The Heart

1
The Heart

• Control of the Heart Beat

2
The Heart Beat

• The heart is made up of cardiac muscle.
• Cardiac muscle is myogenic, which means it
  naturally contracts and relaxes.
• Therefore, it receives no impulse from a nerve to
  make it contract.

3
The Heart Beat

• The cardiac cycle is initiated by a small patch
  of muscle called the Sinoatrial node (SAN) or
  pacemaker.
• This node sets the rhythm for all the other
  cardiac muscle.
• Pacemaker cells have an inbuilt rhythm that is
  faster than the other cells in the heart.

4
The Heart Beat

• The SA node sends out an excitation wave of
  electrical activity over the atrial walls.
• The cardiac muscle responds to this wave by
  contracting at the same speed as the SAN.
• This results in both atria contracting
  simultaneously.

5
The Heart Beat

• There is a delay between atrial contraction and
  ventricular contraction.
• Fibres between the two chambers that do not
  conduct the excitation phase cause this delay.
• Therefore, the wave is conducted through a patch
  of fibres in the septum known as the
  atrio-ventricular node or AVN.

6
The Heart Beat

• After about a delay of 0.1 seconds the AV node
  passes the wave onto another set of conducting
  fibres that run down the centre of the septum
  between the ventricles called the Purkyne fibres.

7
The Heart Beat

• The wave is then transmitted (very rapidly) down
  to the bottom of the septum, where it spreads
  through the ventricles walls in an upward
  direction.
• This movement causes the muscle to contract and
  the ventricles squeeze the blood out of the heart.

8
The Heart Beat

• Fibrillation is the result of the contractions of
  the atria and ventricles becoming erratic and out
  of sync.
• Using electric shock treatment to restart the SA
  node can rectify the problem.

9
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