Figure 1.

Schematic representation of the systemic circulation as a network of resistances. A. Basic elements of the systemic circulation. The pressure gradient between arterial pressure P_A generated by the left heart and venous pressure P_V drives blood through a network of blood vessels, consisting of the arteries, the microcirculation and the veins. Vascular segments are indicated by zigzag symbols, as in electrical circuits. The pulmonary circulation (not shown) has the same overall structure. B, C. Hemodynamic interactions within a network of resistances, with flow driven by a pressure difference P₁ − P₂. Arrays of dots signify additional levels of branching in the network. B. Increased flow resistance in one segment (*) (e.g. due to constriction or occlusion) causes a decrease in flow along all flow pathways containing that segment (dashed lines). C. Decreased flow resistance along one flow pathway (*) (e.g. due to formation of a shunt pathway) causes increased flow on that pathway (heavy black lines) but reduced flow on parallel pathways (dashed lines).