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HUMAN PHYSIOLOGY 20201
,CHAPTER 1
HOMEOSTASIS
Definition of Homeostasis
Homeostasis is a key concept in understanding how our body works. It means
keeping things constant and comes from two Greek words: 'homeo,' meaning
'similar,' and 'stasis,' meaning 'stable.' A more formal definition of homeostasis
is a characteristic of a system that regulates its internal environment and tends
to maintain a stable, relatively constant, condition of properties.
Homeostasis is happening constantly in our bodies. We eat, sweat, drink, dance,
eat some more, have salty fries, and yet our body composition remains almost
the same.
If someone were to draw your blood on ten different days of a month, the level
of glucose, sodium, red blood cells and other blood components would be pretty
much constant, regardless of your behavior (assuming fasting before drawing
blood, of course).
No matter how much water you drink, your body doesn't swell up like a balloon if
you drink tons, and it doesn't shrivel like a raisin if you drink very little. Have
you ever wondered about this?
Somehow, our bodies know how much fluid we need to keep, and then maintain a
constant level regardless of how much water we drink.
This maintenance of body size is an example of homeostasis. And we don't even
have to think about it for this to happen! Aren't our bodies amazing?
There are several other examples of homeostasis. For example, our
concentration of salts and glucose (sugar) is constant; our body temperature is
usually around 37 degrees Celsius (98.6 degrees Fahrenheit); the amount of
blood in our bodies is about 5 liters, the osmolarity (number of solutes) of our
blood remains about 300mOsm. The normal value of a physiological variable is
called its set point.
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,This adjusting of physiological systems within the body is called homeostatic
regulation.
Homeostatic regulation involves three parts or mechanisms:
1) The receptor,
2) The control
center
3) The effector.
The receptor receives information that something in the environment is
changing.
The control centre or integration centre receives and processes information
from the receptor.
And lastly, the effector responds to the commands of the control centre by
either opposing or enhancing the stimulus.
This is an ongoing process that continually works to restore and maintain
homeostasis.
For example, in regulating body temperature there are temperature receptors
in the skin, which communicate information to the brain which is the control
centre, and the effector is our blood vessels and sweat glands in our skin.
Because the internal and external environment of the body are constantly
changing and adjustments must be made continuously to stay at or near the set
point, homeostasis can be thought of as a synthetic equilibrium.
Since homeostasis is an attempt to maintain the internal conditions of an
environment by limiting fluctuations, it must involve a series of negative
feedback loops.
Positive and Negative Feedback
When a change of variable occurs, there are two main types of feedback to
which the system reacts:
Positive Feedback Mechanisms Homeostatic systems utilizing positive feedback
exhibit two primary characteristics: 1. Time limitation – Processes in the body
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, 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 Sensor
Control Center EffectorIntensifies
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 Production and Release of Oxytocin into the
Blood Increasing strength of uterine contractions Intensifies The birth of the
child will bring this process to a close.
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: Feedback in Coagulation
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 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.
Home Heating System vs. Negative Feedback
When you are at home, you set your thermostat to a desired temperature. Let's
say today you set it at 70 degrees.
The thermometer in the thermostat waits to sense a temperature
change either too high above or too far below the 70 degree set point.
When this change happens the thermometer will send a message to the "Control
Center", or thermostat,
Which in turn will then send a message to the furnace or heater to either shut
it off if the temperature is too high or kick back on if the temperature is too
low.
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