Modeling exchange in capillary beds

Analysis of the handling of a material by the tissues of an organ after injection into the arterial inflow requires models of the functional units as well as of the distribution of the blood flow to the units. The major models of functional units are reviewed as are the effects of transit time distributions in the large vessels and capillaries. Multiple-indicator methods generally do not provide sufficient information to solve the inverse problem, i.e., to calculate the kinetics of handling the injected material in the tissue from the venous outflow curves. However, the use of simple, plausible assumptions allows one to extract considerable information from the venous outflow curves. These assumptions are 1) independence of the probability density functions of transit times for the arteries, veins, and capillaries, and 2) uniformity of handling of materials that do not leave the capillaries.