When one is exposed to microgravity, fluid which is normally pooled in the lower extremities is redistributed headward and weight bearing bones begin to demineralize due to reduced mechanical stresses. The kidney, which is the primary regulator of body fluid volume and composition, responds to the fluid shift and bone demineralization by increasing the urinary output of water, sodium, and calcium. This research involves developing a mathematical description of how water and electrolytes are internally redistributed and exchanged with the environment during space flight. Various levels of activity are also considered, since it is believed that exercise can prevent some of the undesired effects of space flight such as muscle atrophy and bone demineralization. This model involves mathematically describing kidney function and the associated endocrine system. The model agrees well with actual data, including that a low sodium diet can prevent bone demineralization, therefore, assumptions made to develop the model are most likely valid. Since a life support system is responsible for controlling the environment, results from the model can aid in the design and development of such systems.