What is the primary function of the digestive system?

The primary function of the digestive system is to break down food into smaller components, allowing for the absorption of nutrients, particularly fats, into the body while also facilitating the elimination of waste products. This process involves several organs working together to ensure that the body receives the essential nutrients it needs to function properly. The digestive system starts from the mouth, where food is mechanically broken down, and continues through the esophagus, stomach, and intestines, where the absorption of nutrients occurs. The remaining undigested matter is then formed into waste for elimination.

In contrast, the other options highlight functions that do not pertain directly to the digestive system. Respiration is primarily concerned with the intake of oxygen and release of carbon dioxide, which is not a function of digestion. The mobilization of joints is related to the skeletal and muscular systems, while the regulation of body temperature falls under the body's homeostatic mechanisms rather than digestion. Understanding these differences clarifies why the absorption of fats and elimination of waste products correctly identifies the core duties of the digestive system.