Heart as a Factory

Cardiology often compares the heart to a factory, a system with inputs and outputs. The inputs arrive via the veins, while the outputs leave through the arteries. Just as a factory’s production can be calculated by multiplying the number of employees by their productivity, the heart’s performance can be quantified by multiplying the heart rate (beats per minute) by the stroke volume (the amount of blood ejected per beat). This yields the cardiac output.

Cardiac Output and Cardiac Index

• Cardiac output (CO): HR × Stroke Volume (SV)

  • Normal ≈ 5 L/min

• Both the right and left ventricles pump about 5 L/min, circulating the entire blood volume once per minute.

• Cardiac index (CI): CO ÷ Body Surface Area (BSA)

  • This normalizes output across body sizes, allowing fairer comparisons.

The Two Hearts

• Right heart: Pumps deoxygenated blood via the pulmonary artery to the lungs.

• Left heart: Pumps oxygenated blood via the aorta to the systemic circulation.

• All valves have three cusps except the mitral valve, which has two.

Output and Pressure

• Output drives arterial blood pressure.

• Blood pressure (BP) = Cardiac Output × Total Peripheral Resistance (TPR).

• Resistance is inversely related to vessel radius (to the fourth power).

  • Vasoconstriction → ↑ resistance → ↑ BP

  • Vasodilation → ↓ resistance → ↓ BP

Key Volumes and Fractions

• End-Diastolic Volume (EDV): Volume in ventricle at the end of filling (diastole).

• End-Systolic Volume (ESV): Volume left after contraction.

• Stroke Volume (SV): EDV – ESV

• Ejection Fraction (EF): SV ÷ EDV (normal ≈ 60%)

  • Absolute measure: SV (e.g., 60 mL)

  • Relative measure: EF (e.g., 60%)

Determinants of Cardiac Output

1. Preload (Venous return/EDV):

   • ↑ Preload → ↑ Stroke Volume → ↑ Cardiac Output

   • Enhanced by fluids, pregnancy, venoconstriction, muscle pump, negative intrathoracic pressure, or atrial contraction.

   • Decreased by pericardial effusion, tamponade, constrictive pericarditis, restrictive cardiomyopathy.

2. Afterload (Arterial resistance):

   • ↑ Afterload (e.g., vasoconstriction) → harder to eject blood → ↓ Stroke Volume → ↑ ESV → ↓ Cardiac Output.

3. Contractility (Inotropy):

   • Stronger contraction → ↑ SV, ↑ EF, ↓ ESV → ↑ Cardiac Output.

4. Heart Rate (HR):

   • Mild increase → ↑ CO.

   • Extreme tachycardia → ↓ filling time → ↓ EDV → ↓ SV → ↓ CO.

Clinical Correlations

• Pregnancy: Plasma volume rises, preload increases, HR increases → higher cardiac output.

• Pathology: Stiff ventricles, increased afterload, or impaired contractility can reduce cardiac output.

Parallel Concept in Lungs

• Minute Ventilation = Respiratory Rate × Tidal Volume

• Same principle as Cardiac Output = HR × SV

Summary

• Input = Venous return

• Output = Arterial flow

• Preload = Venous (input)

• Afterload = Arterial (output resistance)

• Cardiac output is central to blood pressure, tissue perfusion, and overall cardiovascular health.

For further exploration, cardiac output dynamics are directly relevant to conditions like arrhythmias, myocardial infarction, stroke, and pregnancy-related cardiomyopathies.