THE HEART

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THE HEART
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Blood Flow Through Heart
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Histology of heart
Striations Intercalated discs Gap
junctions Desmosomes Functional syncytium
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Comparison Skeletal and Cardiac Muscle

• Means of stimulation
• Skeletal stimulated by nerve ending
• Cardiac Self-excitable, initiate own
  depolarization
• Organ vs motor unit contraction
• Skeleta all cells of a motor unit contract
• Cardiac organ contracts as a unit
• Length of absolute refractory period
• Skeletal 1-2 ms contraction 15-100 ms
• Cardiac 250 ms contraction 300 ms

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Action Potentials of Cardiac Tissue
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Pacemaker Potential
Marieb and Hoehn, 2007, Human Anatomy
Physiology,
Pearson/Benjamin Cumming
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Channels responsible for pacemaker potential
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Action Potential of SA Node

• Pacemaker potential has slow depolarization
• Progressive reduction in K permeability
• K channels close
• Depolarization due to influx of Ca
• Depolarization due to influx of Na
• Repolarization due to efflux of K
• Repolarization of previous potential triggers
  following AP

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CARDIAC OUTPUT

• The volume of blood pumped by each ventricle per
  minute.
• Volume that flows through pulmonary or systemic
  circulation per minute
• CO HR x SV
• 5.0L 72beats/min x 0.07L/beat

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Control of Heart Rate

• SA nodes autonomous discharge is 100 beats/min.
• Parasympathetic stimulation slows discharge to 72
  beats/min.
• Sympathetic stimulation increases discharge.

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Effect of Cholinergic Stimulation

• ACh binds to muscarinic receptor
• h K conductance of SA node
• Leads to hyperpolarization of potential
• Slows opening of Ca channels of next potential
• iFiring rate of SA node
• Vagus nerve (vagal stimulation

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Sympathetic Stimulation to SA Node

• NE increases spontaneous firing rate
• Binds to B receptors
• Opens Ca channels
• h rate of depolarization
• h strength of contraction of muscle due to
• h Ca in fibers

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Control of Stroke Volume

• The volume of blood ejected by each ventricle
  during each contraction.
• Stroke volume is changed by changing force of
  contraction by
• Changes in end-diastolic volume (Frank Starling
  Mechanism)
• Changes in magnitude of sympathetic stimulation
  (contractility)
• Afterload

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(via SA Node)
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