A computer simulation model of the human metabolic system describing the massflow rates of the input and output metabolic elements was developed. The input metabolic elements consist of oxygen, dry food, and water, while the output elements include carbon dioxide, feces, urine and insensible water. Energy expenditure rate was introduced as the fundamental quantity, and its dependence upon the body mass and energy activity category was modeled. Four energy activity categories, i.e., work, exercise, leisure, and sleep, were considered. All the metabolic massflow rates were expressed in terras of the energy expenditure rate by means of linear relations. Integration and demonstration of the model of the metabolic system was accomplished using a scenario of daily distribution of energy activity categories and dry food composition for a given body mass. The results indicate the sensitivity of the calculated input/output metabolic massflow rates to the body mass and the selected scenario.