Cardiac Output
Cardiac output is the amount of blood ejected from the heart.The amount of blood ejected by ventricles by each contraction is Stroke volume.Cardiac output is expressed in litres/minute.Cardiac output is obtained by multiplying heart rate(measeured in beats/minute) with stroke volume.
Cardiac output = Stroke volume x Heart Rate
In a healthy individual at rest , stroke volume is 70 ml (approximate)and if his heart rate is 72 beats/min then his stroke volume is 5 Litres/minute [ Cardiac output = 70ml (stroke volume) x 72( heart rate) ]
This can be increased 25 litres/minute during exercise and in athletes it may increase upto 35 litres/minute.This increase known as “cardiac reserve”
Factors affecting cardiac output
1.Ventricular end diastolic volume(VEDV)
2.Venous return
3. Blood Volume
4.Heart rate
1.Ventricular end diastolic volume(VEDV)
It is the amount of blood in the ventricles just before their contraction.increased VEDV leads to stronger contraction and volume of blood expelled is more Hence the cardiac output and stroke volume is more.
Increase in stroke volume with increase in VEDV is finite and after reaching the limit i.e cardiac output cannot match the venous return,cardiac output decreases and heart begins to fail
2. Venous return
Venous return means the flow of blood from the periphery back to the right atrium. cardiac output and venous return are interdependent .The contraction force of left ventricle pumping the blood in to aorta is not sufficient to return the blood through veins and back to the heart.
Factors involved in venous return are
a.Gravity – Effect of gravity on venous return is paradoxical because in standing postion right atrial pressure decreases and venous pressure in dependent limbs increases due to the effect of hydrostatic forces.
It increases the pressure gradient for venous return from the dependent limbs to the right atrium; but, venous return contradictorily decreases.
The reason is when a individual stands and before the baroreceptor reflex is activated, arterial pressure and cardiac output decreases because right atrial pressure and ventricular preload decreases, which causes fall in stroke volume by the Frank-Starling mechanism. The flow through the whole systemic circulation decreases as arterial pressure falls more than right atrial pressure, hence the pressure gradient driving flow throughout the circulatory system falls.
b.Muscular Contraction –Back flow of blood in viens of limbs especially is standing posture is prevented by valves.Rhythamic contraction of skeletal muscles around the deep viens during normal locomotory activity compresses the viens and pushes the blood towards heart. In lower limbs it is termed as Skeletal muscle pump.
c.Respiratory activity –During inspiration venous return increases temporarily due to decrease in right atrial pressure. Opposite process occurns in expiration.
3.Blood Volume
Blood volume is depends on amount of sodium and water ingested, excreted by the kidneys into the urine, and lost through the gastrointestinal tract, skin and lungs . To maintain blood volume at constant level kidneys regulate the amount of water and sodium lost into the urine.
If the blood volume is low -This results in decrease in the stroke volume , venous return ,and cardiac output .
4.Heart Rate
If heart rate increases cardiac output also increases. If heart rate falls , cardica output also falls.
Factors affecting heart rate are as follows
a.Autonomic nervous system – The rate at which the heart beats is a balance between sympathetic and parasympathetic activity.
Sympathetic nervous system(SNS) releases hormones catecholamines-epinephrine , nor epinephrine which increases heart rate
Parasympathetic nervous system(PNS) releases Acetylcholine hormone which reduces the heart rate.
b.Position – When the individual is upright the rate of heart beat is faster than when lying down.
c.Exercise- Physical exercise for a duration will increase heart rate and heart rate will remain elevated as long as physical activity is continued . At the beginning of exercise the body gets rid of parasympathetic stimulation which increases heart rate gradually.
d.Emotional condition – Fear ,anxiety ,excitement leads to increase of heart rate
e.Body temperature – A very important factor affecting exercise heart rate is temperature. Rise in body temperatures cause the higher heart rate and heart rate falls with fall in body temperature
Submitted by
Imroz.s.m , M.pharm
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