Pulmonary Circulation: Step-by-Step Blood Flow Through Heart and Lungs

Deoxygenated Blood → Lungs → Oxygenated Blood → Body

This cycle (pulmonary + systemic) occurs ~70 times/minute at rest.

Pulmonary Circulation (Right Heart → Lungs → Left Heart)

1. Superior/Inferior Vena Cava → deoxygenated blood from body → Right Atrium.
2. Tricuspid Valve → Right Ventricle.
3. Pulmonary Valve → Pulmonary Artery (only artery carrying deoxygenated blood).
4. Pulmonary Capillaries (alveolar interface): CO₂ diffuses out, O₂ diffuses in → binds hemoglobin.
5. Pulmonary Veins (only veins carrying oxygenated blood) → Left Atrium.
6. Mitral Valve → Left Ventricle.
7. Aortic Valve → Aorta → systemic arteries.

Systemic Circulation (Left Heart → Body → Right Heart)

1. Aorta → arteries → arterioles → systemic capillaries: O₂ delivered, CO₂ picked up.
2. Venules → veins → Superior/Inferior Vena Cava → back to Right Atrium.

Key Pressure Gradients

• Right ventricle: 25/5 mmHg (systolic/diastolic)
• Pulmonary artery: 25/10 mmHg (low resistance)
• Left ventricle: 120/80 mmHg (high resistance systemic)

Clinical Pearls

• Right heart failure → elevated JVP, peripheral edema, hepatomegaly.
• Left heart failure → pulmonary edema, dyspnea.
• Shunts (ASD, VSD, PDA) alter flow patterns.

The heart is a dual pump in series: right side handles low-pressure pulmonary circuit; left side powers high-pressure systemic circuit.