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Introduction to Homeostasis and Feedback Mechanisms

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.

house with air conditioner and furnace. air conditioner is in a box entitled hot weather and points to house. Furnace is in a dotted box entitled cold weather and points to house.

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.

Source: 
Images from Microsoft Word 2010.

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.

(A) a graph of temperature and a balance at the set point. an arrow points to B where the temperature increased and unbalanbed. The air conditioner triggers and points to C where the air conditioner cools and the balance returns back to normal.

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.

 balance of house temperature at set point. Arrow points to B where hot weather upsets the balance to increased temperature. Arrow points to C where increased temperature triggers furnace and furnace heats house, which increases the temperature further (D). Arrow points back to increased temperature triggers furnace.

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. 

Source: 
Images from Microsoft Word 2010.

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.

photo of a thermocouple with arrow pointing to a thermostat with arrow pointing to air conditioner and heater. The thermocouple is labelled sensor, the thermostat is labelled integrator, and the air conditioner and furnace are labeled effectors.

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.

Source: 
Images from Microsoft Word 2010.

Examples of Homeostasis and Feedback Mechanisms

Body Temperature Regulation

Consider that you are exercising in the hot sun. When your body temperature increases, you have a homeostatic mechanism to lower it:

diagram showing homeostatic mechanism to respond to increasing body temperature.

  1. Receptors in the skin detect the increased temperature. They send the information by nerve impulses to temperature centers in the hypothalamus.
  2. The hypothalamus integrates the information, and sends nerve impulses to the capillaries in the skin and to the sweat glands.
  3. The blood vessels in the skin become wider, which diverts more blood flow to the skin. The increased blood flow transfers heat, which is lost to the air through the skin. The sweat glands secrete sweat, which evaporates and cools the skin.
  4. The body temperature decreases to near normal. This continues until you leave the sunny area and stop exercising. The body temperature then returns to normal, and you stop sweating.

Note that this is a negative feedback mechanism.

Conversely, if you go out on a cold day, your body temperature decreases.

  1. Receptors in the skin detect the decreased temperature. They send the information by nerve impulses to temperature centers in the hypothalamus.
  2. The hypothalamus integrates the information, and sends nerve impulses to the capillaries in the skin and to skeletal muscles.
  3. The blood vessels in the skin become narrower, which reduces blood flow to the skin. The decreased blood flow reduces heat lost to the air through the skin. The muscles contract rapidly (shiver), which produces heat.
  4. These actions increase the body temperature.

Gas exchange in leaves

Homeostatic mechanisms are not limited to animals. Plants have them too.

Here's one example. There are openings in leaves called stoma (plural: stomata). The stoma have guard cells around them. Gases like carbon dioxide, oxygen, and water vapor enter and exit the leaf through these openings. Here's how the mechanism works:

During the day there is enough sunlight for photosynthesis. Photosynthesis in the guard cells causes them to accumulate potassium ions and water (by osmosis). The guard cells swell and open the stoma. As a result, gases can flow into and out of the leaf. Conversely, at night photosynthesis stops. The guard cells remove potassium and water (by osmosis). The guard cells shrink and close the stoma. Subsequently, no gases can move into or out of the leaf.

Source: 
Images from Microsoft Word 2010.

Homeostasis and Feedback Mechanisms Review

Properties of Homeostatic Mechanisms

When identifying or evaluating a homeostatic mechanism, look for these properties:

  • The mechanism should be triggered by some change in the environment—either internal environment or external environment.
  • The system should have a receptor or sensor, an integrator, and an effector. Sometimes these parts may be molecules or chemical processes.
  • The mechanism is negative feedback if it corrects the initial change. In other words, the response is in the opposite direction of the initial change in the environment.
  • The mechanism is positive feedback if it makes the initial change more intense. In other words, the response is in the same direction as the initial change in the environment.

Let's practice. Below are some feedback mechanisms. Identify the receptors, integrators, and effectors. Determine if it is a negative or positive feedback mechanism.