Any system where there is a constant exchange of matter and energy from the surrounding environment is called an open system. This makes the human body itself an open system because there is a continuous input of essentials such as oxygen and water as well as output of carbon dioxide and waste. Essential inputs and outputs also include the body’s specific responses (via afferent neurons) to stimuli (as perceived by sensory neurons).
The nervous system is an important key in this process. It is driven by the brain to sense the state of the body and act upon it in order to maintain homeostasis. The response may be controlled by the conscious mind, such as visually observing one’s hair out of place and making the effort to fix it. Or it may be an action of the subconscious mind, like an increase in heart rate when facing a stressful situation. Regardless of the change, the nervous system is always working to maintain equilibrium in the body. Without it, our internal systems would be launched into complete mayhem after just one stressful event and this could ultimately cause irreversible damage.