Stroke Volume, Cardiac Output + Ejection Fraction

Heart Rate (HR)

Heart undergoes an average of 60–80 cardiac cycles or beats per minute

Stroke Volume (SV)

Amount of blood pumped in one heartbeat

May range from 50–120 ml, but is difficult to measure directly

Subtract the amount of blood in the ventricle at the end of a contraction (ESV) from the amount of blood in the ventricle after it has filled during diastole (EDV)

ex. EDV – ESV = Stroke Volume (SV)

120 ml – 50 ml = 70 ml

Cardiac Output (CO)

Amount of blood pumped into the pulmonary and systemic circuits in one minute

Multiply heart rate by stroke volume

ex. HR × SV = Cardiac Output (CO)

72 beats/min × 70 ml/beat = 5040 ml/min (~5 liters/min)

Cardiac Output of the Right and Left Ventricles

Right and left ventricles pump about 5 liters into the pulmonary and systemic circuits each minute, which is the body’s entire supply of blood

Ejection Fraction

Percentage of blood that is ejected with each ventricular systole

equal to the stroke volume divided by the EDV

normal ejection fraction is 50–65% and should be equal for each ventricle

Factors That Influence Stroke Volume

Three factors influence stroke volume:

Preload imposed on the heart before it contracts

Heart’s contractility, or ability to generate tension

Afterload against which the heart pumps as it contracts

Preload: Degree of Cardiac Muscle Cell Stretch

Determined by the EDV, or amount of blood that has drained into the ventricle by the end of the filling stage

Influenced by the length of time the ventricle spends in diastole and the amount of blood returning to the right atrium from the systemic circuit, called Venous Return

Frank-Starling Law

The more the ventricular muscle cells are stretched, the more forcefully they contract

Stretching forms more crossbridges between actin and myosin and more calcium ions enter the cytosol

important during exercise to meet the body’s needs

Contractility: Heart’s Pumping Ability

Ability to generate tension in the absence of external forces, such as stretch

Can be estimated clinically by examining velocity of blood being ejected from ventricles

increasing contractility causes more blood to be ejected each cardiac cycle and increases stroke volume

Inotropic Agents

Agents that affect contractility

Physiological processes, such as sympathetic nervous stimulation, often affect contractility, increasing the force of contraction

Afterload: Amount of Back Pressure

Refers to the force that the ventricles must overcome in order to eject blood into their respective arteries

Determined by the blood pressure in the arteries in the pulmonary and systemic circuits

If afterload increases, ventricular pressure must increase to exceed pressure in the vessels and open the semilunar valves

Causes a decrease in stroke volume, which leads to a rise in the ESV of the ventricles

High Stroke Volume

High preload leads to a high EDV which stretches the muscle cells

High contractility causes a more forceful contraction which isn’t pumping against a lot of resistance

results in a high stroke volume

Low Stroke Volume

Low preload leads to a low EDV which reduces stretch of the muscle cells

Low contractility causes a less forceful contraction which is trying to pump against a lot of resistance

results in a low stroke volume