Biomedical Engineering Reference
In-Depth Information
RESPONSE
RECEPTOR
INTEGRATOR
EFFECTOR
STIMULUS
FIGURE 3.39
Feedback mechanisms are used to help maintain homeostatis. A stimulus is received by a recep-
tor that sends a signal (messenger) to an effector or to an integrator that sends a signal to an effector. The effector
responds to the signal. The response feeds back to the receptor and modifies the effect of the stimulus. In negative
feedback, the response subtracts from the effect of the stimulus on the receptor. In positive feedback, the response
adds to the effect of the stimulus on the receptor.
integrator that can send messages to muscles or glands or both. The messages result in
some type of response from the effectors. The brain receives information about how parts
of the body are operating and can compare this to information about how parts of the body
should be operating.
Positive feedback mechanisms are those in which the initial stimulus is reinforced by the
response. There are very few examples of this in the human body, since it disrupts homeo-
stasis. Childbirth provides one example. Pressure from the baby's head in the birth canal
stimulates receptors in the cervix, which send signals to the hypothalamus. The hypothala-
mus responds to the stimulus by releasing oxytocin, which enhances uterine contractions.
Uterine contractions increase in intensity and force the baby further into the birth canal,
which causes additional stretching of the receptors in the cervix. The process continues
until the baby is born, the pressure on the cervical stretch receptors ends, and the hypothal-
amus is no longer stimulated to release oxytocin.
Negative feedback mechanisms result in a response that is opposite in direction to the
initiating stimulus. For example, receptors in the skin and elsewhere in the body detect
the body's temperature. Temperature information is forwarded to the hypothalamus in the
brain, which compares the body's current temperature to what the temperature should be
(approximately 37
C). If the body's temperature is too low, messages are sent to contract
the smooth muscles in blood vessels near the skin (reducing the diameter of the blood vessels
and the heat transferred through the skin), to skeletal muscles to start contracting rapidly
(shivering), and to the arrector pili muscles (see Figure 3.16a) to erect the hairs and form
“goose bumps.” The metabolic activity of the muscle contractions generates heat and warms
the body. If the body's temperature is too high, messages are sent to relax the smooth muscles
in the blood vessels near the skin (increasing the diameter of the blood vessels and the
amount of heat transferred through the skin) and to sweat glands to release moisture and
thus increase evaporative cooling of the skin. When the temperature of circulating blood
changes to such an extent in the appropriate direction that it reaches the set point of the
system, the hypothalamus stops sending signals to the effector muscles and glands.
Another example of a negative feedback mechanism in the body involves the regula-
tion of glucose in the bloodstream by clusters of cells, the pancreatic islets (Figure 3.40).
There are between 2
10
5
and 2
10
6
pancreatic islets scattered throughout the adult
