BIOL 1051H Chapter Notes - Chapter 1: Thyroid, Positive Feedback, Homeostasis
Homeostasis, primary tissue types and important basic cellular
mechanisms.
Homeostasis is the maintenance of a normal state of function within a narrow range.
This is a state of dynamic constancy in that there is continual fluctuation within a set of narrow
parameters. The stabilization of this range above and below a physiological set point is
maintained by negative feedback loops. These feedback loops involve some kind of sensor that
responds to changes in the system. When the sensor detects a change out of range, these
sensors will send a signal to an integrating system, such as the brain or spinal cord, or an
endocrine gland. The integrating system then causes a change in that system to keep it within
the set range. The cells that make the change are called effectors, because this is what makes
a change. An effector can be a muscle or gland.
These regulatory mechanisms can be intrinsic or extrinsic to the organ. An
intrinsic mechanism is built into the organ being regulated. For instance, perivascular
sphincters located on the small arteries leading to capillary beds can open or close
blood flow to a muscle or patch of skin. An extrinsic mechanism is a mechanism located
outside the organ. A good example of this would be a hormone produced in a gland
elsewhere in the body that changes the conditions in that organ. For example, thyroid
hormone produced in the thyroid gland will affect the heart by causing an increase in
heart beat rate, thereby increasing muscle flow to muscles.
For instance, we mammals regulate our body temperature within very narrow
range. If we get hot blood vessels in our skin will dilate to allow heat to escape, we may
also start sweating. If we get cold our metabolism will increase generating more heat; if
we get really cold our muscles will start contracting in small units to generate heat, this
is shivering. These opposing mechanisms are used to keep our body temperature within
the normal range. As you can see these mechanisms do not really keep the body at one
temperature. The temperature does fluctuate, only within a narrow range. This dynamic
consistency is homeostasis.
Another example of negative feedback in homeostasis is shown with changes in
blood pressure with body position. When you stand after lying down you experience a
sudden drop of blood pressure in the brain. This stimulates blood pressure receptors in
the brain. The brain then sends signals to the heart which causes it to beat faster,
thereby increasing pressure back to the normal range.
Contrast this with positive feedback. Positive feedback is very different from
negative feedback in that the changes that take place in the effector amplify the
Document Summary
Homeostasis, primary tissue types and important basic cellular mechanisms. Homeostasis is the maintenance of a normal state of function within a narrow range. This is a state of dynamic constancy in that there is continual fluctuation within a set of narrow parameters. The stabilization of this range above and below a physiological set point is maintained by negative feedback loops. These feedback loops involve some kind of sensor that responds to changes in the system. When the sensor detects a change out of range, these sensors will send a signal to an integrating system, such as the brain or spinal cord, or an endocrine gland. The integrating system then causes a change in that system to keep it within the set range. The cells that make the change are called effectors, because this is what makes a change. An effector can be a muscle or gland . These regulatory mechanisms can be intrinsic or extrinsic to the organ.
