17-06-2013, 01:01 PM
Homeostasis
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Introduction:
Homeostasis may be thought of as the central theme in physiology. All, or nearly all body systems
function to maintain a state of dynamic constancy within the internal environment, a state known as
homeostasis. The mechanism of maintaining this state of homeostasis typically involves negative
feedback control of effectors (muscles & glands).
The maintenance of homeostasis is a prerequisite for life. When homeostasis is disturbed the
disturbance must be corrected or the survival of the organism may be at risk. For example, the
symptoms and risk factors associated with most illnesses are due to uncorrected disturbances in
homeostasis. Thus, under normal circumstances all disturbances are quickly corrected. When the
disturbance is detected (by a sensor) this information is routed to an integrator for comparison to
homeostatic values and corrective instructions are delivered to effectors as necessary.
Exercise A: Heart Rate & Negative Feedback Control
The dynamic constancy of the internal environment, or homeostasis, can be evaluated by measuring
and recording physiological values such as heart rate, body temperature, blood pH etc... over a
period of time. Due to the dynamic nature of homeostasis, fluctuations in measured values are
normally seen during the measurement time period.
In this procedure we are going to measure heart rate which is determined by two antagonistic
effectors. Sympathetic nerves stimulate an increase in heart rate while parasympathetic nerves
produce an inhibitory effect that slows the heart rate.
Exercise A - Procedure
1. Select a healthy team mate with a strong pulse to serve as subject in this procedure. You will be
determining heart rate by palpating (feeling) the radial pulse at the wrist or the carotid pulse in
the neck.
2. To determine the pulse gently press your index and middle fingers (not your thumb) against the
radial artery in the subjects wrist or the carotid artery in the neck until you feel a pulse.
3. Count the number of pulses in a 15 second interval, record this number in the data table
provided.
4. Wait 15 seconds and take the subjects pulse during the next 15 seconds. Repeat this
procedure, taking a pulse during every other 15 second interval, for a 5-minute period. A total of
ten measurements will be obtained.
5. Since heart rate is expressed as beats or pulses per minute the values you recorded will need
to be corrected by multiplying each pulse count by 4 (each 15 second measurement represents
1/4 of the heart beats in a minute). Record the corrected heart rates (expressed as beats per
minute) in the data table provided. You now have a record of heart rate once every 30 seconds
for 5 minutes.
6. Graph the corrected heart rates (expressed as beats per minute) in your report by placing a dot
at the point corresponding to the heart rate for each measurement interval, then connect the
dots. Note: you will first need to establish the scale on the y axis. (Here is one way to do this:
write the lowest heart rate value at the bottom of the scale (by the L) and the highest value at
the top of the scale (by the H). Divide the difference between the high and low value by 6 to
determine the value of each interval on the Y axis.)
Exercise B: Calculation of Normal Homeostatic Values
Normal homeostatic values are based on a statistical average among normal healthy people. Because
people tend to differ, no single value (average or not) could apply to every person. As a result
“normal values” are often expressed over a range that encompasses the measurements of most
normal healthy people. Even with this adjustment perfectly healthy people may fall outside of
published normal ranges. Keep in mind, normal values are based on statistical averages and are
subject to the limitations of statistical analysis. In addition, the determination of what constitutes a
healthy individual from whom the normal range is determined is subject to interpretation.
For example, endurance athletes typically have a low heart rate while healthy moderately active
individuals often have a much higher heart rate. A normal range of heart rates based on both types of
individuals would differ dramatically from normal values determined from a population of athletes
alone. Thus the makeup of the population and how you determine which individuals are healthy can
significantly effect the “normal range” that is established.