Further drive to ventilation comes from lactic acidosis. This has been shown to stimulate chemoreceptors. Heart rate continues to rise with increasing severity of exercise, but SV levels off when exercise reaches half maximum. SV is usually less than doubled. Together the increase in HR and SV gives an increase in Cardiac Output of 5 fold .
Active muscle increases oxygen extraction.
Furthermore, at low tissue PO 2 the Hb dissociation curve is steep, and larger amounts of O 2 are liberated per unit drop in PO 2 . HR and O 2 consumption increase together up to a point, but O 2 consumption levels off off as heart rate approaches maximum. A healthy subject is limited not by ventilation but by his capacity to transport oxygen to the tissues. This is why trained athletes with a low resting heart rate can do so well. Anaemia reduces oxygen carrying ability of blood, and anaemic subjects work capacity is thus limited.