ABSTRACT
The human body is built for physical activity, not for rest. Throughout the ages humans have had to expend great physical effort simply to survive; today, however, much of our life is sedentary. A human heart at rest pumps out blood at 4-5 1/min (8-11 US pints per minute); during physical effort the volume might increase to 10-40 l/min (21-84 pt/min). When breathing normally at rest 61 (0.2 ft3) of air is exchanged per minute, but the volume can increase to over 100 1/min (3.5 ft3/min) if necessary, and a well-trained athlete can exchange up to 200 l/min (7 ft3/min).
