What general benefit does the spleen provide during exercise in horses?

The spleen plays a crucial role in enhancing the oxygen-rich blood supply during exercise in horses. When a horse exercises, the spleen contracts and releases red blood cells into the bloodstream. This release increases the oxygen-carrying capacity of the blood, which is vital for sustaining physical performance and endurance. The additional red blood cells allow more efficient oxygen delivery to the muscles, helping them function optimally. This function is particularly important during intense activities, as the muscles require more oxygen to generate energy.

The other options, while they pertain to important physiological processes, do not directly relate to the spleen’s specific role during exercise. For instance, calming the nervous system is more associated with the brain and central nervous system functions. Muscle flexibility is influenced by joint and muscle health, not directly by the spleen, and hormone regulation primarily involves endocrine glands rather than the spleen. Hence, the role of the spleen in supporting oxygen-rich blood supply is the best answer in the context of exercise in horses.