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 THE MAJOR FACTORS CONTROLLING THE HEARTBEAT

Schematic pdf download available

  1. The heart muscle tissue shows an inherent contraction without connection to nerves.
     
    Evidence: (a) remove a heart from the body and. it will continue to beat, in some animals for several hours.
      (b) the heart in a chick embryo begins to beat before the nerve connections are made.

     

  2. The heart beat is initiated. and controlled. at the sino-atrial node (the pacemaker)

Control of heart beat frequency

Nervous control

Stimuli from the vagus nerve will slow the heart beat Its terminal buttons release acetylcholine.

Stimuli from the accelerans nerve will speed up the heart beat. Its terminal buttons release noradrenalin.

Hormonal control

Adrenalin in increased concentration in the blood stream speeds up the heart beat (direct effect). Noradrenalin in the blood causes vasoconstriction of blood vessels, which raises the blood pressure. This stimulates the baroreceptors (blood pressure sensors), which in turn stimulate the cardioinhibitory centre in the brain slowing the heart beat down via the vagus nerve.

The control of stroke volume (the volume of blood pumped per beat)

The stroke volume may be increased by:

  1. using adrenalin, which promotes a more fierce contraction of the muscle.
  2. increasing the venous return (the amount of blood. flowing back to the heart). The fuller the ventricle becomes the more stretched the muscle fibres and the more fiercely they will contract (positive feedback)

Summary table

FACTORS DECREASING HEART BEAT (BRADYCARDIA)

FACTORS INCREASING HEART BEAT (TACHYCARDIA)
Increased stimulation of baroreceptors in the aorta, carotid. arteries and heart. Decreased stimulation of baroreceptors.
Decreased stimulation of chemoreceptors. Increased stimulation of chemoreceptors by change in pH or composition of blood:
  1. increased CO2
  2. decreased O2 (less pronounced)
  3. increased lactic acid.
Decreased stimulation of stretch receptors in the lungs (expiration) Increased stimulation of stretch receptors in the lungs (inspiration)
Fear and grief Excitement and pain.
Noradrenaline blood levels (not pronounced) Circulating adrenaline and thyroid hormones, increase the potency of noradrenaline released at the pacemaker.
  Fever causes a temperature rise of the pacemaker.

© Paul Billiet 2004