The Heart: Conduction System

1
The Heart Conduction System

• Intrinsic conduction system (nodal system)
• Function coordinates the action of the heart
  chambers and causes the heart to beat faster than
  it would otherwise
• Heart muscle cells contract, without nerve
  impulses, in a regular, continuous way
• The heart has an intrinsic ability to contract
  that is different from all other muscles in the
  body. Whereas the nervous system my increase of
  decrease its rate, the heart continues to beat
  even if all nervous connections are cut.

2
Elements of the Intrinsic Conduction System

• Special tissue sets the pace
• 1. Sinoatrial node SA node (pacemaker),
• is in the right atrium
• 2. Atrioventricular node AV node,
• is at the junction of the atria and
  ventricles
• 3. Atrioventricular bundle AV bundle (bundle
  of His),
• is in the interventricular septum
• 4. Bundle branches
• are in the interventricular septum
• 5. Purkinje fibers
• 
  spread within the ventricle wall muscles

3
Heart Contractions

• Sinoatrial Node (SA node)- starts the contraction
• Sequential stimulation
• Force cardiac muscle depolarization in one
  directionfrom atria to ventricles

4
Heart Contractions

• Once SA node starts the heartbeat
• Impulse spreads to the AV node ?Then the atria
  contract
• At the AV node,
• Impulse passes through the AV bundle, bundle
  branches, and Purkinje fibers? wringing
  contraction of ventricles
• Result Blood is ejected from the ventricles to
  the aorta and pulmonary trunk

5
Heart Contractions
Figure 11.6
6
Heart Contractions

• Tachycardiarapid heart rate over 100 beats per
  minute
• Bradycardiaslow heart rate less than 60 beats
  per minutes

7
The Heart Cardiac Cycle

• Cardiac cycle events of one complete heartbeat,
  during which both atria and ventricles contract
  and then relax
• Atria contract simultaneously
• Atria relax, then ventricles contract
• Since most of the pumping is done by ventricles
  the following terms refer to the ventricles
• Systole contraction
• Diastole relaxation

8
Filling Heart Chambers Cardiac Cycle
DUB
LUB
LUB DUB is caused by the closing of the heart
valves
Figure 11.7
9
Filling Heart Chambers Cardiac Cycle
Atria relaxed! Ventricles relaxed!
AV valves open! Semilunar valves closed!
Figure 11.7, step 1a
10
Filling Heart Chambers Cardiac Cycle
Atria contract! Ventricles relaxed!
AV valves open! Semilunar valves closed!
Figure 11.7, step 1b
11
Filling Heart Chambers Cardiac Cycle
Close AV valves!
AV valves closed! LUB Semilunar valves closed!
Figure 11.7, step 2a
12
Filling Heart Chambers Cardiac Cycle
Ventricles contract!
AV valves closed! Semilunar valves open!
Figure 11.7, step 2b
13
Filling Heart Chambers Cardiac Cycle
Ventricles relax!
AV valves closed! Semilunar valves closed! DUB
Figure 11.7, step 3
14
The Heart Cardiac Cycle

• Cardiac cycleevents of one complete heart beat
• Mid-to-late diastoleblood flows from atria into
  ventricles
• Ventricular systoleblood pressure builds before
  ventricle contracts, pushing out blood
• Early diastoleatria finish refilling,
  ventricular pressure is low

15
Abnormal Heart Sounds

• Murmurs inicate valve problems
• Blood flows silently as long as the flow is
  smooth and uninterrupted. If it strikes
  obstructions, its flow becomes turbulent and
  generates sounds.
• If a valve does not close tightly, a swishing
  sound will be heard as the blood flows back
  through the partially open valve.
• If valves are narrowed, distinct sounds will also
  be heard.

16
The Heart Cardiac Output

• Cardiac output (CO)
• Amount of blood pumped by each side (ventricle)
  of the heart in one minute
• Stroke volume (SV)
• Volume of blood pumped by each ventricle in one
  contraction (each heartbeat)
• Usually remains relatively constant
• About 70 mL of blood is pumped out of the left
  ventricle with each heartbeat
• Heart rate (HR)
• Typically 75 beats per minute

17
The Heart Cardiac Output

• CO HR ? SV
• CO HR (75 beats/min) ? SV (70 mL/beat)
• CO 5250 mL/min
• Starlings law of the heartthe more the cardiac
  muscle is stretched, the stronger the contraction
• Changing heart rate is the most common way to
  change cardiac output

18
The Heart Regulation of Heart Rate

• Increased heart rate
• Sympathetic nervous system
• Crisis
• Low blood pressure
• Hormones
• Epinephrine
• Thyroxine
• Exercise
• Decreased blood volume

19
Increased heart rate

Crisis stressors
Low blood pressure
Hormones
Exercise
Decreased blood volume
20
The Heart Regulation of Heart Rate

• Decreased heart rate
• Parasympathetic nervous system
• High blood pressure or blood volume
• Decreased venous return

21
Cardiac Output Regulation
Figure 11.8
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