Final answer:
The heart is the organ responsible for the long-distance transport of oxygen to cells in animals, as part of the circulatory system that includes a network of blood vessels.
Step-by-step explanation:
Among the options provided, the heart is the organ primarily responsible for facilitating the long-distance transport of oxygen to the cells of animals. This vital organ works in conjunction with the circulatory system to pump oxygen-rich blood, obtained from the lungs, to the various tissues and cells throughout the body. The circulatory system serves as an extensive network of blood vessels that delivers nutrients and oxygen to cells and removes waste products, including carbon dioxide.
While the brain, stomach, and mouth play important roles in other bodily functions, they are not directly involved in the transportation of oxygen. The lungs are where red blood cells pick up oxygen, which is then used by the heart to pump these oxygenated cells throughout the body. Animals have evolved various proteins like hemoglobin and myoglobin to effectively carry and store oxygen, ensuring that metabolic functions continue even during periods of increased demand, such as during physical activity.