Chapter 18' The Cardiovascular System: The Heart

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Chapter 18. The Cardiovascular System The Heart

• Part II Physiology

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Electrical Events

• Cardiac muscle depolarization/contraction is
  rhythmic and intrinsic
• Gap junctions and the intrinsic cardiac
  conduction system
• Self depolarizing cardiac cells possesses an
  unstable resting potential

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Parts

• There are 4 areas that contain these cells
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• Atrioventricular bundle/bundle branches
• Purkinje fibers

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SA node

• Located in right atrium where superior vena cava
  empties
• Called the pacemaker
• Rhythm it generates called the sinus rhythm
• Averages 75 beats per minute w/ modification

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AV node

• Located superior to tricuspid valve
• Receives depolarization wave via gap junctions by
  way of the internodal pathway
• Impulses delayed at AV node
• Intrinsic depolarization 50 beats/ minute

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Atrioventricular bundle / branches

• Also called Bundle of His
• Located superior part of interventricular septum
• Bundles connected to SA node indirectly through
  AV node
• Branches travel down either side of septum

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Purkinje fibers

• Complete interventricular pathway, travel through
  apex, and up the ventricular walls
• More elaborate on left side
• Directly innervate papillary muscles
• Ventricles contract as a unit due to gap
  junctions

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more

• Time b/t SA depolarization and last of
  ventricular cells is .22 sec.
• This system allows coordination, otherwise
  different parts of the heart would contract at
  different times

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C
BC
A
AB
D
E
AB
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Homeostatic Imbalances

• Arrhythmia irregular heart rhythms
• Fibrillations rapid out-of-phase contractions
• Ectopic focus abnormal pacemaker
• Junctional rhythm
• Extrasystole
• Heart block

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External control

• As you remember, the ANS affects heart rate
• Centers located in the medulla
• Parasympathetic via the vagus, sympathetic via
  the chain
• e.g., intrinsic rate of SA node is 100 bpm,
  parasympathetic modifies this to 75 bpm

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ECGs

• Electrical depolarizations can be recorded
• ECG is a summation of all impulses
• Typically 3 deflection waves are recorded
• P wave
• QRS complex
• T wave

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• P wave SA depolarization
• QRS ventricular depolarization
• T wave ventricular repolarization

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• P-Q interval beginning of atrial excitation to
  beginning of ventricular excitation
• S-T segment entire myocardium depolarized
• Q-T interval ventricular depolarization through
  ventricular repolarization

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Irregular ECGs

• Normal

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Junctional rhythm
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Heart block
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Ventricular fibrillation
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Lub-dub

• Auscultation listening
• Lub AV closure, beginning of contraction
• Dub SL closure beginning of ventricular
  relaxation

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Homeostatic imbalance

• Murmurs generation of sound due to turbulence
  caused by obstructed blood flow
• Incompetent valve swishing sound
• Stenotic valve high-pitched/clicking

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Mechanical events

• Heart alternates b/t contraction and relaxation
• Contraction systole
• Relaxation diastole
• Cycle 1 heart beat atrial systole/diastole
  ventricular systole/diastole

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Lets put it together

• Arbitrary starting point beginning of
  ventricular filling (1.)
• mid-to-late diastole
• Pressure is low
• AV valves open SL valves closed
• Towards the end, SA node depolarizes, causing
  the P wave

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• Just after this, atrial systole.
• Atrial pressure maximizes, atria empty
• Ventricles, at the last part of diastole, contain
  maximum volume of blood (EDV)
• Atrial diastole for rest of cycle

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• Beginning of AV node/bundle/Purkinje
  depolarization causing QRS complex

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Phase 2

• Completion of QRS complex
• Ventricular systole
• 1st heart sound as AV valves shut
• Ventricular pressure maximizes
• SL valves open
• T wave seen as ventricles repolarize
• Also called isovolumetric phase

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Phase 3

• Also called isovolumetric relaxation phase
  beginning of diastole
• Occurs immediately after the T wave is seen
• Ventricles relax
• SL valves close, causing 2nd heart sound

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• Remaining blood volume in ventricles called ESV
• Pressure on aorta greatest right after
  ventricular systole
• As pressure in ventricles, backflow that closes
  SL valves causes brief increase in aortic
  pressure called the dicrotic notch

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• During ventricular systole, atria have been
  filling.

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P wave
Atrial pressure
Atrial volume
Ventricle volume