O2 transport in the horse during rest and exercise

We studied mechanisms of O2 transport in 6 adult (2-5 year old) horses at rest and during steady-state exercise on a treadmill (0% slope) at 12 m/s (a submaximal gallop). Oxygen consumption was measured using an open-flow system. Arterial and mixed venous blood samples were simultaneously obtained for measurement of O2 content and hemoglobin concentration. VO2 increased from 1.5 +/- 0.2 L/min at rest to 46.2 +/- 4.8 L/min during exercise. HR increased from a resting value of 36.9 +/- 2.5 bpm to 196.5 +/- 10.9 bpm and the arterio-venous O2 content difference (a-v O2) increased from 4.2 +/- 0.8 ml O2/100 ml blood to 20.3 +/- 1.6 ml O2/100 ml blood. The 30.4-fold increase in oxygen consumption in the horse at submaximal VO2 versus only a 10-fold increase in man at VO2 max demonstrates the marked ability of the horse to transfer O2 at each step in the O2 transport pathway.