chapter 1 part 2 positive feedback Flashcards
Positive-feedback mechanisms occur
when a response to the original stimulus results in the deviation from the set point becoming even greater
positive means
increase
At times, this type of response is required to
return to homeostasis.
during blood loss, a chemical responsible for blood clot formation,
called thrombin, stimulates production of even more thrombin.
What prevents the entire vascular system from clotting?
The clot formation process is self-limiting. Eventually,
the components needed to form a clot will be depleted in the damaged area and more clot material cannot be formed.
__ is another example of a normally occurring
positive-feedback mechanism.
birth
- Near the end of pregnancy,
the baby’s large size stretches the uterus.
- This stretching, especially around the opening of the uterus
stimulates contractions of the uterine muscles.
- The uterine contractions push the baby against the opening of the uterus,
which results in additional stretching.
This positive-feedback sequence ends
when the baby is delivered from the uterus and the stretching stimulus is eliminated
Two basic principles about homeostatic mechanisms to remember are that
a. many disease states result from the failure of negative-feedback mechanisms to maintain homeostasis,
b. some positive-feedback mechanisms can be detrimental instead of helpful.
One example of a detrimental positive-feedback mechanism is
inadequate delivery of blood to cardiac (heart) muscle.
Contraction of cardiac muscle generates blood pressure. The heart pumps blood to itself through a system of blood vessels on the outside of the heart. Just as with other tissues, blood pressure must be maintained to ensure adequate delivery of blood to the cardiac muscle. Following extreme blood loss, such as a severe cut on the body, not enough
blood is delivered to cardiac muscle. As a result, the heart cannot function
normally. The heart pumps less blood, which causes the blood pressure to
drop even further, which causes even less blood to be delivered to the heart.
The process continues until the heart stops beating, and death results.
In this example, we see the deviation from the heart-rate set point becoming larger
and larger—this is the hallmark of positive feedback.
Thus, if blood loss is severe,
negative-feedback mechanisms may not be able to maintain homeostasis and the positive feedback of ever-decreasing blood pressure can develop.
On the other hand, following a moderate amount of blood loss (e.g., after
donating a pint of blood),
negative-feedback mechanisms result in an increase in heart rate that restores blood pressure.
