A close relationship exists between the local capillary density in different brain structures and their local blood flow and metabolism. Capillary density appears to have developed depending on local functional demands. Investigation of single capillary perfusion has shown that all capillaries are perfused with plasma in the brain at any time point. Theories of capillary cycling and capillary recruitment have been based on experimental artifacts. Indirect evidence exists for a heterogeneity of perfusion under normal conditions, especially with respect to erythrocyte flow. The capillary diffusion capacity depends on, among other things, the available capillary surface area, which would increase with recruitment of capillaries. In the case of capillary perfusion heterogeneity, the capillary diffusion capacity may also be increased by homogenization of the perfusion rate (slowly perfused capillaries becoming faster perfused). This could give a physiological impression of an "apparent" increase in the capillary surface area. It is recommended that the terms "capillary cycling" and "recruitment" should be used in conjunction with more specific explanations, like "recruitment of erythrocytes" and "recruitment of previously nonperfused capillaries".