The impact of gravitational loading on the cardiac shape is studied numerically through application of two different constitutive models for the cardiac tissue. Transversely isotropic and orthotropic cardiac materials models are incorporated into a finite element heart model that includes a realistic 3D cardiac geometry based on detailed cardiac fiber and sheet micro-architecture. The models are used to predict the cardiac sphericity ratio for different gravitational loadings of Earth, Mars, Moon, and Microgravity. Both the transversely isotropic and the orthotropic material models correctly predict the linear dependence of cardiac sphericity on the gravitational force. However, in comparison to the experimental results, only the orthotropic model predicts the rate and magnitude of the cardiac shape change with gravity accurately. The transversely isotropic tissue model produces a stiffer heart that significantly under-predicts the ultrasound measurements of heart shape change during parabolic flight experiments.