Cardiac Output Control

This answer is oriented to dentistry students.

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)

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

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

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

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

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