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Understanding Homeostatic Mechanisms

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            The purpose of this lab exercise is to show how homeostatic mechanisms are controlled by negative feedback, and how hyperventilation can be controlled in the human body.


            Negative feedback mechanisms consist of restoring an organ or system’s activity to normal. In this exercise, a water bath was used to demonstrate how negative feedback mechanisms control homeostasis.

Hyperventilation is a non-physiological ventilation that is accompanied with the modification of normal partial pressures of blood gases. When hyperventilating, the partial pressure of CO2 in arterial blood decreases.

Hypothesis / Predicted outcome

            The results of this lab are expected to show that negative feedback does control homeostatic mechanisms, and that the acid-base balance in the human body can also control ventilation.

Method / Procedure

Water bath experiment

A water bath was set at 50°C; it was turned on and allowed to reach the needed temperature. After the heater was shut off, the temperature was monitored for 5 minutes with a thermometer. Then, the ice water (8.6°C) was added to the bath; the time needed for the temperature to return to the set point was calculated, and the water bath was shut off.

Buffer experiment

            50 ml of fresh water is poured in a beaker, after the pH is measured; one drop of HCL is added, the mixture is swirled, and two more drops are added, then pH is measured. 50 ml of fresh water are poured in a new beaker, and the same procedure is done with drops of NaOH, instead of HCl.

Exercise and CO2 production experiment

            100 ml fresh water is poured in a beaker, and 2 ml of the 10 N NaOH are added to it. 1-2 drops of phenolphthalein are then added to the solution; thus, it turns pink. After mixing, divide the solution equally into two beakers, then have the subject blow through the straw at a steady rate. Repeat the procedure and record the time spent exhaling through the straw.

Hyperventilation and CO2 production experiment

            The resting breathing rate of a subject is measured by counting the number of breaths in 30 seconds, then multiplying it by two. Then, count the number of breaths after the subject hyperventilated.

Results / Outcome

The outcome of the lab procedures generated data showing in the first place that the automatic system raised the water’s temperature by 0.2°C in 5 minutes, while it took 47 minutes for the temperature to rise from 37.6°C to 50°C, after adding the ice water.

The personal heart rate did not drastically change when at rest as seen on table 1. The average personal heart rate can be calculated as follows:

The group heart rate data shows afterwards that the non-exercise group has a higher heart rate at rest than the exercise group, providing an average heart rate for the exercise group of , and  for the non-exercise group, according to the table 2.

The data of the acid-base buffer experiment shows that water’s pH changes drastically, when mixed with HCl or NaOH as compared to the buffer’s one.

Then, the exercise and CO2 production experiment showed that the time to color change post-exercise is shorter than pre-exercise; and, finally, the last experiment showed that the respiratory rate after hyperventilation is also smaller than the rate after normal breathing.


            Regarding negative feedback, the water bath experiment clearly shows that the heating time is significantly increased after adding the ice water than after it was first turned on; this works analogically to the way the human body maintains homeostasis when exposed to cold weather, for example, and clearly shows how negative feedback helps in controlling homeostasis of body temperature.

            As for the personal heart rate data, there is not a substantial difference between average and single heart rate. This means that in resting conditions, the homeostasis is constantly maintained if no influence is made by external factors. On the other hand, data of the table 2 show that the average heart rate for the exercise group (70.8BPM) is lower than the one for non-exercise group (80.9BPM). As a matter of fact, the subject of the exercise group are more used to effort; therefore, homeostasis is more effectively regulated, and the average heart rate of the group is less variable than the second group.

            In terms of the acid-base buffer experiment, the results clearly show that a buffer (a solution that maintains approximately the same pH, despite the addition of small amounts of an acid or base or in spite of dilution) undergoes less pH variation than the fresh water. The table 3 shows exactly how water pH drops to 1.8, when adding a single drop of HCL, while the buffer only changes from 7.0 to 6.7. By analogy to the human body, the bicarbonate in the plasma serves as a buffer, constantly stabilizing blood pH.

            Finally the CO2 production experiments showed that it took less time for the color of the phenolphthalein solution to change post-exercise (18s) as compared to the time needed pre-exercise (32s) in the table 4. The respiratory rate also decreased from 16 bpm, after normal breathing, to 12 bpm after hyperventilation in the table 5. Those two tables confirm the hypothesis that after exercise, or hyperventilation, the CO2 production in the body is increased, which causes the body to regulate it by decreasing the respiration rate.


            It was shown that any change in the body’s constants (temperature, pH, CO2 production, heart rate, etc.) is regulated by negative feedback, the mechanism being called homeostasis.


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