Two pumps in series, each through own resistance in connecting vessels ('right heart/left heart', common septum and pacemaker).

Blood pressure higher on arterial side, lower on venous; thus arterial resistance greatest (ca. 40% in arterioles, 30% capillaries).

Symmetrical arrangement of components -

- decreasing, then increasing diameter

- decreasing, then increasing length

- increasing, then decreasing number

- increasing, then decreasing x - sectional area

- increasing, then decreasing surface area

Pattern of number and surface area consistent with exchange function.

Pattern of x-sectional area dictates decrease, then increase in blood flow velocity (i.e. reciprocal relationship).

Blood volume greatest on venous side (55 - 65%), nil in capillaries.

Blood O₂ concentration; high on arterial side, low on venous (a-v O₂ difference).