Most multicellular organisms have most of their cells protected from the external environment, having them surrounded by an aqueous internal environment. This internal environment must be maintained in such a state as to allow maximum efficiency. The ultimate control of homeostasis is done by the nervous system. Often this control is in the form of negative feedback loops. Heat control is a major function of homeostatic conditions that involves the integration of skin, muscular, nervous, and circulatory systems.
Multicellular organisms have a series of organs and organ systems that function in homeostasis.
Changes in the external environment can trigger changes in the internal environment as a response. Most physiological systems in the body use feedback to maintain the body's internal environment. Endocrine and nervous systems are the major control systems in higher animals. The nervous system depends on sensors in the skin or sensory organs to receive stimuli and transmit a message to the spinal cord or brain. Sensory input is processed and a signal is sent to an effector system, such as muscles or glands, that effects the response to the stimulus.