Blood Transformation Across Pulmonary, Coronary, and Systemic Circulations

Blood undergoes sequential, purposeful changes as it traverses the three circulatory circuits.

1. Pulmonary Circulation (Right Heart → Lungs → Left Heart)

• Input: Mixed venous blood (SvO₂ ~70%, PaCO₂ 45 mmHg).
• Gas exchange: CO₂ offloaded, O₂ loaded at alveolar-capillary membrane.
• Output: Oxygenated arterial blood (SaO₂ >95%, PaO₂ 80–100 mmHg, PaCO₂ 35–45 mmHg).

2. Coronary Circulation (Left Main → Coronary Arteries → Coronary Sinus)

• Perfusion: Occurs mainly in diastole (aortic pressure drives flow).
• Extraction: Myocardium extracts 70–80% of delivered O₂ (highest in body).
• Venous return: Coronary sinus → RA (mixed with systemic venous return).
• Metabolic exchange: Lactate, CO₂, K⁺, adenosine released → local vasodilation.

3. Systemic Circulation (Left Heart → Aorta → Arteries → Capillaries → Veins → Right Heart)

• Delivery: O₂, nutrients, hormones to all tissues.
• Extraction: Variable by tissue (brain/heart high; skin/resting muscle low).
• Waste removal: CO₂, lactate, urea, heat.
• Return: Deoxygenated (SvO₂ ~70%) via SVC/IVC → RA.

Integrated Homeostasis

These circuits are in series: pulmonary output = systemic input. Cardiac output adjusts to metabolic demand via heart rate, contractility, preload, afterload. Baroreceptors, chemoreceptors, and local autoregulation maintain perfusion pressure and matching.

Disruption in any circuit (PE, MI, shock) cascades systemically.