A mathematical model of substantial exchange between three capillaries and the surrounding tissue is presented.We suppose that three capillaries, which are parallel to each other and have the same radius and circular section, are arranged in equilateral triangle.Fluid movement across the capillary walls is assumed to be governed by the generalized Starling’s law.The motion of the interstitial fluid obeys Darcy’s law.The calculated result obtained in this paper shows that in order to study the effect of capillary networks on the tissue space, as a whole, we have found a concept of equivalent pressure, concentration and source strength for the networks. A mathematical model of substantial exchange between three capillaries and the surrounding tissue is presented. We suppose that three capillaries, which are parallel to each other and have the same radius and circular section, are arranged in equilateral triangle. Fluid movement across the capillary walls is assumed to be governed by the generalized Starling’s law. The motion of the interstitial fluid obeys Darcy’s law. calculated result obtained in this paper shows that in order to study the effect of capillary networks on the tissue space, as a whole, we have found a concept of equivalent pressure, concentration and source strength for the networks.