What part of the horse's heart is responsible for pumping blood away from the heart?

The ventricles of the heart play a crucial role in the circulatory system of a horse, as they are the chambers responsible for pumping blood away from the heart. More specifically, the right ventricle pumps deoxygenated blood to the lungs for oxygenation, while the left ventricle pumps oxygenated blood out to the rest of the body through the aorta. The thick muscular walls of the ventricles enable them to generate sufficient pressure necessary to propel blood through the circulatory system.

The other components listed serve different functions: the atria collect blood returning to the heart; the valves ensure unidirectional blood flow and prevent backflow; and the aorta is a large artery that carries blood away from the heart but is not a chamber that pumps blood. Thus, the ventricles are distinctly responsible for the active process of pumping blood from the heart, making them the right answer in this context.