A mathematical model of a biological system, the first in a series of simulations which will become successively more complex and, hence, more realistic representations of the biological system. The paper demonstrates a method for simulating complex chemical equilibria and uses the respiratory function of the blood at the lung surface as an example. The analog computer is used because its characteristic parallel computation and its fast solution-time enable the simulation of dynamic systems in real time. The results obtained for a small model indicate that the accuracy and stability are sufficient for analysis within the laboratory experimental error. The method is flexible, and basic models may be expanded to incorporate more complex phenomena.