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a feedback loop in which the output of a system is increased by the mechanism's own influence on the system that creates it that output.
When an individual doesn't have enough to eat, the body adjusts to this by slowing down metabolism so that the individual expends fewer calories in order to conserve the limited energy available from the inadequate diet.
Homeostasis regulates an organism's internal environment and maintains a stable, constant condition of properties such as temperature or pH. It can be influenced by either internal or external conditions. Many different mechanisms make maintaining homeostasis possible. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated.
A sensor (or receptor) that detects changes in the internal or external environment. One example are the peripheral chemoreceptors, which detect changes in blood pH.
The integrating center (or control center) receives information from the sensors and initiates the response to maintain homeostasis. The most important example is the hypothalamus, a region of the brain that controls everything from body temperature to heart rate, blood pressure, satiety, and circadian rhythms.
An effector is any organ or tissue that receives information from the integrating center and acts to bring about the changes needed to maintain homeostasis. An example could be the kidney, which retains water if blood pressure becomes too low.
The sensors, integrating center, and effectors are the basic components of every homeostatic response. Positive and negative feedback are more complicated mechanisms that enable these three basic components to maintain homeostasis for more complex physiological processes.
Positive feedback is a mechanism in which an output is enhanced in order to maintain homeostasis. Positive feedback mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated. Positive feedback mechanisms are designed to push levels out of normal ranges. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial, but is rarely used by the body due to risks of the acceleration becoming uncontrollable. One positive feedback example in the body is blood platelet accumulation and aggregation, which, in turn, causes blood clotting in response to an injury in the lining of the blood vessels.
Negative feedback mechanisms reduce output or activity of any organ or system back to its normal range of functioning. A good example of this is regulating blood pressure. Blood vessels have sensors called "baroreceptors", which can detect if blood pressure is too high or too low, and sends a signal to the hypothalamus. The hypothalamus then sends a message to the heart, blood vessels, and kidneys which all act as effectors in blood pressure regulation. If blood pressure is too high, then the heart rate would decrease as the blood vessels increase in diameter (known as vasodilation) while the kidneys would retain less water. These changes would cause the blood pressure to fall back to its normal range. The opposite happens when blood pressure decreases, and causes vasoconstriction and the kidney to increase water retention. Similarly, the release of glucocorticoids by the adrenal cortex is stimulated by ACTH release by the anterior pituitary. As glucocorticoid levels rise, they prevent further release of hormones by the hypothalamus and pituitary, acting as a negative feedback mechanism .
Another important example of negative feedback mechanism is temperature control. Nervecells in the body act as sensors by relaying information about body temperature to the hypothalamus. The hypothalamus then signals several effectors to maintain the human body's temperature set point (37 degrees celcius). Examples of actions done by effectors to reduce body temperature if its above the set point include signaling the sweat glands to cool the skin, and vasodilation (relaxation) of blood vessels, which allows the body to give off more heat. Conversely if the body's temperature is below the set point, effectors such as muscles will shiver to generate heat and the blood vessels undergo vasoconstriction to retain heat in the body. This important example is very complex because the hypothalamus can change the body's temperature set point, such as raising it during a fever to help fight off an infection. Therefore, both internal and external events can induce negative feedback mechanisms.