Blood sugar level
The sugar content in the blood, the blood glucose level, changes constantly: after horses have been fed, it is elevated. During exertion, it falls, for example since muscle cells consume blood sugar to generate energy.
Also because the brain and red blood cells cover their energy requirements exclusively via glucose, a slide into dangerously low or dangerously high ranges must be avoided. A number of regulating systems are responsible for keeping the blood sugar level constant.