Cardiac Output Control

This answer is oriented to dentistry students.

Contents 1 Cardiac Output 2 Heart Rate 3 Stroke Volume 4 Frank-Starling Mechanism of the heart 5 Efficiency of the Heart 6 Mechanism of the increase in the arterial pressure during exercise 7 References

edit Cardiac Output

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)

edit Heart Rate

• 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

edit Stroke Volume

• 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

edit Frank-Starling Mechanism of the heart

• 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
  • Central Venous Pressure (CVP)
  • Compliance of the ventricles
  • Filling time
• Ejection of Blood is determined by
  • Arterial Pressure
  • Contractility of myocardium

edit Efficiency of the Heart

• 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

edit Mechanism of the increase in the arterial pressure during exercise

• 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
  • Increased heart rate
  • Large vein constriction
  • Increased contractility of the heart
• These actions cause an increase in the arterial pressure above normal, which causes increased blood flow through the active muscles

edit References

Prof. Jaroslav Pokorný Lecture & Notes