Blood Circulation: Why Blood Goes to the Lungs First
The Continuous Loop
Blood does not "skip" the body—it flows in a continuous figure-eight circuit:
- Pulmonary circulation: Right atrium → right ventricle → pulmonary arteries → lungs (CO₂ off, O₂ on) → pulmonary veins → left atrium.
- Systemic circulation: Left atrium → left ventricle → aorta → systemic arteries → capillaries (O₂/nutrient delivery, CO₂/waste pickup) → systemic veins → right atrium.
Why Lungs First?
Deoxygenated blood returning from the body must pick up oxygen in the lungs before it can supply tissues. The heart acts as a dual pump: right side for pulmonary, left side for systemic. Both circuits operate simultaneously.
Key Points
- One circuit cannot function without the other.
- Transit time: ~1 minute for full circuit at rest.
- Shunts (fetal: foramen ovale, ductus arteriosus) close after birth; persistence = pathology.
Understanding this dual-circuit design clarifies why lung disease affects systemic oxygenation and why heart failure backs up into lungs or periphery.
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