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Oxygen Saturation in the Heart: Why the Right Side Is 'Blue' (Low O2) and Left Side 'Red' (High O2)

Right Heart (Systemic Venous Return)

  • Receives deoxygenated blood from SVC/IVC (body) and coronary sinus.
  • Mixed venous O2 saturation (SvO2) ≈ 70–75% (not 5% — that is O2 extraction; arterial is ~95–98%, venous ~70–75%).
  • Pulmonary artery carries this blood to lungs.

Lungs

Gas exchange: O2 diffuses into pulmonary capillaries, binds hemoglobin → saturation rises to 95–100% (PaO2 80–100 mmHg).

Left Heart (Oxygenated Blood)

  • Pulmonary veins → left atrium → left ventricle → aorta.
  • Systemic arterial saturation (SaO2) ≈ 95–100%.

Why the "Blue/Red" Model?

Anatomical diagrams color deoxygenated pathways blue, oxygenated red. In reality, venous blood is dark maroon; arterial blood is bright red. The 5% figure in the original text likely reflects a misunderstanding — mixed venous O2 content is ~15 mL/dL vs arterial ~20 mL/dL (25% extraction), not 5% saturation.

Clinical Relevance

  • Cyanosis appears when reduced hemoglobin >5 g/dL (~85% saturation).
  • Right-to-left shunts (e.g., Tetralogy of Fallot, Eisenmenger) lower systemic saturation.
  • Pulse oximetry measures peripheral SpO2 (surrogate for SaO2).

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