Extra Vehicular Activities (EVA) are very safety and mission critical. Therefore the thermoregulation and cardio-pulmonary responses of an astronaut to the environmental constraints during EVA have to be simulated to support various activities: human physiology research, procedures definition, astronaut training, space suit design, and prediction of astronaut status. A software model allowing the simulation of all these human physiological functions has been developed. The model has been connected to a draft model of the space suit and to various environmental conditions, allowing evaluation and testing. The paper presents the method and assumptions made to define the various algorithms of the mathematical models. Results from the test and validation sessions, which were based on the comparison with real data acquired during actual experiences, are also covered.