EndocrineSystem

Made up of glands that secrete hormones

Positive feedback (click link)

(less common)

Positive FeedbackWhen a change occurs in the body, there are two general ways that the body can respond. In negative feedback, the body responds in such a way as to reverse the direction of change. This is the type of response that is involved in maintaining constant conditions inside the body (see Homeostasis.) The second type of response is positive feedback. This means that if a change occurs in some variable, the response is to change that variable even more in the same direction. The result is a continuing spiral of change. (Eventually, negative feedback may take over to put a limit on things.) An example of positive feedback is the phenomenon of autacatalysis, which occurs in some digestive enzymes such as pepsin. Pepsin is a protein-digesting enzyme that works in the stomach. However, the stomach does not secrete pepsin; it secretes an inactive form, called pepsinogen. When one pepsinogen molecule becomes activated, it helps to activate other pepsinogens nearby, which in turn can activate others. In this way, the number of active pepsin molecules can increase rapidly, by using positive feedback. The pepsinogen-pepsin system is thought to be one of the ways that the stomach avoids digesting itself. Pepsin is a powerful enzyme for breaking down proteins, and could damage the very cells that produce it. By secreting it in an inactive form, the cells in the stomach's lining play it safe. Other cells, which produce mucus, help protect the stomach lining once pepsin is activated.

Negative Feedback (click link)

Used to maintain homeostasis

Homeostasis and Negative FeedbackHomeostasis is one of the fundamental characteristics of living things. It refers to the maintenance of the internal environment within tolerable limits. All sorts of factors affect the suitability of our body fluids to sustain life; these include properties like temperature, salinity, acidity, and the concentrations of nutrients and wastes. Because these properties affect the chemical reactions that keep us alive, we have built-in physiological mechanisms to maintain them at desirable levels. When a change occurs in the body, there are two general ways that the body can respond. In negative feedback, the body responds in such a way as to reverse the direction of change. Because this tends to keep things constant, it allows us to maintain homeostasis. On the other hand, positive feedback is also possible. This means that if a change occurs in some variable, the response is to change that variable even more in the same direction. This has a de-stabilizing effect, so it does not result in homeostasis. Positive feedback is used in certain situations where rapid change is desirable (see Positive Feedback for an example). To illustrate the components involved in negative feedback, we can use the example of a driver trying to stay near the speed limit. The desired value of a variable is called the set point. Here, the set point is a speed of 55 mph; in controlling body temperature, the set point would be 98.6 degrees. The control center is what monitors the variable and compares it with the set point. Here, the control center is the driver; for body temperature, it would be the hypothalamus of the brain. If the variable differs from the set point, the control center uses effectors to reverse the change. Here, the effector is the foot on the accelerator pedal; in controlling body temperature, it would include the glands that sweat and the muscles that shiver.

Autocatalysis

• Autocatalysis occurs in some digestive enzymes such as pepsin. Pepsin is a protein-digestive enzyme that is located in the stomach where it works. The stomach does not release pepsin but rather it secretes an inactive form, called pepsinogen. When one pepsinogen is activated, it activates other pepsinogen around it. Thus, the number of active pepsin molecules radically increases by using positive feedback.

Pepsin

Negative Positive Feedback

• Positive feedback is the opposite of negative feedback which creates homeostasis. An example of this is if the death of a small area of the heart stimulates a heart attack the heart pumps an inadequate amount of blood. Then, the heart muscle itself is deprived of blood which leads to “more” death of it.

Is Global Warming Positive Feedback?

Global warming can be a positive feedback. For example, if the tundra was to be one more degrees higher, then it would melt more, releasing more greenhouse gases, creating more heat, which would melt more tundra etcetera, etcetera.

Also, the ocean is a darker color which absorbs sun, which makes the surface temperature rise. With the surface water being warmer (even if by 1 degree) the glaciers melt and recede, which exposes more water.

Tundra

Ocean

Can Positive Feedback be…well…positive?

• Yes, in fact blood clotting is an example of positive feedback. The enzyme that speeds up the process of clotting also speeds up the process of thrombin. Thrombin, is a self-catalytic, self-accelerating effect, so that once the clotting begins, it runs faster and faster until, bleeding stops.

• Also, in childbirth there is positive feedback. When the uterus stretches which triggers the secretion of the hormone oxytocin which stimulates faster contractions which then speeds up labor.

Real examples…..

Goiter

ExopthalmosCommon symptom of Graves disease which can cause an overactive Thyroid gland