MDCAT Circulation & Blood 2026

Heart Structure

• 4 chambers: 2 atria + 2 ventricles. Right pumps to lungs (pulmonary); left pumps systemic.
• Valves: tricuspid (R atrium-ventricle), bicuspid/mitral (L), pulmonary semilunar, aortic semilunar.
• Pacemaker: SA node (R atrium); AV node; Bundle of His; Purkinje fibres.

Blood

• Plasma (55%) + cells (45%): RBC (no nucleus in mammals), WBC (5 types), platelets.
• Hemoglobin binds O₂ (and CO₂ as carbaminohemoglobin).
• Blood groups: ABO (codominance) + Rh (D antigen).

Blood Vessels

Arteries thick muscular wall, no valves, away from heart. Veins thinner with valves, toward heart. Capillaries: single endothelial layer, gas exchange.

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