Cardiac Output

The following variables are measures of the capacity of the heart:
  • Stroke volume (SV) is the volume of blood ejected by each ventricle during a single contraction.

  • Heart rate (HR) is the number of heartbeats per minute.

  • Cardiac output (CO) is the volume of blood pumped out of the right or left ventricle per minute. CO = SV × HR.

Cardiac output varies widely with the health of the individual and the state of activity at the time of measurement. Cardiac output in exercising athletes may exceed their resting cardiac output seven times. The ratio between the maximum and resting cardiac output of an individual is the cardiac reserve. Note that cardiac output changes when either stroke volume or heart rate changes.

Stroke volume is regulated by the following three factors:

  • Preload is the degree to which cardiac muscle cells are stretched by the blood entering the heart chambers. According to the Frank‐Starling law of the heart, the more the chamber is stretched, the greater the force of its contraction. Because the end‐diastolic volume (EDV) is a measure of how much blood enters the ventricles, the EDV is an indicator of ventricle preload.

  • Contractility is the degree to which cardiac muscle cells contract as a result of extrinsic influences. Positive inotropic factors, such as certain hormones (epinephrine or thyroxin), drugs (digitalis), or elevated levels of Ca 2+, increase contractility, while negative inotropic factors, such as certain drugs (calcium channel blockers) or elevated levels of K +, decrease contractility.

  • Afterload is a measure of the pressure that must be generated by the ventricles to force the semilunar valves open. The greater the afterload, the smaller the stroke volume. Arteriosclerosis (narrowing of the arteries) and high blood pressure increase afterload and reduce stroke volume.

Heart rate is regulated by the following three factors:

  • The autonomic nervous system may influence heart rate when the sympathetic nervous system stimulates cardiac muscle contractions or when the parasympathetic system inhibits cardiac muscle contractions.

  • Chemicals such as hormones and ions can influence heart rate. Epinephrine, secreted by the adrenal medulla, and thyroxin, secreted by the thyroid gland, increase heart rate. Abnormal blood concentrations of Na +, K +, and Ca 2+ interfere with muscle contraction.

  • Other factors such as age, gender, body temperature, and physical fitness may influence heart rate.