considerations:
• pressure rises in a chamber when blood is entering it or the chamber is
contracting or recoiling
• pressure falls in a chamber when. blood is leaving it or chamber is relaxing
Cardiac Cycle (fig. 18 - 20):
a. Systole: contraction of ventricles - ventricular P rises: AV valves close
isovolumetric contraction (IVC) - both sets of valves closed
ventricular ejection - ventricular P exceeds arterial P and opens SL valves
b. Diastole: relaxation of ventricles - ventricular P drops: SL valves close
isovolumetric relaxation (IVR) - both sets of valves closed
ventricular filling - ventricular P falls below atrial P: AV valves open again
Cardiac Output (fig. 18 - 22):
• cardiac output (CO) = heart rate (HR) x stroke volume (SV)
• Heart Rate regulated by:
• Epinephrine: hormone with positive chronotropic effect
• Autonomic Nervous System: sympathetic division (like epinephrine)
• parasympathetic division has negative chronotropic effect
• Stroke Volume regulated by:
• Venous Return: amount of blood returned by veins (preload)
• Contractility ( strength of contractions):
a. Starling’s law of the heart
b. Epinephrine: hormone with positive inotropic effect
c. Autonomic Nervous System: sympathetic: positive inotropic effect
• parasympathetic: negative inotropic effect
• Afterload (factors which obstruct effectiveness of pumping):
a. High Arterial Pressure: opposes opening of SL valves
b. Enlarged Ventricle: once ventricular radius exceeds certain threshold
value, wall tension falls off for a given contractile force
c. Valve Disorders: stenosis (improper opening) stifles blood flow;
insufficiency (improper closing) allows retrograde flow
• detectable by presence of “heart murmur”