Transcript Homeostasis - Faculty Home

Homeostasis
A condition in which the internal
environment of the body remains
relatively constant despite changes in
the external environment. Examples
would be the maintenance of body
temperature and levels of glucose in
the blood
Homeostatic mechanisms are designed to reestablish
homeostasis when there is an imbalance.
The Home Heating System
1. When the temperature of a room decreases below a set point, the
thermostat electrically starts the furnace.
2. As the temperature of the room rises to the set point, the thermostat
shuts down the furnace.
3. As the room cools, step one is repeated.
There are three components to this system:
1. The Sensor which detects the stress.
2. The Control Center which receives information from the sensor and
sends a message to the Effector.
3. The Effector which receives the message from the control center
and produces the response which reestablishes homeostasis.
There are three components to a homeostatic system:
1. The Sensor which detects the stress.
2. The Control Center which receives information from the sensor
and sends a message to adjust the stress.
3. The Effector which receives the message from the control
center and produces the response which reestablishes
homeostasis
It should be noticed that
1. the heat produced by the furnace shuts the furnace down
through the thermostat.
2. the original stress is reduced, i.e., the room warms up.
Homeostatic mechanisms that show these two
characteristics are operating by negative feedback
Homeostatic Regulation of Body Temperature through
Negative Feedback
Hyperthermia
Heat receptors
in the skin
Hypothalamus
Sensors
Control Center
Stress
Stress is reduced
shutting down
mechanism
Perspiration
evaporates
cooling the skin
Increased
activity of
sweat glands
Increased blood
flow to the skin
Effect
Effectors
Homeostasis Using a Neural Pathway
Control center
Many homeostatic
mechanisms use a
nerve pathway in which
to produce their effects.
These pathways
involve an afferent path
which brings sensory
messages into the
brain and an efferent
path which carries
outgoing nerve
messages to effectors.
Homeostatic Regulation of Blood Sugar through
Negative Feedback
Hyperglycemia
Stress
Pancreas-beta cells
Sensor and Control center
Stress is reduced
shutting down
mechanism
Blood glucose
is reduced
Liver and Muscle cells
take up glucose from
the blood
Effectors
Insulin is released
into blood
Negative Feedback Via a Hormonal Pathway
Regulation of Blood Sugar
Hormones play an important role in many homeostatic pathways.
Hormones are produced by endocrine glands. They enter the
blood after being produced and travel throughout the body.
However, hormones have their effect on specific target tissues.
Positive Feedback Mechanisms
Homeostatic systems utilizing positive feedback exhibit two primary
characteristics:
1. Time limitation – Processes in the body that must be completed within a
constrained time frame are usually modified by positive feedback.
2. Intensification of stress – During a positive feedback process, the initial
imbalance or stress is intensified rather than reduced as it is in negative
feedback.
Typical Positive Feedback Process
Stress
Intensifies
Sensor
Control Center
Effector
Homeostatic Regulation of Child Birth through
Positive Feedback
Pressure of Fetus on
the Uterine Wall
Nerve endings in the uterine
wall carry afferent messages
to the Hypothalamus
Intensifies
Increasing strength of
uterine contractions
Production and Release
of Oxytocin into the
Blood
The birth of the child will bring this process to a close. Other
examples of positive feedback regulation occur during milk
letdown and blood clotting.
Feedback in Coagulation
Positive feedback “mini-loops” are built into pathway to speed up
production of chemicals needed to form the clot. Entire sequence of
clotting is a negative feedback pathway:
Harmful Effects of Positive Feedback
Positive feedback can be harmful. Two specific
examples of these harmful outcomes would be:
1. Fever can cause a positive feedback within
homeostasis that pushes the body temperature
continually higher. If the temperature reaches 45
degrees centigrade (113 degrees Fahrenheit) cellular
proteins denature bringing metabolism to a stop
and death.
2. Chronic hypertension can favor the process of
atherosclerosis which causes the openings of blood
vessels to narrow. This, in turn, will intensify the
hypertension bring on more damage to the walls of
blood vessels.
1. What is homeostasis? Why is it called a dynamic equilibrium?
2. Describe these components of a homeostasis loop: stress, receptor,
control center, effector, response. Using an example, put them in order.
3. What are the benefits of a negative feedback response? In what
direction does a variable change as a result of a negative feedback
response to a stress?
4. What are the benefits of a positive feedback response? In what
direction does a variable change as a result of a positive feedback
response to a stress?
5. What are the risks associated with positive feedback responses?
6. Draw 2 graphs to show negative & positive feedback responses.
7. Explain how positive feedback events can be “built into” a negative
feedback loop.
8. What is homeostasis failure? What has happened with this occurs?