Nutrients and wastes are transported one way or another between the two fluids. Therefore, there is constant motion in the body and the environment is constantly the same for cells in the body. Because of this, the ECF is often called the body's internal environment. Homeostasis may be effected by stress, which is any stimulus that creates an imbalance in the internal environment. External sources of stress include heat, cold, loud noises, lack of oxygen.
Output is the response, i.e., the return to normal.
The regulation of homeostasis comes about through feedback systems. Nerves send messages to and from organs responding to the stress when deviations in homeostasis are detected. Nerves work rapidly, i.e., in seconds. Hormones work slowly, i.e., they take minutes. Figure 10 illustrates an example of negative feedback.