Blood Vascular System The blood vascular system is conceptually
divided into two compartments: the central pool and the marginal
pool. The marginal pool consists of the microcirculation, ie, at
the capillary–tissue interface. The central pool consists of
larger vessels. Blood samples taken by venipuncture are
inherently representative of the central pool. Flow rate, fluid
movement to the extravascular space, and selective WBC adhesion
to endothelium are factors that may contribute to marked
differences in cell concentration in the two pools. Furthermore,
these pools are in hemodynamic equilibrium with each other and
the extravascular space. Therefore, WBC concentration may change
appreciably due to movement of cells and/or fluid from one pool
to the other as a result of a change in equilibrium. In most
species, WBC are roughly equally distributed between the two
pools. Cats have a greater distribution of leukocytes within the
marginal pool. WBC in the circulating pool can be increased by
certain mechanisms: e