Cardiac Output

  • a measure of what the heart is actually putting out

  • measured in ml/min (mils/minute)

  • the amount of blood pumped by the heart in one minute

  • must adapt constantly to the changing demands of the body


Two factors that influence cardiac output

  • heart rate (bpm)

    • nerves that speed up and slow down the heart

    • hormones that also have an influence on overall heart rate

  • stroke volume (ml/beat)

    • determined by the blood volume

      • how much blood is actually in the heart in order to contract

    • vascular resistance

      • what you’re fighting against in order to push the blood into the cardiovascular system

Cardiac Output = Heart Rate x Stroke Volume


  • The SA node paces the heart at about 100 bpm on its own

  • The parasympathetic nervous system innervation slows the pace of the heart when the pace is less than 100 bpm

    • when the heart goes from 70 to 100, it’s not you telling the heart to speed up but instead telling the heart to stop slowing down

  • But when you go higher than 100 bpm, the sympathetic nervous system kicks in telling the heart to increase its rate further to meet the body's demands during activities like stress or exercise

  • heart rate is also affected by some endocrine hormones

    • adrenaline (epinephrine and norepinephrine) from the adrenal gland increases heart rate


  • Stroke Volume

    • the amount of blood pumped by both ventricles per beat

    • affected by:

      • Preload - the amount of blood in the heart prior to contraction

        • increase stroke volume:

          • fast filling time

          • increased venous return

        • decrease stroke volume:

          • fluid electrolyte imbalance

      • Contractility - the force of contraction (speed)

        • increase stroke volume:

          • adrenaline hormones

          • sympathetic stimulation

        • decrease stroke volume:

          • parasympathetic stimulation

          • hypoxia

      • Afterload - the pressure in large arteries the ventricles must work against in order to force blood movement

        • increase stroke volume :

          • increased vascular resistance

          • semilunar valve damage

        • decrease stroke volume:

          • decreased vascular resistance


  • Cardiac output is monitored and adapted by the body as needed

    • mechanoreceptors - sense stretch and send information to the nervous system (how much the aorta stretches)

    • chemoreceptors - monitor different chemicals that are in the blood (how much CO2, O2, pH, etc is in the blood)