1. The ratio of the surface of the capillary bed to the volume of tissue supplied by the vessels (S/V ratio) for both open and closed capillaries in fat-rich tissue of the rat is 51.9, in fat-poor tissue of the same sort 222.2. About one-half of the capillaries in fat-rich tissue, to one-fourth in fat-poor tissue, are open during ordinary activity. The total capillary bed of fat-rich tissue is one-third as great as in muscle; the total capillary bed of fat-poor tissue has about the same density as that of the most poorly supplied muscle. This establishes the fact quantitatively that the capillary bed of fat is relatively inadequate, compared to other tissues, for transferring inert gases from fat tissue at a sufficiently rapid rate to prevent the occurrence of extravascular bubbles following rapid decompression from high pressure atmospheres. It also explains the greater distention of the blood vessels in fat tissue, due to gas, than in any other tissue following decompression. The observations have a bearing also on the estimation of the permeability of the blood-fat barrier to inert gases.
2. The volume of protoplasm of fat cells may be very small; a method is presented for estimating it quantitatively. Since it alone is important in metabolism, recalculation of the basic data on a basis of the ratio of surface area of capillaries to volume of protoplasm of fat cell in fat tissue yields a more useful figure. For fat-rich fat tissue S/V (protoplasm) = 2159.0 (for total capillary surface) or 977.6 (for open capillary surface). This means that for purpose of metabolism, the capillary bed is far richer than that of muscle.