The routine determination of the oxygen flux into body's tissues (= resting oxygen uptake) should gain more attention in practice, because its magnitude can be considered a kind of characteristic value for the energetic state of the entire organism. The determination of this value has deserved special interest since we discouvered a "switch mechanism" of blood microcirculation, which depends on the oxygen state of the body. A high value of pO2 (greater than or equal to 50 mm of Hg) at the venous ends of the capillaries, attainable by the procedures of the Oxygen Multistep Therapy and by powerful physical exercise as well, results in an increase of the blood microcirculation and, consequently, in a permanent elevation of the oxygen influx and uptake, respectively. This effect can persist for weeks, months or even years. If the oxygen state gets worse and declines below a certain threshold, e. g. in progressing age or after long-term distress, the cross sections of the capillaries shrink by swelling of the endothelial cells, and the blood microcirculation is diminished for an extended period of time. The utilization of the above-mentioned switch mechanism for permanent improvement of the oxygen flux into all the tissues of the organism is therefore of decisive importance for fighting against the common cause of many diseases, disorders and complaints going often along with increasing age due to insufficient oxygen (energy) supply for general metabolism. As can be taken from measurements described in this paper, the main contribution to this deterioraction comes from the age-dependent diminution of the cardiac minute or stroke volume, respectively, since eta--the percent exhaustion of the oxygen-binding capacity of blood--increases even with progressing age, as was found by us. Numerous determinations of the arterial and venous pO2 resting levels have shown that the values of eta can almost be doubled easily by the procedures of the Oxygen Multistep Therapy, whereby--as a rule--the age-dependent decrease of the cardiac output is overcompensated. Within the schedule of the Oxygen Multistep Therapy, the third step prescribes a temporary enhancement of the blood circulation by physical exercise. With respect to this measure, there was made progress of principal importance by the idea to increase the minute volume by application of sympathicomimetics during the single treatment units under moderately enhanced oxygen inhalation, but with due consideration of the known contraindications.(ABSTRACT TRUNCATED AT 400 WORDS)