Mechanisms include Increasing heart rate Increasing systolic and diastolic volumes Increasing stroke volume Increasing tissue extraction of oxygen (the difference between oxygen content in arterial blood and in mixed venous or pulmonary artery blood) In well-trained young adults during maximal exercise, heart rate may increase from 55 to 70 beats / minute at rest to 180 beats / minute, and CO may increase from 6 to ≥ 25 L / minute.