Splanchnic tissues are a major part of the rapid distribution spaces of inulin, urea and theophylline.

Distribution of kinetics of inulin, [14C]urea and theophylline were studied in five anesthetized dogs after splenectomy and gastrointestinal resection. Distribution was modeled with three-compartment mammillary systems in which the central compartment corresponds to intravascular space and the two peripheral compartments have different rates of transcapillary exchange. Compared with results in intact dogs, the surgical procedure removed between 41 and 55% of the rapidly equilibrating tissues and reduced the permeability coefficient-surface area products for the rapidly equilibrating inulin and urea compartments proportionately. This is consistent with the concept that splanchnic organs equilibrate rapidly with inulin and urea because they are supplied by fenestrated and discontinuous capillaries that are prominent in the splanchnic vascular bed. However, splanchnic organs probably do not contain all rapidly equilibrating tissues, and somatic tissues may contribute as much as 36 and 22%, respectively, of the rapidly equilibrating inulin and urea compartments. Cardiac output averaged 2.87 +/- 0.86 liters/min and was similar to the sum of compartmental blood flows estimated from the intercompartmental clearances of urea and inulin (2.74 +/- 0.96 liters/min) and to the sum of theophylline intercompartmental clearances (2.62 +/- 0.74 liters/min). Theophylline intercompartmental clearance to each peripheral compartment was similar to estimated compartmental blood flow.