How do you determine the body volume of a horse? The only way I can think of is to immerse the horse in water and see how much water is displaced. There would be a significant difference between the horse in an exhaled state vs. the horse in an inhaled state.
A more likely measurement is picograms per liter of blood or picograms per kilogram (mass of the horse).
sorry, autocorrect, that's supposed to be BLOOD volume.