There are few similarities between horses and automobiles, but in a manner of speaking, the horse's circulatory and respiratory systems constitute its engine. The food a horse consumes is its fuel. The fuel is converted into nutritional energy that powers the muscles. The respiratory system provides oxygen to facilitate metabolism, and the cardiovascular or circulatory system delivers the oxygen and nutrients to tissues as well as carrying off waste products that are produced when the "engine" is running.
Air passes through the nostrils and along a lengthy nasal cavity, then it passes over the larynx and pharynx. After passing through the trachea, the inspired air arrives at the bronchial trees within the lungs. Gas exchange occurs in the alveoli, where the circulatory system receives oxygen and delivers it to tissues throughout the body, along with nutrients absorbed from the digestive system.