Cardiac Output and Its Determinants

Cardiac Output

• Cardiac output is the amount of blood pumped by each ventricle per minute.

• It is calculated as the product of stroke volume and heart rate.

  • Cardiac Output = Stroke Volume \times Heart Rate

• An increase in either stroke volume or heart rate leads to an increased cardiac output, and vice versa.

• During physical exercise, the heart rate increases to meet the body's higher demand for blood.

Stroke Volume

• Stroke volume is the amount of blood pumped by each ventricle per heartbeat.

• Ventricles do not eject all the blood they contain during a beat.

• For example, a ventricle might contain 100 ml of blood (end-diastolic volume) but eject only 60 ml.

• Ejection Fraction:

  • Ejection fraction is the percentage of blood ejected from the ventricle with each contraction.

  • In the example above, the ejection fraction is 60%.

  • Ejection Fraction = \frac{Stroke Volume}{End-Diastolic Volume}

End-Diastolic Volume (EDV)

• The amount of blood in the ventricle at the end of its filling phase (diastole).

• In the example, EDV = 100 ml.

End-Systolic Volume (ESV)

• The amount of blood remaining in the ventricle after contraction (systole).

• In the example, ESV = 40 ml.

Stroke Volume Calculation

• Stroke volume is the difference between EDV and ESV.

• Stroke Volume = EDV - ESV

• In the example, Stroke Volume = 100 ml - 40 ml = 60 ml.

Factors Affecting Stroke Volume

• Stroke volume depends on three main factors: contractility, preload, and afterload.

Contractility

• Contractility is the force of the heart muscle's contraction.

• A more forceful contraction results in more blood ejection.

Preload

• Preload is the degree of stretch of cardiac myocytes at the end of ventricular filling.

• It is often represented by the end-diastolic volume (EDV) because direct measurement of myocyte stretch is difficult.

• The greater the EDV, the more the ventricular wall is stretched.

• Frank-Starling Mechanism: The greater the stretch (preload), the greater the force of contraction.

• Analogy: Like a balloon, the more it's inflated, the more forcefully it releases air when deflated.

Afterload

• Afterload is the resistance the ventricle must overcome to eject blood.

• Major components of afterload:

  1. Vascular Pressure:

     • The pressure in the left ventricle must exceed systemic pressure for the aortic valve to open.

     • The pressure in the right ventricle must exceed pulmonary pressure to open the pulmonary valve.

     • In hypertension, higher vascular pressures increase afterload, making it harder for the valves to open and reducing the amount of ejected blood.

  2. Valve Damage:

     • Conditions like valve stenosis increase resistance, leading to lower blood output.