The amount of oxygen used by muscle cells increase with exercise. The two body systems that influence the amount of oxygen delivered are the cardiovascular system and the pulmonary system. Two groups of patients were studied, those that were mildly hypertensive (but are not taking prescription medication for hypertension) and the control or normal group. The two groups exercised on a bicycle ergometer. Work rate was increased in steps (multiples of thirty watts) until anaerobic threshold (as measured by an ear oximeter) was reached. Heart rate, blood pressure, oxygen uptake and carbon dioxide production were measured during exercise. Their data were then fitted to a mathematical model developed by Kyuichi Niizeki, Tatsushisa Takahashi, and Yoshimi Miyamoto. The model consisted of three compartments (the lungs, muscle tissue involved in exercise, and the inactive tissues and organs). The assumptions made for the model are: (1) that the dynamic responses of pulmonary blood flow and the muscle VO₂ obey first-order kinetics; (2) the ratio of blood flow in the muscle compartment to total cardiac output was assumed to increase linearly with work rate; (3) the muscle compartment and the inactive tissue compartment were assumed to be connected in parallel to the lung compartment and perfused by separate regional circulations. The model is able to predict the volume of oxygen consumed by the cells as a function of time during incremental step exercise.