Assignment Homeostasis and Feedback Loops I Homeostasis is

Assignment: Homeostasis and Feedback Loops

I. Homeostasis is the maintenance of a constant internal environment A. Homeostasis works to maintain the organism's internal environment within tolerance limits - the narrow range of conditions where cells are able to function.

B. Homeostasis is a self-adjusting mechanism involving feedback where the response alters the internal conditions based on the stimulus.

C. The body responds to: 1. The changing conditions of the external environment 2. The changing conditions of the internal environment

II. How homeostasis is achieved A. Structural: physical features specific for survival

B. Functional: metabolism adjusts to changes in conditions (controlled by hormones from the endocrine system)

C. Behavioral: the actions help an organism to survive in its environment.

III. Feedback mechanisms and loops A. Feedback is when the response to a stimulus has an effect of some kind on the original stimulus 1. Stimulus - any factor that causes a receptor to trigger impulses in a nerve pathway

B. Negative feedback is when the response stops the stimulus 1. Negative feedback is the most common type

1. Examples: a. Blood glucose concentrations rise after a sugary meal (the stimulus), the hormone insulin is released and it speeds up the transport of glucose out of the blood and into selected tissues (the response), so blood glucose concentrations decrease (thus decreasing the original stimulus).

b. Exercise creates metabolic heat which raises the body temperature (the stimulus), cooling mechanisms such as vasodilation (flushed skin) and sweating begin (the response), body temperature falls (thus decreasing the original stimulus).

C. Positive feedback is when the response enhances the original stimulus. 1. Positive feedback is not common

1. Examples: a. A ripening apple releases the volatile plant hormone ethylene (the stimulus). Ethylene accelerates the ripening of unripe fruit in its vicinity so nearby fruit also ripens, releasing more ethylene (the response). All the fruit quickly becomes ripe together. ("One 'bad' apple has ruined the whole lot. " The biological explanation - positive feedback - for an old saying!)

ADD THIS IN: The endocrine system controls homeostasis through hormones. Hormones are chemical messengers that have an effect on organs far from the gland.

1. Examples: b. A baby begins to suckle her mother's nipple and a few drops of milk are released (the stimulus). This encourages the baby and releases a hormone in the mother which further stimulates the release of milk (the response). The hungry baby continues to suckle, stimulating more milk release until she stops. (Positive feedback, it would not have helped the baby if suckling decreased milk flow, as in negative feedback!)

c. An example of positive feedback can be found in childbirth. The hormone oxytocin stimulates and enhances labor contractions. As the baby moves toward the birth canal, pressure receptors within the cervix (muscular outlet of uterus) send messages to the brain to produce oxytocin. Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract stronger. The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin production stops and labor contractions cease.

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