Consider how you heat and cool your home. You have heating and air conditioning systems. When the temperature outside is hot, the air conditioner cools your home. Conversely, when the temperature outside is cold, the furnace heats your home.
The goal of these systems is to maintain the inside of your home at a constant, comfortable temperature despite changes in the temperature outside.
This state of constant conditions can be called equilibrium, or steady state. In this example, the state of equilibrium is a constant temperature inside the house.
Similarly, your body has systems in place to maintain a steady state of its internal environment for many different parameters, like body temperature, or the level of glucose in your blood. The ability of an organism to maintain a constant internal environment in response to environmental changes is called homeostasis.
Like the heating and air conditioning systems in the house, your body has systems that respond to changes in the environment to maintain homeostasis. These mechanisms are called feedback mechanisms.
Focus on Feedback Mechanisms
1. Negative feedback
Now consider what happens when hot weather warms a house. The temperature inside becomes out of balance. It rises above the set point of the thermostat. This increase in temperature triggers the the thermostat, which turns on the air conditioner. The air conditioner cools the house until it reaches the set temperature point, and then shuts off. The house is now back to its original temperature.
This is an example of negative feedback. In negative feedback, the mechanism acts in the opposite direction of the initial change so that the steady state is restored. Most homeostatic mechanisms are negative feedback.
2. Positive feedback
Now suppose a HVAC technician crossed the wires in the system. The wires were crossed so that when the house temperature increased it triggered the furnace instead of the air conditioner. The furnace heated the house, which increased the temperature further. The increased temperature continued to keep the furnace on, which heated the house even further. You can see from this scenario that the house gets increasingly hotter, rather than returning to the set point of the thermostat.
This is an example of positive feedback. In positive feedback, the mechanism acts in the same direction as the initial change. The steady state is never restored, and extreme conditions are favored. There are only a few instances in which homeostatic mechanisms are positive feedback loops. One example is labor and delivery in childbirth. During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. The birth ends the release of oxytocin and ends the positive feedback mechanism.
Parts of a Feedback Mechanism
Let's continue thinking about the heating and cooling systems of a house. A sensor, such as a thermocouple, detects the temperature inside the house and sends the information to the thermostat. The thermostat compares the temperature to the set point. If the temperature is higher than the set point, the thermostat turns on the air conditioner. If the temperature is lower than the set point, the thermostat turns on the furnace. If the temperature matches the set point, nothing is done.
Feedback mechanisms consist of sensors, integrators, and effectors:
- Sensor: Monitors the environment (external or internal) and sends information to the integrator.
- Integrator: Processes information from the sensors and sends signals or commands to effectors.
- Effector: Takes actions to restore the environment to a steady state.
Note: The sensors are called receptors in biological systems.