Flow-metabolism coupling in brain is different from flow-metabolism coupling in other vascular beds. In the classic description of Krogh, the capillary bed is a system of parallel tubes serving cylinders of tissue known as Krogh's cylinders. This simple arrangement yielded a quantitative expression of oxygen delivery to the tissue. However, in brain tissue, the arrangement is so disorderly that no prediction of oxygen tensions in the tissue is possible. Only two claims of the capillary bed in the brain appear to be indisputable, i.e., the capillaries have a common arterial source and a common venous terminus, and their density is proportional to the average regional rates of metabolism at steady-state. The following revision of the mechanism of flow-metabolism coupling in brain arose from the simple assumption, first introduced by Erwin R. Weibel in The Pathway for Oxygen, that every segment of the capillary bed "feeds" the same amount of brain tissue, i.e., that every fraction of the tissue is served by commensurate fractions of capillary density and oxygen diffusibility and accounts for the same fraction of the total oxygen consumption.