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. The body prefers to be kept around 37 C, more or less. When the body begins to heat up, signals are sent via nerve endings in the skin to the brain. The brain sends the signals to the cooling portion of the brain and signals are sent back to the skin. These signals stimulate sweat glands to secrete sweat, which causes the body to cool back down to normal.