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Understanding Blood Circulation: How the Circulatory System Works

The circulatory system is a closed, continuous loop delivering oxygen and nutrients to tissues while removing carbon dioxide and waste. The heart acts as a dual pump driving two serial circuits.

Heart Chambers & Valves

ChamberReceivesPumps ToValve
Right Atrium (RA)SVC (upper body), IVC (lower body), Coronary sinusRight VentricleTricuspid
Right Ventricle (RV)RAPulmonary Artery → LungsPulmonic
Left Atrium (LA)4 Pulmonary VeinsLeft VentricleMitral
Left Ventricle (LV)LAAorta → SystemicAortic

Pulmonary Circuit (Right Heart → Lungs → Left Heart)

  1. Deoxygenated blood (low O₂, high CO₂) enters RA via SVC/IVC.
  2. RA contracts → blood through tricuspid valve → RV.
  3. RV contracts → blood through pulmonic valve → pulmonary artery.
  4. Pulmonary artery branches → pulmonary capillaries (alveoli).
  5. Gas exchange: CO₂ diffuses out, O₂ diffuses in (hemoglobin binding).
  6. Oxygenated blood → pulmonary veins → LA.

Systemic Circuit (Left Heart → Body → Right Heart)

  1. LA contracts → blood through mitral valve → LV.
  2. LV contracts → blood through aortic valve → ascending aorta.
  3. Aorta → arteries → arterioles → capillaries (exchange site).
  4. O₂ delivery to tissues; CO₂/waste pickup.
  5. Deoxygenated blood → venules → veins → IVC/SVC → RA.

Key Hemodynamic Principles

  • Pressure gradient: Mean arterial pressure (MAP) ~90 mmHg → right atrial pressure ~2-5 mmHg drives flow.
  • Flow = ΔP / Resistance (Poiseuille's law: R ∝ ηL/r⁴). Arterioles = main resistance vessels.
  • Cardiac output (CO): HR × SV. Normal ~5 L/min at rest; up to 25 L/min in athletes.
  • Autoregulation: Tissues match flow to metabolic demand (myogenic, metabolic, neural, hormonal).

Special Circulations

  • Coronary: Perfused during diastole (aortic root pressure).
  • Hepatic portal: GI → liver (nutrient processing) → IVC.
  • Fetal: Placental gas exchange; foramen ovale, ductus arteriosus, ductus venosus shunt blood.

Regulation

  • Baroreflex: Carotid sinus/aortic arch → medulla → HR, contractility, vascular tone.
  • Chemoreflex: Carotid bodies (O₂, CO₂, pH) → respiratory & cardiovascular drive.
  • Hormonal: RAAS, ADH, ANP, catecholamines.

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