Feedback and Homeostasis How organisms respond to changing environments and situations.

As we recall from September, in order to stay alive all organisms must maintain homeostasis. They are able to do this because their bodies are able to adjust to changes in their internal and external environments.

Stimulus vs. Response. A stimulus is an event or condition that initiates a response. A response is the reaction to a specific stimulus. For example placing your hand on a hot stove (stimulus) will cause you to quickly move your hand away (response). The lack of food (stimulus) causes your stomach to grumble (response).

Dynamic Equilibrium. The condition of balance between varying, shifting, and opposing forces that is characteristic of living processes Your body is continually adjusting to changes in temperature, blood sugar levels, pH levels, fluid levels, etc. When you ride a bicycle, you are continually making minor adjustments to keep yourself upright. This is an example of dynamic equilibrium.

Positive and negative feedback Feedback means the return of information about the result of a process or activity. This feedback can be either negative or positive.

Negative Feedback = Agent of stability. A negative feedback system maintains stability by opposing the change caused by the stimulus. A familiar example of a negative feedback system would be a thermostat. When the temperature drops below the selected threshold, the furnace goes on, and the house heats up. Once the threshold temperature is reached, the thermostat senses this and turns off the thermostat.

A negative feedback mechanism in humans. When your blood sugar levels get to high, your pancreas releases insulin. Insulin removes sugar and lowers your blood sugar level. When your body sugar levels get too low, your pancreas releases glucagon. Glucagon releases store sugar which raises your blood sugar level. When blood sugar level gets too high, insulin is once again released.

Negative feedback in Plants On the lower surface of plant leaves are tiny pores called stomates, or stomata. The purpose of these pores is for the release of excess water vapor (via transpiration). On either side of these pores are two specialized cells called guard cells. When the plant is well watered, the guard cells are relaxed and the stomata are open. However, if the plant begins to dry up, the guard cells will expand and close the stomata. In this way the plant retains the water.

Negative feedback cont. Negative feedback systems are responsible for maintaining our dynamic equilibrium. Without them homeostasis would not be maintained, and we would die.

Positive Feedback = Agent of change A positive feedback system further disrupts stability by increasing the change caused by the stimulus. This type of feedback system does not lead to stability, and will continue until the system is completely disrupted.

Positive feedback examples Childbirth is an example of a positive feedback system. Pressure caused by the baby’s head on the mothers uterus causes an increase in the production of the hormone, oxytocin. Oxytocin causes more intense uterine contractions which in turns causes more pressure on the wall of the uterus. This leads to the release of even more oxytocin. This feedback loop continues until the child is born.

Positive feedback loop cont. Blood clotting is another example of a positive feedback loop. Not all positive feedback loops are beneficial. An illness can cause your body to increase the temperature in order to try to kill a pathogen. Until the pathogen is killed, the body will continue to increase the temperature. This can lead to a dangerous fever in which the body heats up to the point where the proteins begin to denature. This can lead to death.