goldstandard_mcat wrote:Which of the following is an example of positive feedback?
A. A body temperature of 39 C causes a further increase
B. Elevated TSH results in elevated thyroxine
C. Calcitonin and parathormone regulate calcium levels
D. Increased TBG leads to an increase in TSH
Endocrine glands are often linked to neural control centers by homeostatic feedback mechanisms. The two types of feedback mechanisms, which happen to be frequent visitors to the real MCAT, are negative feedback and positive feedback. Negative feedback decreases the deviation from the normal value, and is important in maintaining homeostasis and thus controls most endocrine glands.
The classic example of negative feedback is the thermostat in your home. As the temperature rises (deviation from the ideal normal value), the thermostat detects the change and triggers the air-conditioning to turn on and cool the house (or, it turns off the heating). Once the temperature reaches its thermostat setting (normal value), the air conditioning turns off.
Positive feedback controls self-perpetuating events that can be out of control and do not require continuous adjustment. For example, in positive feedback the original stimulus is promoted rather than negated, thus the deviation from the normal value increases. Unlike negative feedback that maintains hormone levels within narrow ranges, positive feedback is rarely used to maintain homeostatic functions.
If calcium decreases, the parathyroid glands sense the decrease and secrete more parathyroid hormone. The parathyroid hormone stimulates calcium release from the bones and increases the calcium uptake into the bloodstream from the collecting tubules in the kidneys. Conversely, if blood calcium increases too much, the parathyroid glands reduce parathyroid hormone production. Both responses are examples of negative feedback because in both cases the effects are negative (opposite) to the stimulus.
If your increased body temperature triggers a further increase in body temperature (getting further away from the normal), then this is a classic example of positive feedback.
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