The INTEGUMENTARY SYSTEM is the largest organ system of the body that consists of the skin, hair, nails, glands and nerves. It functions as a barrier to protect the body from the external environment. It also regulates body temperature which is achieved through its homeostatic relationship with the digestive system that includes the digestion and assimilation of dietary fats and oils since they can be deposited to the dermis.
Therefore, the system to which the digestive system forms a homeostatic relationship is the INTEGUMENTARY SYSTEM.