Heart Rate

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•  The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the 

    medulla of the brain

While the basal heart rate is determined within the heart by the pacemaker, it can be regulated by external signals

  • Nerve signals from the brain can trigger rapid changes, while endocrine signals can trigger more sustained changes
  • Changes to blood pressure levels or CO2 concentrations (and thereby blood pH) will trigger changes in heart rate

Nerve Signalling

The pacemaker is under autonomic (involuntary) control from the brain, specifically the medulla oblongata (brain stem)

Two nerves connected to the medulla regulate heart rate by either speeding it up or slowing it down:

  • The sympathetic nerve releases the neurotransmitter noradrenaline (a.k.a. norepinephrine) to increase heart rate
  • The parasympathetic nerve (vagus nerve) releases the neurotransmitter acetylcholine to decrease heart rate

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•  Epinephrine increases the heart rate to prepare for vigorous physical activity

Hormonal Signalling

Hormones are chemical messengers released into the bloodstream that act specifically on distant target sites (like the heart)

Heart rate can undergo a sustained increase in response to hormonal signalling in order to prepare for vigorous physical activity

  • The hormone adrenaline (a.k.a. epinephrine) is released from the adrenal glands (located above the kidneys)
  • Adrenaline increases heart rate by activating the same chemical pathways as the neurotransmitter noradrenaline

Regulation of Heart Rate

heart rate