The forgotten organs
By the time a horse crosses the finish line in a five-furlong race, has completed a Grand Prix show jumping round, or gone one-sixth of the way round a 3-star cross-country course, he will have moved somewhere around 1,800 liters of air in and out of the lungs. If you find 1,800 liters hard to visualize, then think of six bathtubs full of air. This equates to moving two five-gallon buckets of air into and out of the lung every second.
The air inhaled during a race will consist of around 380 liters of oxygen (the rest being made up of the gas nitrogen). The horse will take up into the blood and use around a quarter of this oxygen, i.e., 95 liters.
Of the total amount of energy the racehorse needs to get from the starting gate to the finish in the five-furlong race, around 70% will come from aerobic (oxygen-based) metabolism. The same can be said for the show jumping horse (70%) and a horse completing the cross-country portion of a three-day event (90%).
Aerobic metabolism is essentially the process of using oxygen to get energy from glucose (carbohydrate) in structures inside the muscle cells known as mitochondria. The remainder of the energy comes from anaerobic metabolism, which also breaks down glucose to generate energy. The anaerobic process can work whether oxygen is present or not.
The main difference here is the anaerobic system is very fast, but inefficient, and it can be used for only a short period of time due to the build up of lactic acid. Aerobic metabolism is not so fast, but it is very efficient at generating the energy to run.
So, even in a race or jumping round lasting less than a minute,