Internal organs are heterogeneous structures, both on the macro- and on the micro-scale. However, they are often modeled as homogeneous solids with uniform material properties. In this light, this work investigates the impact of the liver capsule on the integral behavior of the organ by means of in vitro tests and computer simulations. The stiffness of bovine liver obtained in tissue aspiration experiments differed by a factor of 2 to 3 when the capsule was removed. As a first step, the capsule was implemented as separate structure in a finite element model of the organ undergoing tissue aspiration. The finite element simulations are in good agreement with the experimental results.