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Introduction: All living things need to maintain a constant internal environment in order to survive. When an organism is affected by either a biotic (living) or abiotic (nonliving) stimulus that changes its internal environment in some way, it needs to respond accordingly. Examples of biotic stimuli might be the presence of a predator or the infection of a virus. Abiotic stimuli might be an increase in body or environmental temperature or a lack of water.

To respond to these stimuli, an organism uses a series of behavioral and physiological adaptations. Let's say that you are feeling warm while sitting on the beach. An example of a behavioral adaptation that would help to change this situation would be for you to move into the shade (like going under a beach umbrella, as seen in the image) or go into the water. Behavioral adaptations involve the entire organism. A physiological response involves parts of the organism. Using the beach example, to compensate for the increased body temperature, your body will dilate its blood vessels in order to increase the amount of heat loss. Additionally, your sweat glands will activate in order to increase the amount of heat lost from the surface of your skin.

The maintaining of a constant internal environment is called homeostasis. There are two mechanisms that drive the behavioral and physiological adaptations mentioned above. These are called feedback loops and they can be either negative or positive. Unlike its name, negative feedback is usually a beneficial thing. In the beach example, sweating is considered a negative feedback response. This is because it is trying to return the body back to a set point. As your body temperature increases from sitting in the Sun, negative feedback tries to return it to normal.

Conversely, positive feedback is usually bad. However, there are instances when it does benefit the organism. For example, during human childbirth (see image), a hormone called oxytocin is created. When the child presses down against the lower uterine wall, it causes contractions to start. As a result, more oxytocin is produced, which causes more contractions. This then creates more oxytocin, and so on, and so on. In this case, positive feedback is taking the body away from its set point.

Directions for this Lesson: Answer the practice questions and then watch the videos to learn more about how living things maintain their internal environments.

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