Laboratory for Physiology
Homeostasis: The Unifying Concept of Physiology
Objectives:
1) To define and illustrate the concept of homeostasis using pulse rate as a
model.
2) To understand how negative feedback mechanisms work to maintain
homeostasis.
3) To demonstrate how homeostatic mechanisms are characterized.
Introduction:
The concept that unifies the field of physiology is that biological systems
maintain their internal environment despite changes in the external environment. This
concept is known as homeostasis. Homeostasis can be defined as a state of dynamic
equilibrium. This means that in biological systems, the internal environment is not held at
an absolute, fixed state, but instead it is allowed to fluctuate around a set point. For
example, body temperature in humans is maintained around 98.6oF. However, body
temperature is not held rigidly at this temperature. If you measured your body
temperature over the period of a day or week, you would find that it varies around the set
point of 98.6oF. It is also important to understand that, the set point is just an average
value for the population and may vary from individual to individual. That is to say, what
is “normal” may be different for each person.
Homeostasis of the internal environment is maintained by what are known as
negative feedback mechanisms. Negative feedback mechanisms involve sensors that
monitor the internal environment, and activate mechanisms that act to bring the internal
environment back to the set point. Because drifts from the set point activate mechanisms
that lead to changes that oppose the drift, the mechanisms are termed negative feedback
mechanisms. For example, when body temperature falls below 98.6oF, temperature
sensors in the hypothalamus activate the sympathetic nervous system, which in turn
causes vasoconstriction of cutaneous blood vessels. This decreases blood flow to the skin
and keeps warmer blood away from the skin, thus minimizing heat loss and bringing
body temperature back up. On the other hand, if body temperature rises above the set
point, the temperature sensors in the hypothalamus initiate responses that lead to
vasodilation of the cutaneous blood vessels, which increases blood flow to the skin. The
increased blood flow to the skin serves to increase heat loss to the environment and body
temperature comes back down. This balancing between heat conservation and heat
dissipation (vasodilation and vasoconstriction) goes on throughout a person’s life.
In studying homeostasis, physiologists attempt to characterize the parameters of
the control system. These parameters are:
1) The set point.
2) The sensitivity of the system is a measure of the variation in values that is allowed
before negative feedback mechanisms are activated.
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Laboratory for Physiology
Collected data is usually summarized in terms of statistical values. The values commonly
determined are:
Mean (X): The mean is defined as the average value of a set of numbers. The set point is
generally estimated as being equal to the mean.
Standard Deviation (SD): Standard deviation is defined as the square root of the
following formula:
SD= Xi - X)2
n-1
In this formula:
SD is standard deviation
represents summation
Xi = each individual data point
X = the mean or average of all the data points
n = the total number of data points
In a normally distributed data set, 68% of the data points fall within one standard
deviation of the mean. One standard deviation from the mean is sometimes referred to as
the range. The sensitivity of a physiological system is estimated by the standard
deviation.
This lab will illustrate the concept of homeostasis.
Negative Feedback Control of Pulse Rate
In living organisms, homeostasis is maintained by negative feedback mechanisms.
In most cases, to maintain homeostasis in the internal environment, living organisms
use negative feedback mechanisms that act antagonistically to each other. The
antagonistic effects of the parasympathetic and sympathetic divisions of the nervous
system largely determine resting heart rate. An increase in activity of the sympathetic
division of the nervous system causes an increase in heart rate. Whereas, an increase
in parasympathetic activity causes a decrease in heart rate. The balance between
sympathetic and parasympathetic inputs to the heart is maintained in a state of
dynamic equilibrium by negative feedback loops initiated by sensory receptors that
monitor blood pressure in the aorta and carotid arteries, and by other factors.
Therefore, resting heart rate is not constant, but varies around a set point. This part of
the lab will demonstrate that your pulse rate is in a state of dynamic equilibrium
(implying that negative feedback mechanisms are at work).
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Laboratory for Physiology
Each member of your team will monitor the heart rate of 4 other class members by
taking his/her pulse every 15 seconds for 5 minutes.
The pulse may be monitored by palpating the radial artery in the wrist or the carotid
artery in the neck (see Fig. 1). The carotid pulse can be palpated just below the angle
of the mandible. The radial pulse can be palpated on the wrist just medial to the radial
bone. You should not monitor your own pulse, as this will affect the results.
A
B
Figure 1. A. Palpating the pulse in the carotid artery. B. Palpating the pulse in the
radial artery.
Pulse rate is usually expressed as pulses per minute. However, you will only monitor
the pulse per 15-second intervals so you will need to multiply your count by 4 to get
pulses per minute.
Starting at T=0, you should take your teammate’s pulse for the first 15 seconds,
record your finding and pause 15 seconds. Then take your next reading for 15
seconds, record your finding and pause 15 seconds. Carrying this procedure out for 5
minutes will give you a total of 10 measurements that you can convert to pulses per
minute after you have finished.
You will need to gather data from four people in the class, and at least one of your
subjects must be of the opposite gender.
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Laboratory for Physiology
DATA SHEET
Data for Negative Feedback Control of Pulse Rate
Subject’s initials:_______
Measurement
1
2
3
4
5
6
Pulses/15secs
Pulses/min
Average pulse rate (set point): ______ pulses/min
Standard Deviation (sensitivity): ______
Subject’s initials:_______
Measurement
1
2
3
4
Pulses/15secs
Pulses/min
Average pulse rate: ______pulses/min
Standard Deviation: ______
Subject’s initials:_______
Measurement
1
2
3
4
Pulses/15secs
Pulses/min
Average pulse rate: ______ pulses/min.
Standard Deviation: ______
Subject’s initials:_______
Measurement
1
2
3
4
Pulses/15secs
Pulses/min
Average pulse rate: ______ pulses/min
Standard Deviation: ______
7
8
9
10
5
6
7
8
9
10
5
6
7
8
9
10
5
6
7
8
9
10
4