Cardiac Output Control

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This answer is oriented to dentistry students.

Cardiac Output[edit | edit source]

is the amount of blood pumped by the heart per unit of time and it depends on

  1. Stroke volume (SV) = the amount of blood ejected per beat of the heart (Normally, 70 ml/beat)
  2. Heart rate (HR) = heart beats per minute (normally, 70 beats/min)

Heart Rate[edit | edit source]

  • Varies between about 60 to 180 (200) beats/minute
  • Marked increases in Heart Rate cause a proportional decrease in diastolic filing time which could result in lower volume of blood ejected (unless venous return is increased)
  • Elevated Heart Rate can increase contractility, independent of sympathetic effect
  • Trained individuals can maintain the same cardiac output during rest and exercise with lower Heart Rate (bradycardia)
  • Sympathetic stimulation causes increase of Heart Rate
  • Parasympatheric stimulation causes decrease of Heart Rate
  • In experiments, an increase in pressure in the atria (particularly in LA) produces tachycardia (Bainbridge reflex) which is a 100% Vagal (Parasympathetic) reflex

Stroke Volume[edit | edit source]

  • Stroke volume of the left and right chamber over any significant period of time are the same
  • In the Right Ventricle (consider it a segment of a sphere thus V= 4/3πR3) relatively large volumes of blood are ejected as a result of small changes in fiber length. At the same time, the pressure is low.
  • The Left Ventricle is thick-walled (consider it a cylinder thus V=πR2h), so greater fiber shortening must occur to eject the same volume of blood, but the pressure reached in higher
  • Exercise induced hypertrophy
    • causes an increase of the heart’s capacity to generate a large stroke volume

Frank-Starling Mechanism of the heart[edit | edit source]

  • The length of cardiac fibers at the end of the diastole determines the stroke volume independently of neural regulation
  • When the muscle length is stretched prior to contraction (the end diastolic volume is larger), the heart contracts more forcibly (thus increasing stroke volume)
    • Neural (sympathetic & parasympathetic) and humoral (epinephrine) factors can substantially influence this parameter of the heart activity
  • When the muscle length is stretched beyond at critical point, the developed force in decreased
  • The end diastolic volume is determined by
  • Ejection of Blood is determined by

Efficiency of the Heart[edit | edit source]

  • The external work done by the heart is much less than the energy equivalent of the oxygen consumed by the myocardium
  • The greater part of the energy is used on internal work that is dissipated in heat
  • The ratio of useful work performed to energy requirement is referred to as the efficiency which is about 10-20%
  • Decreased when heart when there is increased resistance
  • Increased when stroke volume increases

Mechanism of the increase in the arterial pressure during exercise[edit | edit source]

  • Due to exercise, the metabolism in the active skeletal muscles causes the local muscle arterioles to relaxing allowing an influx of nutrients and oxygen for the tissue
  • This greatly decreases the total peripheral resistance, which should decrease the arterial pressure too. This does not happen since the nervous system instantly compensates.
  • The brain activity that send the signals to the skeletal muscles simultaneously activates the autonomic nervous system centers of the brain to stimulate the circulatory system, initiating
  • These actions cause an increase in the arterial pressure above normal, which causes increased blood flow through the active muscles

References[edit | edit source]

Prof. Jaroslav Pokorný Lecture & Notes